Chapter 2 Force and Motion STUDENT MODULE 2009

JPN Pahang Physics Module Form 4Students’ Module Chapter 2 : Force and Motion

2. FORCE AND MOTION

2.1 ANALYSING LINEAR MOTION

Distance and displacement

1. Types of physical quantity:

(i) Scalar quantity: ………………………………………………………………….

(ii) Vector quantity: …………………………………………………………………

2. The difference between distance and displacement:

(i) Distance: …………………………………………………………………………

(ii) Displacement: ……………………………………………………………………

3. Distance always longer than displacement.

4. Example: The following diagram shows the location of Johor Bahru and Desaru. You can travel by car using existing road via Kota Tinggi, or travel by a small plane along straight path. Calculate how far it is from Johor Bahru to Desaru if you travelled by:a. The car

b. The plane

Solution:

Hands-on Activity 2.2 pg 10 of the practical book.Idea of distance and displacement, speed and velocity.

Speed and velocity

1. Speed is ..…………………………………………………………………………………

2. Velocity is: ..……………………………………………………………………………...

3. Average of speed: ………………………………………………………………………

4. Average of velocity: ……………………………………………………………………...

1

Kota Tinggi

60 km

41 km 53 km

DesaruJohor Bahru


5. Example:

An aero plane flies from A to B, which is located 300 km east of A. Upon reaching B, the aero plane then flies to C, which is located 400 km north. The total time of flight is 4 hours. Calculate

i. The speed of the aero plane ii. The velocity of the aero plane

Solution:

Acceleration and deceleration

1. Study the phenomenon below;

Observation: ………………………………………………………………………………

2. Acceleration is, ……………………………………………………………………….

Then, a =

3. Example of acceleration;

2

20 m s-10 m s-1 40 m s-1

Or, a = v – u t

20 m s-10 m s-1 40 m s-1

A B Ct = 2 s t = 2 s


Calculate the acceleration of car;

i) from A to B

ii) From B to C

4. Deceleration happens ...…………………………………………………………………

………………………………………………………………………………………………

5. Example of deceleration;

A lorry is moving at 30 m s-1, when suddenly the driver steps on the brakes and it stop 5 seconds later. Calculate the deceleration of lorry.

Analyzing of motion

1. Linear motion can be studied in the laboratory using a ticker timer and a ticker tape.

Refer text book photo picture 2.4 page 26.

(i) Determination of time:

(ii) Determination of displacement as the length of ticker tape over a period of time. x y

(iii) Determine the type of motion;

………………………………………………………………………………………..

...……….……………………………………………………………………………..

.……………………………………………………………………………………..

3

20 – 0 2= 10 m s-2

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .


(iv) Determination of velocity

displacement = ……………………… time = ………………………………..

Velocity, v =

(v) Determine the acceleration

The equation of motion

1. The important symbols : ………………………………………………………………..

………………………………………………………………………………………………

2. The list of important formula;

3. Example 1 : A car travelling with a velocity of 10 m s-1 accelerates uniformly at a rate of

3 m s-2 for 20 s. Calculate the displacement of the car while it is accelerating.

4

Length/cm

8

7

6

5

4

3

2

1

0

u

v

ticks

. . . . . . . .


Example 2 : A van that is travelling with velocity 16 m s-1 decelerates until it comes to rest. If the distance travelled is 8 m, calculate the deceleration of the van.

Exercise 2.1

1. Figure 2.1 shows a tape chart consisting of 5-tick strip. Describethe motion represented by AB and BC.In each case, determine the ;

A to B acceleration, BC uniform velocity(a) displacement

(b) average velocityFigure 2.1

(c) acceleration

2. A car moving with constant velocity of 40 ms-1 . The driver saw and obstacle in front and he immediately stepped on the brake pedal and managed to stop the car in 8 s. The distance of the obstacle from the car when the driver spotted it was 180 m. How far is the obstacle from where the car has stopped?

5

Length / cm

16

12

8

4

0A B C Time/s


2.2 ANALYSING MOTION GRAPHS

The data of the motion of the car can be presented………………………………….

