FORCE, WORK, ENERGY & POWER - WordPress.com · dr. mohammed mostafa emam inaya medical college (imc) phys 101 - lecture 5 force, work, energy & power 1

DR. MOHAMMED MOSTAFA EMAM

INAYA MEDICAL COLLEGE

(IMC)PHYS 101 - LECTURE 5

FORCE, WORK, ENERGY & POWER

1

FORCE

• What change the state of object is called “force”.

• We mean by saying state, shape or position of the

object.

2

Newton's law of Motions

(All objects save their state of motion)

(Force causes acceleration)

(Every actions cause reactions)

3

Newton's law of Motions

(All objects save their state of motion)

1st law of motion


2ed law of motion

(Every actions cause reactions)

3rd law of motion

4

• In summary, you observe and apply force every day.

• Force is a vector quantity having both magnitude

and direction.

• We understand that cause of motion is “force”.

• The unit of force is Newton or kg.m/s².

5

FORCE

• You can kick the ball and apply a force then you give

speed to the ball and changes its position or you can

press the spring apply a force and give potential energy

to it, we can increase the number of example.

6

FORCE

• Newton's First Laws of Motion

(all objects save their state of motion)

• In his first law of motion Newton stated that

all objects save their state of motion.

• In other words, if an object is at rest it

continuous to be at rest and if it is moving it

continuous to move unless a nonzero force

is applied on it.

• What we mean by a word “nonzero force”? Look at

the following picture to understand what we mean.7

We called nonzero force “net force” also

8

FORCE

• Newton’s Second Law of Motion

9

FORCE

• In the previous topics we said that force causes acceleration.

• Moreover, we also learned the net force concept in the last

section.

• Now, we deal with the relation between force and acceleration.

• Newton’s Second Law of Motion


10

FORCE

• As you remember, acceleration is the rate of change in the

velocity of the object. This change occurs because of the net

force. Thus, we can say that there is a linear relation

between the net force acting on the object and acceleration.

We show this relation like;

11

FORCE

• If we increase the amount of net force than the acceleration

also increases in the same amount.

• If we decrease the net force than acceleration also

decreases. Let’s see it from the following pictures.

12

FORCE

• If we increase the amount of net force than the acceleration

also increases in the same amount. If we decrease the net

force than acceleration also decreases. Let’s see it from the

following pictures.

13

FORCE

• From this relation:

•

14

FORCE

•Example

• Find the acceleration (a) of the block given in the picture below.

where, F is the force and its unit is Newton, m is mass and has the unit

kg and a is the acceleration has unit m/s².

15

FORCE

•Example


where, F is the force and its unit is Newton, m is mass in Kg, and has

the unit kg and a is the acceleration has unit m/s².

16

FORCE

•Example


where, F is the force and its unit is Newton, m is mass in Kg, and has

the unit kg and a is the acceleration has unit m/s².

17

FORCE

•Example

Given in the picture below, a horse is pulling the horsebox having

8 kg mass in it with a force of 40N; if the applied force has an

angle of 37º to the horizontal; calculate the acceleration of the

horsebox.

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FORCE

19

FORCE

20

FORCE

21

FORCE

22

FORCE

23Direction of the acceleration is in the direction of

Fx which is the net force in direction of motion.

FORCE

•Example

Find the weight (W), of the object having mass 15 kg.

24

FORCE

•Example

Find the weight of the object having mass 15 kg.

25

FORCE

• Equilibrium Conditions:

Equilibrium in physics means, forces are in balance. The net

force should be zero.

In other words, forces acting downward and acting upward,

and forces acting right and acting left should be equal in

magnitude. Look at the illustration shape to understand that.

26

FORCE

In the given picture there are five forces acting on our block.

Let us analyze them, whether this block is in equilibrium or not.

Forces acting downward are; 10N+5N=15N Force acting on

upward is; 15N.

Thus, we can say that,

27

FORCE

In the given picture there are five forces acting on our block.

Let us analyze them, whether this block is in equilibrium or not.

Forces acting downward are; 10N+5N=15N Force acting on

upward is; 15N.


our block is in equilibrium in +y, -y direction.

28

FORCE

• Now, look at the forces acting right and left. As you see,

they are also equal in magnitude and opposite in direction.


29

FORCE

• Now, look at the forces acting right and left. As you see,

they are also equal in magnitude and opposite in direction.


our block is in equilibrium in +y, -y direction.

our block is in equilibrium in +x, -x direction.

