Demonstrate understanding of aspects of mechanics.

Science A.S. 1.1

2014

FORCES

Forces are pushes or pulls (a combination is a twist).  

Forces act in pairs.

Objects are stationary or travelling at a constant speed when forces are balanced.

Unbalanced forces cause changes to objects motion (speed or direction), or shape.

The unit of force is the Newton (N)

FORCE PAIRS

Forces act in pairs (e.g. thrust and friction, Weight and support).

Force diagrams show the direction of forces acting on an object and whether they are balanced or unbalanced.

Arrow size represents force size if no measurements are available.

Weight

Thrust

Support

Friction

Force pairs

• What are the missing terms?

• Support (Buoyancy)

• Friction(Drag)• Thrust• Weight

TYPES OF FORCESIf a force acts on a stationary object and causes motion, the object has gained kinetic (movement) energy.

Friction will stop the object moving.

Types of force:

Gravity

Electrostatic

Tension – the force in rope, etcMagnetism

Friction – the force that opposes motion

Support

Bouyancy – in the water

Lift – in the air (planes/birds)

CONTACT FORCESSome forces only act on contact, others can act from a distance.

Which are which?

Contact DistanceGravity

Electrostatic

Tension

Magnetism

Friction

Support

Balanced and Unbalanced Forces

What do the size of the top and bottom arrows say about the forces and how do the vertical arrows differ?

Label the forces

Net force is the overall force acting on the object

Thrust

Support

Weight

Friction

If the force of friction is 10N and Thrust is 22N what is the net force?

What is the vertical net force and what does this say about the motion of the object?

BALANCED OR UNBALANCED?

Explain whether the forces in the following scenarios balanced or unbalanced.

1. The international space station is orbiting Earth at about 28,000kmhr-1.

2. A can is being crushed.

3. A car is travelling at a constant speed.

4. A skydiver has just jumped from a plane.

5. A car stays at 50kmhr-1 as it turns a corner.

Motion of objects• Balanced forces

– Net force is 0N– Motion is stationary or travelling at a constant

speed

• Unbalanced forces– Motion is accelerating or decelerating

• The rate of acceleration depends on the mass of the object and force applied…Force = mass × acceleration

(F = m × a).

F

m aIf m is constant and a increases what happens to F?

FORCE AND MOTIONWhat happens when you apply (using a Newton meter) a small constant force to a trolley and time it over a set distance?

Small constant force

An unbalanced force causes acceleration.

The trolley should accelerate because…

Set distance

F

m a

FORCE AND MOTIONWhat happens when you apply (using a Newton meter) the same small constant force to a trolley carrying a 1kg mass and time it over a set distance?

Small constant force

The larger the mass the slower the acceleration

The trolley should accelerate but slower than previously because…

Set distance

1Kg

F

m a

FORCES AND ACCELERATION

Given the formula F = ma try the following questions.

1. What are the names and units of F, m and a?

2. Complete the table….

3. The rider and cycle are 150kg:

a. What is the Nett force?

b. What is the cyclist’s acceleration?

4. A bike accelerates at 10ms-2 using a force of 900N. The rider is 70kg. What is the mass of the bike?

F m A

a. 9kg 0.5ms-2

6N 0.2kg b.

c. 800g 1.5ms-2

350N d. 15ms-2

e. 1200kg 0.015ms-2

800N150N

Pressure• Pressure is the measure of the

force that acts on an area.

• Pressure (Pa) = Force (N) / area (m2)

• 1Pa is the same as 1N/m2

• The larger the area the lower the pressure

F

P a

Pressure CalculationsPressure (Pa) = Force (N) / area (m2)

A skier weighs 500N, each of

her skis has an area

of 0.05m2.

Calculate the total pressure exerted by her skis on the snow.

P=F/A

P =500/ (0.05 +0.05)

P= 5000Pa

Pressure Calculations

• If her weight increases to 600N, what area would each ski have to exert to have the same pressure as before?