The displacement-time Graph

6

0m 100m 200m 300m 400m 500m displacement0s 10s 20s 30s 40s 50s time

a) displacement (m) Graph analysis:

………………………………………………………………

………………………………………………………………

time (s) ……………...………………………………………………

b) displacement (m) Graph analysis:

……..…………………………………………………………

…………………………………………………………………

time (s) ……….…………………………………………………………

c) displacement (m) Graph analysis:…….……………………………………………………………

…………………………………………………………………

time (s) ..…………………………………………………………………

d) Displacement (m) Graph analysis:

…………………………….………………………………………

……………………………………………..………………………

time (s) ………………………………………………………………………

…………………………………………………………………


The velocity-time Graph

c) v (m s-1) Graph analysis:

…………………………………..………………….

………………………………………………………

………………………………………………………

t1 t2 t (s)

7

e) displacement (m) Graph analysis:

…………………………………………………………..

…………………………………………………………..

………………………………………………………….. time (s)

…………………………………………………………..

f) displacement (m) Graph analysis:

A B …………………………………………………………..

…………………………………………………………..

…………………………………………………………… O C time (s)

a) v/ m s-1 Graph analysis:

…………………………………………………………..

…………………………………………………………..

……………………………………………………………

t t / s

b) v/ m s-1 Graph analysis:

………………………………………..………………..

…………………………………………………………

…………………………………………………………

t t / s …………………………………………………………


d) v (m s-1) Graph analysis:

...…………………………………..………………..

……………………………………………………..

………………………………………………………

t (s) ………………………………………………………

.……………………………………………………...

e) v (m s-1) Graph analysis:

………..…………………………..………………..

……….

……………………………………………..

………………………………………………………

t (s) ………………………………………………………

Examples ………………………………………………………

1. s/m

20

10

-10

2.

8

O

P Q

R

S

0 2 4 6 8 t/s

O

P Q

R

v/m s-1

10

5

0 2 4 6 8 10 t/s

Calculate:- (i) Acceleration, a over OP, PQ and QR(ii) Displacement

Solution :

Calculate:- (i) Velocity over OP, QR and RS(ii) Displacement

Solution :


Exercise 2.2

1. (a) s/m (b) s/m (c) s/m

10

t/s 0 2 4 t/s t/s

-5 -10

Figure 2.21

Describe and interpret the motion of a body which is represented by the displacementtime graphs in Figure 2.21

2. Describe and interpret the motion of body which is represented by the velocity-time graphs shown in figure 2.22. In each case, find the distance covered by the body and its displacement

(a) v/m s-1 (b) v/m s-1

10

t/s 0 2 4 t/s

-5 -10

Figure 2.22

9


2.3 UNDERSTANDING INERTIA

Idea of inertia

1. ………………………………………………………………………………………………

2. ………………………………………………………………………………………………

3. ………………………………………………………………………………………………

Hand-on activity 2.5 in page 18 of the practical book to gain an idea of inertia

4. Meaning of inertia :

…………..………………………………………………………………………………….

………………………………………………………………………………………………

Mass and inertia

1. Refer to figure 2.14 of the text book, the child and an adult are given a push to swing.

(i) which one of them will be more difficult to be moved ……………………...

(ii) which one of them will be more difficult to stop? …………………………….

2. The relationship between mass and inertia : ……………………………….

……………………………………………………………..

3. The larger mass ………………………………………………………………………….

………………………………………………………………………………………………

Effects of inertia

1. Positive effect : …………………………………………………………………………

(i) ………………………………………………………………………………………

(ii) ………………………………………………………………………………………

(iii) ………………………………………………………………………………………

2. Negative effect : ………………………………………………………………………….

(i) ……………………………………………………………………………………...

……………………………………………………………………………………..

(ii) ………………………………………………………………………………………

………………………………………………………………………………………

(iii) ………………………………………………………………………………………

10


………………………………………………………………………………………

(iv) ………………………………………………………………………………………

Exercise 2.3

1. What is inertia? Does 2 kg rock have twice the inertia of 1 kg rock?

………………………………………………………………………………………………

………………………………………………………………………………………………

………………………………………………………………………………………………

2.

Figure 2,3

A wooden dowel is fitted in a hole through a wooden block as shown in figure 2.31. Explain what happen when we

(a) strike the top of the dowel with a hammer,

………………………………………………………………………………………

………………………………………………………………………………………

(b) hit the end of the dowel on the floor.