30

FORCE

•Example

If the boy is in equilibrium, find the G from the given data in picture.

31

FORCE

•Example


As we see;

T at each arm is = 30 N

32

FORCE

•Example


33

FORCE

34

FORCE

35

FORCE

36

FORCE

37

(sin 30º =0,5)

FORCE

38

(sin 30º =0,5)

FORCE

39

(sin 30º =0,5)

FORCE

• Newton’s Third Law of Motion:

(Every actions cause reactions / normal force)

• In this law Newton states that, when we apply a force on an object then it also apples force in same magnitude but opposite in direction.

• In general, all actions have reactions in the same

magnitude but opposite direction. Suppose that, when

you swim you push the water to the backward direction

and water also pushes you to the forward direction.40

FORCE

• We can increase the examples;

1- When birds fly they push the air with their wings and the

air also push the bird in the opposite direction, in this way

birds can stay in the air and fly.

• In every where, we can see this couple of forces. Everyactions cause reactions.

41

FORCE

• 2- Force exerted by the rifle to the bullet gives acceleration

to the bullet, moreover, the bullet also exert a force to the

bullet which is the reaction of action force and as a result

rifle is recoiled.

42

FORCE

• Look at the picture given above.

• 3- The book applies a force because of its weight, and the

table shows a reaction to this action. These force pairs are

same in magnitude but as you see their directions are

opposite. In the next section we examine this example in

detail.

43

FORCE

Normal Force:

• We call this force as “normal force” which is same in

magnitude and opposite in direction with the applied force

(weight of the book).

• For different situations; in general, force is the reaction to

the perpendicular force exerting on it.

44

FORCE•Example

Find the normal force that the inclined plane exerts on the box.

(sin37º=0,6 cos37º=0,8),

(m=4kg, g=10m/s²).

45

FORCE

46

FORCE

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FORCE

48

FORCE

49

FORCE

50

CHECK YOUR UNDERSTANDING

• 1. Who was the scientist who gave us the Laws of Motion?

51


• 1. Who was the scientist who gave us the Laws of Motion?

• Answer: Sir Isaac Newton

52


• 2. How many Laws of Motion are there?

53


• 2. How many Laws of Motion are there?

• Answer: three

54


• 3. What is another name for the first law of motion?

55


• 3. What is another name for the first law of motion?

• Answer: Law of Inertia

56


• 4. Which law explains why we need to wear seat belts?

57


• 4. Which law explains why we need to wear seat belts?

• Answer: First Law of Motion

58


• 5. Which law says that force is equal to mass times acceleration (F=MA)?

59


• 5. Which law says that force is equal to mass times acceleration (F=MA)?

• Answer: Second Law of Motion

60


• 6. Which law says that heavier objects require more force than lighter objects to move or accelerate them?

61


• 6. Which law says that heavier objects require more force than lighter objects to move or accelerate them?

• Answer: Second Law of Motion

62


• 7. Which law explains how rockets are launched into space?

63


• 7. Which law explains how rockets are launched into space?

• Answer: Third Law of Motion

64


• 8. Which law says that for every action there is an equal and opposite reaction?

65


• 8. Which law says that for every action there is an equal and opposite reaction?

• Answer: Third Law of Motion

66

Work, Energy & Power

67

Work

• The work done by force is defined

as the product of that force times

the parallel distance over which it

acts.

cosFsWThe unit of work is the newton-meter,

called a joule (J).

Energy

• The amount of energy transferred

to the object is equal to the work

done.

• Types of Energy

• Kinetic Energy = “Motion Energy”

• Potential Energy = “Stored Energy”

Kinetic Energy

• Kinetic Energy is the energy

possessed by an object

because it is in motion.

2

21 mvKE

Gravitational

Potential Energy• Gravitational Potential Energy is

the energy possessed by an

object because of a gravitational

interaction.

mghPEG

Conservation of Energy

• Energy can neither be

created nor destroyed, but

only transformed from one

kind to another.

finalinital )PEKE(W)PEKE(

Power

• Power is the time rate of doing work.

SpeedForce

work thisdo taken totime

force aby donework erAveragePow

Power

• The unit of power is;

a joule per second, called a Watt (W).

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FORCE, WORK, ENERGY & POWER - WordPress.com · dr. mohammed mostafa emam inaya medical college (imc) phys 101 - lecture 5 force, work, energy & power 1