A = F/P

A = 600/5000

A = 0.12m2 in total

• Therefore 0.12/2 = each ski needs an area of 0.06m2

Explain using pressure why trying to use the sharp end to push the round end into the board is not a good idea. P=F/A. The force applied remains constant but the round end has a larger area so applies a low pressure to your thumb, but the sharp end has a smaller area and applies a high pressure to the notice board, so it pushes in.

Test Yourself

WEIGHT FORCE

Weight is a force. It is therefore measured in…

An object’s weight depends on two things…

Newtons (N)

Gravity

• varies depending where you are

• 10ms-2 or 10N/Kg on Earth

Mass

• does not vary

• measured in Kg

•A man with mass of 75Kg on earth weighs 750N

BUT on the moon he weighs 125N

Weight Loss

Planet Mass relative to Earth

Radius

Mercury 0.06 2400

Venus 0.82 6000

Earth 1 6400

Mars 0.1 3400

Jupiter 318 71500

Saturn 95 60300

Uranus 14 25600

Neptune 17 24800

Pluto 0.002 1200

MASS AND WEIGHT

1. a. What is mass?

b. What is it’s unit?

2. a. What is Weight?

b. What is it’s unit?

3. ON EARTH: 1N = kg 1kg = N

4. How would your mass and weight change if you went to Jupiter?

FRICTION

Friction is a contact force that opposes motion, it causes heat, damage, wear and slowing

 Friction can be reduced by…

lubrication,

streamlining (aerodynamics),

slowing down,

smoothing surfaces

ENERGYThere are 2 main energy forms…

Moving objects have Kinetic Energy

Stored energy is called Potential Energy

Potential Kinetic

Sound

Light

Gravitational

ElasticChemical

Electrical

HeatSome different types are:

Nuclear

ENERGY CHANGES

Energy cannot be created or destroyed. It can only be converted from one form into another.

Energy changes occur all of the time…

A light bulb glowing

A person climbing a ladder

Something that is energy efficient converts from one form to another very well; it loses very little energy.

Electrical Light + Heat

Chemical Gravitational + Heat (body)

Energy Transformations• Energy can neither be created or destroyed

but it is converted from one form to another.

• What transformations are involved when a long jumper competes

• Chemical kinetic gravitational potentialkinetic sound/heat

What about driving and turning on a light?

ENERGY CALCULATIONSEnergy possessed by objects can be calculated. It is measured in joules.

Ek = ½ mv2

Calculate Ek of a 500Kg car travelling at 10m/s

Ek = ½ mv2

Ek = ½ x 500 x 100

Ek = 25000j

Kinetic Energy (J) Mass (Kg) Velocity (ms-1)

Write Formula

Substitute numbers

Answer with units

ENERGY CALCULATIONS

Calculate Ep of a 500Kg car being lifted 1.5m onto a tow truck

Ep = mgh

Ep = 500 x 10 x 1.5

Ep = 7500j

Gravitational potential Energy (J)

Mass (Kg) Height (m)

Ep = mgh

10ms-1 or 10NKg-1

Write Formula

Substitute numbers

Answer with units

Test Yourself

• A 70kg climber is 50m up a wall when he slips. He falls 30m in 3 sec and remains stationary hanging from his rope.

• Discuss the relative size of the forces acting on the climber as he falls.

• State whether they are balanced or unbalanced.

• Sketch a speed v time graph for his motion during the 4 secs following his slip.

Test yourself (cont.)

• State the energy transformations involved as he falls.

• Calculate the gravitational

potential energy before

and after his fall and why

there is a difference in

these values.

WORKWork is done when a force moves a distance.

When work is done, energy is converted into different forms.

When our skydivers fall energy is converted from…

Gravitational Potential energy into…

Kinetic Energy (and heat).

Work = Force distance

W = Fd W

F dIn Joules (J)

Work Calculations

Work (J) = Force (N) distance (m)

• How much work is done when a crane lifts 1000Kg of concrete 3m?