………………………………………………………………………………………

……………………………………………………………………………………

2.4 ANALYSING MOMENTUM

Idea of momentum

1. When an object is moving, …...…………………………………………………………

2. The amount of momentum ...……………………………………………………………

3. Momentum is defined…………………………………………………………………….

………………………………………………………………………………………………

11


Conservation of momentum

The principle of conservation of momentum :

………………………………………………………………………………………………………

………………………………………………………………………………………………………

1. Elastic collision .…………………………………………………………………………..

12

(mb + mg)

mg vg = 0

mb

vb&g

Momentum = mbvb

Momentum = (mb+mg)vb&g

Starting position before she catches the ball

vb

Receiving a massive ball

mb vb

mg

vg

Momentum = mbvb

Momentum = - mgvg

Starting position before she throws the ball

Throwing a massive ball

m1

m2 m1m2

u2 v2

u1


Before collision after collision

2. Inelastic collision :………………………………………………………………………...

Before collision after collision

3. explosion : …….....…………………………………………………………………...

Before explosion after explosion

Example 1 :

Car A Car B

Car A of mass 100 kg travelling at 30 m s-1 collides with Car B of mass 90 kg travelling at 20 m s-1 in front of it. Car A and B move separately after collision. If Car A is still moving at 25 m s-1 after collision, determine the velocity of Car B after collision.

Solution :

Example 2 :

Car A of mass 100 kg travelling at 30 m s-1 collides with Car B of mass 90 kg travelling at 20 m s-1 in front of it. Car A is pulled by Car B after collision. Determine the common velocity of Car A and B after collision.

Solution :

13

m1 m2 m1 + m2

u2 = 0

u1

v

(m1 + m2), u = 0 v1

m2

v2


Example 3 :

A bullet of mass 2 g is shot from a gun of mass 1 kg with a velocity of 150 m s-1 . Calculate the velocity of the recoil of the gun after firing.

Solution :

Exercise 2.4

1. An arrow of mass 150 g is shot into a wooden block of mass 450 g lying at rest on a smooth surface. At the moment of impact, the arrow is travelling horizontally at 15 ms-1. Calculate the common velocity after the impact.

2. A riffle of mass 5.0 kg fires a bullet of mass 50 g with a velocity of 80 m s-1 .Calculate the recoil velocity. Explain why the recoil velocity of a rifle is much less than the velocity of the bullet.

2.5 UNDERSTANDING THE EFFECT OF A FORCE

Idea of force

1. What will happen when force act to an object?

………………………………………………………………………………………………

14


………………………………………………………………………………………………

………………………………………………………………………………………………

Idea of balanced forces

1. An object is said to be in balance when it is:

………………………………………………………………………………………………

………………………………………………………………………………………………

2. Stationary object

……………………………… explanation :

………………………………………………

………………………………………………

……….……………………………………..

…………………………………………

3. An object moving with uniform velocity

…………………………….. explanation :

…..……………. …………… ……………………………………………..

……………………………………………..

……………………………………………..

……………………………… ………..…………………………………….

……………………………………………..

……………………………………………..

Idea of unbalanced forces

1. A body is said to be in unbalanced..……………………………………………………

2. ……………………….. Explanation;

………………………………………………

………………………………………………

………………………………………………

……… …….. ………………………………………………

Relationship between forces, mass and acceleration (F = ma)

15

Stationary object


Experiment 2.2 page 29.Aim : To investigate the relationship between acceleration and force applied on a constant mass.

Experiment 2.3 page 31Aim: To investigate the relationship between mass and acceleration of an object under constant force.

1. Refer to the result of experiment 2.2 and 2.3,

………………………………………………………………………………………………

………………………………………………………………………………………………

………………………………………………………………………………………………

………………………………………………………………………………………………

………………………………………………………………………………………………

2. 1 Newton (F = 1 N) is defined as the force required to produce an acceleration of 1 m s-2 (a=1 m s-2) when it is acting on an object of mass 1 kg ( m = 1 kg)So, …………………………………………………………………………………………

3. Example 1 : Calculate F, when a = 3 m s-2 and m = 1000 kg

Example 2 :

Calculate the acceleration, a of an object.

Exercise 2.5

1. A trolley of mass 30 kg is pulled along the ground by horizontal force of 50 N. The opposing frictional force is 20 N. Calculate the acceleration of the trolley.

16

m = 25 kgm = 25 kgF = 200 NF = 200 N


2. A 1000 kg car is travelling at 72 km h-1 when the brakes are applied. It comes to a stop in a distance of 40 m. What is the average braking force of the car?

2.6 ANALYSING IMPULSE AND IMPULSIVE FORCE

Impulse and impulsive force

1. Impulse is ……………………………………………………………………………….

2. Impulsive force is ………………………………………………………………………

………………………………………………………………………………………………

3. Formula of impulse and impulsive force:

Refer, F = ma

Example 1; v u

wall

If ; u = 10 m s-1 , v = - 10 m s-1 , m = 5 kg and t = 1 s

Impulse, Ft = and impulsive force, F =

Example 2; v u

Wall with a soft surface

If ; u = 10 m s-1 , v = - 10 m s-1 , m = 5 kg and t = 2 s

Impulse, Ft = and impulsive force, F =

17


4. The relationship between time of collision and impulsive force.

………………………………………………………………………………………………

………………………………………………………………………………………………

Exercise 2.6

1. A force of 20 N is applied for 0.8 s when a football player throws a ball from the sideline. What is the impulse given to the ball?

2. A stuntman in a movie jumps from a tall building an falls toward the ground. A large canvas bag filled with air used to break his fall. How is the impulsive force reduced?

2.7 BEING AWARE OF THE NEED FOR SAFETY FEATURES IN VEHICLES

18


Importance of safety features in vehicles

Safety features Importance

Padded dashboard

Rubber bumper

Shatter-proof windscreen

Air bag

Safety seat belt

Side bar in doors

Exercise 2.7

1. By using physics concepts, explain the modifications to the bus that help to improve that safety of passengers and will be more comfortable.

19

Safety features in vehicles


2.8 UNDERSTANDING GRAVITY

Carry out hands-on activity 2.8 on page 35 of the practical book.

Acceleration due to gravity.

1. An object will fall to the surface of the earth because………………………………...

2. The force of gravity also known ………………………………………………………...

3. When an object falls under the force of gravity only, ………………………………...

………………………………………………………………………………………………

4. The acceleration of objects falling freely ………………………………………………

5. The magnitude of the acceleration due to gravity depends ………………………...

………………………………………………………………………………………………

Gravitational field

1. The region around the earth is ………………………………………………………….

2. The object in gravitational field …………………………………………………………

3. The gravitational field strength is defined ……………………………………………..

4. The gravitational field strength, g can be calculated as;

5. At the surface of the earth, …………….

………………………………………………………………………………..

6. This means

……………………………………………………………………………………………..

20


7. Example 1. Can you estimate the gravitational force act to your body?mass = 60 kg, g = 9.8 N kg-1, F = ?

Example 2,A satellite of mass 600 kg in orbit experiences a gravitational force of 4800 N. Calculate the gravitational field strength.

Example 3,A stone is released from rest and falls into a well. After 1.2 s, it hits the bottom of the well.(a) What is the velocity of the stone when it hits the bottom?

(b) Calculate the depth of the well.

Weight

1. The weight of an object is defined ……………………………………………………..

2. For an object of mass m, the weight can be calculated as :

Example : The mass of a helicopter is 600 kg. What is the weight of the helicopter when it land on the peak of a mountain where the gravitational field is 9.78 N kg-1?

21


Exercise 2.8

1. Sketch the following graphs for an object that falling freely.

(a) Displacement-time graph,(b) Velocity-time graph(c) Acceleration-time graph

2. The following data was obtained from an experiment to measure the acceleration due to gravity.Mass of steel bob = 200 g, distance covered = 3.0 m, time of fall = 0.79 s.Calculate the acceleration due to gravity of steel bob.Give the explanation why your answer different with the constant of gravitational acceleration, g = 9.8 m s-2.