• Weight of concrete = 1000kg x 10N/Kg

= 10000N

• Work = 10000N x 3m

• Work = 30000j

W

F d

Test Yourself• A 1200kg car drives up a 25m slope using a force of

5000N. Calculate the work done.

• W=Fxd• W = 5000N x 25m• W = 125000 j• Calculate the difference between energy gained and

work done and explain what happened to the energy?• Describe the energy transformation which takes place

during the journey.

10m25m

POWERHow Powerful something is depends on 2 things…

How much Work is done (= energy converted)

and the Time it takes.

Power = Work ÷ time

P = W/t

or

P = E/t

How come?

W

P t

Power is measured

in…

Watts

A weightlifter raises 50Kg by 60cm in 1.2s. Calculate his power.

SPEED

Speed is the distance that an object travels in a period of time.

d

t v

Units are meters and seconds (and therefore meters per second). However, sometimes km/hr is more sensible.

A cyclist travels 25 km in ½ an hour.

What is their speed - in kmh-1 V=d/t

V= 25km/0.5hr

V= 50km/h

V=d/t

V= 25000m/1800s

V= 13.89m/s

- in ms-1

DISTANCE/TIME GRAPHSA car takes 1.5 minutes to travel 500m down a busy road. It stops at lights for 30 seconds, then continues on for 1 minute as it goes another 1km.

Plot this on a distance/time graph.

Time (min)1 2 3

Distance

(km)

0.5

1

1.5

Using the distance/time graph:

1.What is the total distance traveled?

2. In what part of the trip is the car going the fastest?

3. What is the fastest speed?

= 1.5 km

= part 3

v = Δd / Δt

v = 1km/1min

v = 1000m/60s

v = 16.67m/s

Steepest section is fastest

Δd = 1km

Δt = 1min

In a distance/time graph the slope of the line = the speed of the object.

SPEED QUESTIONSWhat would these look like on a distance/time graph?

1. stopped

2. slow

3. fast

4. accelerating

ACCELERATIONAcceleration is the change in speed in an object in a period of time.

Δ v

a Δ t

Units ms-2

It takes a cyclist 20 seconds to go from a standing start to 14m/s.

What is their acceleration?

What is 14m/s in km/hr?

a = Δv/Δt

a = 14m/s / 20s

a = 0.7ms-2

= 14 × 60s × 60min 1000m

= 50.4km/hConvert m to Km divide by 1000

Convert Km to m multiply by 1000

Convert hours to seconds multiply by 3600 (60 minx 60 sec)

Convert seconds to hours divide by 3600

SPEED/TIME GRAPHSA runner travels at 4m/s for 10 seconds, then stops suddenly for 5 seconds, then accelerates for 5 seconds to get to 8m/s and continues for 10 seconds.

Plot this on a speed/time graph.

Time (sec)10 20 30

Speed

(m/s)

4

8Using the speed/time graph:

In what part of the trip is the runner going the fastest?

What is the acceleration in part 4?

= part 5

In a speed/time graph the slope of the line = the acceleration of the object.

a = Δv/Δt

a = 8m/s/5s

a = 1.6ms-2

SPEED/TIME GRAPHS

Time (sec)10 20 30

Speed

(m/s)

4

8

What distance is covered in part 1?

What is the total distance covered?

d = v × t

d = 4m/s × 10s

d = 40 m

Part 1 = 40m

Part 2 = 0m

Part 3 = v × t × = 8m/s × 5s × = 20m

Part 4 = v × t = 8m/s × 10 s = 80m

Total = 40 + 20 + 80 =140m

Part 1

Part 2

Part 3

Part 4

In a speed/time graph the distance covered = area under the graph.

Honda

ACCELERATION QUESTIONSWhat would these look like on a speed/time graph?

1. stopped

2. slow

3. fast

4. accelerating