2.9 IDEA OF EQUILIBRIUM FORCES

An object is in equilibrium when :

1. ………………………………………………………………………………………………

2. ………………………………………………………………………………………………

stationary object

22


An object moving with uniform velocity

Addition of Force

1. Addition of force is defined as ...……………………………………………………..

………………………………………………………………………………………………

………………………………………………………………………………………………

Examples : the forces are acting in one direction

F1 = 10 N

F2 = 5 N

Resultant force, F

Example : the forces are acting in opposite directions

F1 = 10 N

F2 = 5 N

Resultant force, F

Example : the forces are acting in different directions

F2 = 5 N

500 F

23


F1 = 10 N

Parallelogram method:

1. Draw to scale.

2. Draw the line parallel with F1 to the edge of F2, and the line parallel with F2 to the

edge of F1

3. Connect the diagonal of the parallelogram starting from the initial point.

4. Measure the length of the diagonal from the initial point as the value of the

resultant force.

F2

F

F1

Triangle method

1. Draw to scale.

2. Displace one of the forces to the edge of another force.

3. Complete the triangle and measure the resultant force from the initial

point.

Example 1: During Sport Day two teams in tug of war competition pull with forces of

6000 N and 5300 N respectively. What is the value of the resultant force?

Are the two team in equilibrium?

Example 2: A boat in a river is pulled horizontally by two workmen. Workmen A

24


pulls with a force of 200 N while workmen while workmen B pulls with a

force of 300 N. The ropes used make an angle 250 with each other. Draw a

parallelogram and label the resultant force using scale of 1 cm : 50 N.

Determine the magnitude of resultant force.

Resolution of a force

1. Resolution of a force is …………………………………………………………………

Refer to trigonometric formula:

Example : The figure below shows Ali mopping the floor with a force 50 N at an angle of 600 to the floor.

F = 50 N

25


Example of resolution and combination of forces

Problem solving

1. When a system is in equilibrium, ……………………………………………………….

2. If all forces acting at one point are resolved into horizontal and vertical

components, ……………………………………………………………………………

3. Example 1; Show on a figure;

a) the direction of tension force, T of stringb) the resultant force act to lampc) calculate the magnitude of tension force, T

a)

mlamp = 1.5 kg

Exercise 2.9

1. Two force with magnitude 18 N and 6 N act along a straight line. With the aid of diagrams, determine the maximum possible value and the minimum possible value of the resultant force.

26

F = ?

200 N

400

T b) T’ T700 700


2. A football is kicked simultaneously by two players with force 220 N and 200 N respectively, as shown in Figure 2.9. Calculate the magnitude of the resultant force.

220 N

900

200 N

2.10 UNDERSTANDING WORK, ENERGY AND EFFICIENCY

Work

1. Work is done, ……………………………………………………………………………..

………………………………………………………………………………………………

2. WORK is the product.…………………………………………………………………….

………………………………………………………………………………………………

3. The formulae of work;

4. Example 1;

Example 2;

27

Force, F

s


80 N

600

s = 5 m

Example 3;

Example 4;

F = 600 N

S = 0.8 m

Energy

1. Energy is .................................................................................................................

28


2. Energy cannot be ....................................................................................................

3. Exist in various forms such as …………………...……………………………………

………………………………………………………………………………………………

4. Example of the energy transformation;

………………………………………………………………………………………………

………………………………………………………………………………………………

5. ………………………………………………………………………………………………

Example :

………………………………………………………………………………………………

Work done and the change in kinetic energy

1. Kinetic energy is …………………………………………………………………………

2. Refer to the figure above,

3. Example 1; A small car of mass 100 kg is moving along a flat road. The resultant force on the car is 200 N.a) What is its kinetic energy of the car after moving through 10 m?b) What is its velocity after moving through 10 m?

29

s

Force, F

Through, v2 = u2 +2as u = 0

and, as = ½ v2


Work done and gravitational potential energy

h = 1.5 m

1. Gravitational potential energy is………………………………………………………...

………………………………………………………………………………………………

2. Refer to the figure above;

3. Example; If m = 10 kg

Principle of conservation of energy

Carry out hands-on activity 2.10 on page 38 of the practical book.

To show the principle of conservation of energy.

1. Energy cannot be ………………………………………………………………………

……………………………………………………………………………………………

2. Example : a thrown ball upwards will achieve a maximum height before changing its direction and falls

3. Example in calculation : A coconut falls from a tree from a height of 20 m. What is the velocity of coconut just before hitting the earth?

30


Power

1. Power is …………………………………………………………………………………

2. A weightlifter lifts 180 kg of weights from the floor to a height of 2 m above his head in a time of 0.8 s. What is the power generated by the weightlifter during this time? g = 9.8 ms-2)

Efficiency

1. Defined……..…………………………………………………………………………….

2. Formulae of efficiency :

3. Analogy of efficiency;

Energy transformation

4. Example; An electric motor in a toy crane can lift a 0.12 kg weight through a height of 0.4 m in 5 s. During this time, the batteries supply 0.8 J of energy to the motor. Calculate(a) The useful of output of the motor.

(b) The efficiency of the motor

31

Device/ mechineDevice/ mechine


Carry out hands-on activity 2.11 on page 39 of the practical book to measure the power.

Exercise 2.10

1. What is the work done by a man when he pushes a box with a force of 90 N through a distance of 10 m? State the amount of energy transferred from the man to the force.

2. A sales assistant at a shop transfers 50 tins of milk powder from the floor to the top shelf. Each tin has a mass of 3.0 kg and the height of thee top shelf is 1.5 m.

(a) Calculate the total work done by the sales assistant.

(b) What is his power if he completes this work in 250 s?

2.11 APPRECIATING THE IMPORTANCE OF MAXIMISING THE EFFICIENCY

OF DEVICES

32


1. During the process of transformation the input energy to the useful output energy,

……………………………………………………………………………………..

2. .……………………………………………………………………………………………..

3. ………………………………………………………………………………………………

Example of wasting the energy;

………..…………………

Input energy output from the petrol energy

…………………… ……………. ……………… …………………….

..………………….. …………….. ………………….. …………………….

..………………….. ……………. …………………. …………………….

4. The world we are living in face acute shortage of energy.

5. It is very important that a device makes

…………………………………………………………………

Ways of increasing the efficiency of devices

1. Heat engines ……………………..………………………………………………………

………………………………………………………………………………………………

2. Electrical devices. ...……………………………………………………………………...

………………………………………………………………………………………………

………………………………………………………………………………………………

………………………………………………………………………………………………

………………………………………………………………………………………………

………………………………………………………………………………………………

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………………………………………………………………………………………………

………………………………………………………………………………………………

………………………………………………………………………………………………

………………………………………………………………………………………………

………………………………………………………………………………………………

………………………………………………………………………………………………

Operation of electrical devices

1. The electrical devices increase the efficiency………………………………….……

2. Proper management ….....………………………………………………………………

3. …………..………………………………………………………………………………

………………………………………………………………………………………………

2.12 UNDERSTANDING ELASTICITY

Carry out Hands-on activity 2.12 page 40 of the practical book.

1. Elasticity is ……………………………………………………………………………...

………………………………………………………………………………………………

2. Forces between atoms …………………………………………………………………..

………………………………………………………………………………………………

3. Forces between atoms in equilibrium condition

Explanation :

………………………………………………………………………………………………

………………………………………………………………………………………………

………………………………………………………………………………………………

4. Forces between atoms in compression

Explanation ;

34

Force of repulsion

Force of attraction

Force of repulsion

compressive force compressive force

Force of repulsion Force of repulsion


………………………………………………………………………………………………

………………………………………………………………………………………………

………………………………………………………………………………………………

5. Forces between atoms in tension

force of attraction

stretching force stretching force

Explanation ;

………………………………………………………………………………………………

………………………………………………………………………………………………

………………………………………………………………………………………………

Carry out Experiment 2.4 on page 41 of the practical bookTo investigate the relationship between force and extension of a springHooke’s Law

1. Hooke’s Law states ………………………………………………………………………

………………………………………………………………………………………………

2. Elastic limit of a spring is defined……………………………………………………….

………………………………………………………………………………………………

3. The spring is said to have a permanent extension,...…………………………………

………………………………………………………………………………………………

………………………………………………………………………………………………

4. The elastic limit is not exceeded,…………………………………………….…………

………………………………………………………………………………………………

………………………………………………………………………………………………

5. Graf F against xGraf F against x

F/ N

35

x (cm)0

EQ P

R


6. Spring Constant, k

F/N

0.8

0 8 x/cm

Example 1; A spring has an original length of 15 cm. With a load of mass 200 g attached, the length of the spring is extended to 20 cm. a. Calculate the spring constant. b. What is the length of the spring when the load is in increased

by 150 g? [assume that g = 10 N kg-1]

Example 2;

36

The graph shows the relationship between the stretching force, F and the spring extension, x. (a) Calculate the spring constant of P and Q.(b) Using the graph, determine the stretching force acts to spring P and spring Q, when their extensions are 0.5 cm

F (N)

x (cm)

P

Q

8

7

6

5

4

3

2

1

0 0.1 0.2 0.3 0.4 0.5

Graph F against x of spring P and spring Q


Elastic potential energy

1. Elastic potential energy ………………………………………………………………..

spring with the original length

F compression

x spring compressed x

F x = compression x

x F spring extended

x = extension F, extension

Other situation where the spring extended and compressed

Relationship between work and elastic potential energy

Graph F against x

Example ;

Factors that effect elasticity

Hands-on activity 2.13 on page 42 the practical book to investigate the factors that affect elasticity.

Type of material different same same same

Diameter of spring wire same different same same

Diameter of spring same same different same

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x / cm

F/N

F x

15 cm

5 kg

8 cm


Length of spring same Same same different

Summarise the four factors that effect elasticity

Factor Change in factor Effect on elasticity

Length

Diameter of spring

Diameter of spring wire

Type of material

Exercise 2.12

1. A 6 N force on a spring produces an extension of 2 cm. What is the extension when the force is increased to 18 N? State any assumption you made in calculating your answer.

2. If a 20 N force extends a spring from 5 cm to 9 cm,(a) what is the force constant of the spring?

(b) Calculate the elastic potential energy stored in the spring.

Reinforcement Chapter 2

Part A : Objective Questions

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1. When a coconut is falling to the ground, which of the following quantities is constant?

A. Velocity B. Momentum C. Acceleration D. Kinetic energy

2. In an inelastic collision, which of the following quantities remains constant before and after the collision?

A. Total acceleration B. Total velocity C. Total momentum D. Total kinetic energy3. Calculate the weight of a stone with

mass 60 g on the surface of the moon.(The gravitational acceleration of the moon is 1/6 that of the Earth.)

A. 0.1 N B. 0.2 N C. 0.4 N D. 0.6 N E. 0.8 N

4. The momentum of a particle is depend on

A. mass and acceleration B. weight and force C. mass and velocity 5. Which of the following diagrams

shows a body moving at constant velocity?

A. 2 N 2N

B. 12 N 7 N

C. 12 N 14 N

D. 20 N 17 N

6. The graph below shows the motion of a trolley with mass 1.5 kg.

Velocity / ms-1

4

0 2 4 6 Time / s Calculate the momentum of the trolley from t = 2s to t = 4s.

A. 1.5 kg m s-1 B. 3.0 kg m s-1

C. 4.0 kg m s-1

D. 6.0 kg m s-1

E. 7.5 kg m s-1

7. This figure shows an aircraft flying in the air.

8. m = 0.3 kg

5 m

What is the momentum of the stone just before it hits the ground?

A. 0.15 kg m s-1 B. 0.3 kg m s-1

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Lift

Thrust Air friction

Weight The aircraft above accelerates if

A. Lift Weight B. Thrust Lift C. Lift Air frictionD. Thrust Air friction


C. 1.5 kg m s-1 D. 3.0 kg m s-1 E. 15.0 kg m s-1

Solution :

9. A big ship will keep moving for some distance when its engine is turned off. This situation happens because the ship has

A. great inertia B. great acceleration

C. great momentum D. great kinetic energy

10. An iron ball is dropped at a height of 10 m from the surface of the moon. Calculate the time needed for the iron ball to land.(Gravitational acceleration of the moon is 1/6 that of the Earth and g = 9.8 N kg-2)

A 0.6 sB 1.4 sC 1.7 sD 3.5 sE 12.0 s

Part B : Structure Questions

1.

(i) Car A (ii) Car B

Diagram 1.1

Diagram 1.1(i) and (ii) show two methods used by the mechanic to move a breakdown car. A constant force, F = 500 N is used to push and pull the car in method A and B. (a) (i) Which method is easier to move the car?

………………………………………………………………………………

(ii) State a reason for your answer in (a)(i).

………………………………………………………………………………

………………………………………………………………………………

(b) The frictional force acting between the car and track surface in both methods is 200 N. Calculate, the

(i) horizontal resultant force in method A.

(ii) horizontal resultant force in method B.

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(iii) acceleration of the car in method B.

( c ) Suggest a method to move Car B so that the acceleration produced is equal to that of method A.……………………………………………………………………………..………..

………………………………………………………………………………………

2. ceiling

Tin water M N hand

P Q R (i) Diagram 2.1 (ii)

a) Diagram 2.1(i) shows tin P that is empty and tin Q that is filled with water. A student find difficult to pushed tin Q. Write the inference about the observation.

………………………………………………………………………………………b) Diagram 2.1(ii) shows a tin being released from the different positions M and N.

The hand of a student at position R needs greater force to stop the motion of the tin falling from position M. Explain this observation.

………………………………………………………………………………………

………………………………………………………………………………………c) Based on the observation (i) and (ii), state two factors that affect the magnitude of

the momentum of the object.

………………………………………………………………………………………d) If water flows out from a hole at the bottom of the tin Q, how would the inertia of

Tin Q depends on time ?

……………………………………………………………………………………

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3. 2 ms-1

P iron ball ( 2 kg ) S T

3.0 m smooth surface 1.0 m 2.0 m

Q R

Diagram 3 Rough surfaceThe figure shows a iron ball that is rolled through PQRST. The rough surface of QR has frictional force of 4 N.a) Calculate

(i) the kinetic energy of the iron ball at P.

(ii) the potential energy of the iron ball at P.

(iii) the total of energy of the iron ball at P.

b)

c) (i) Calculate the total of energy of the iron ball when it reaches at Q ?

(ii) Calculate the work done against friction along QR.

d) Calculate the total kinetic energy of the iron ball at S.

e) Calculate the speed of the ball at position T.

Part C : Essay Questions

1.

42


(i) (ii)Diagram 1.1

Diagram 1.1(i) shows the condition of a car moving at high velocity when it suddenly crashes into a wall.Diagram 1.1(ii) shows a tennis ball hit with racquet by a player. a) (i) What is the meaning of momentum?

(ii) Based on the observations of Diagram (i) and (ii), compare the characteristics of car when it crashes into the wall and the tennis ball when it is hit with a racquet. Hence, relate these characteristics to clarify a physics concept, and name this concept.

b) Explain why a tennis player uses a taut racquet when playing.

c) In launching a rocket, a few technical problems have to be overcome before the rocket can move upright to the sky. By using appropriate physics concepts, describe the design of a rocket and the launch techniques that can launch the rocket upright.

2.

Brand Reaction time / s Mass / kg Engine thrust force / N

Resistance force / N

A 0.3 1.5 10.0 4.0B 0.5 1.8 12.5 2.4C 0.2 0.9 6.5 2.2D 0.6 2.5 16.0 6.5

In a radio-controlled car racing competition, 4 mini-cars branded A, B, C and D took part. The information of the 4 cars is given in the table above. Details of the above information are given as below;Reaction time - Duration between the moment the radio-controlled is switched on and the moment the car starts moving.

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Properties


Resistance - Average value of opposing forces includes the friction between wheels and track, and air resistance.

(a) What is the meaning of acceleration?(b) Draw a graph of velocity against time that shows a car moving initially with

constant acceleration, then moving with constant velocity and followed by constant deceleration until it stops.

(c) Explain the suitability of the properties in the above table in constructing a radio-controlled car racing purpose. Hence, determine which brand of car will win the 50-metre race.

(c) If Car B in the above table is moved up the plane at the angle of 30o to the horizon,(i) Show that the car is able to move up the plane.(ii) Determine the acceleration of the car.

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Documents

Chapter 2 Force and Motion STUDENT MODULE 2009