© Boardworks Ltd 2003 KS4 Forces – Further Forces.

© Boardworks Ltd 2003

KS4 Forces – Further Forces


Ticker timers

A ticker timer is used as an accurate timing device for motion experiments.Ticker timers print dots on paper at a rate of 50 dots per second.

This means the distance between two dots is equivalent to 1/50th of a second or 0.02 seconds.

How much time is shown here?

7/50th of a second or 0.14 seconds


Describe the motion for the ticker tape shown:

Accelerating/decelerating/constant velocity




Plastic

If you put a force on an object making the object change shape and it stays the same shape after you remove the force we say the object is plastic.

Example?

Plasticene


Elastic

If you put a force on an object making the object change shape and it returns to its original shape after you remove the force we say the object is elastic.

Example?

A spring


Elastic and plastic

Elastic Plastic

Write down four examples of plastic and elastic materials.


Match the force with its definition:

Tension

CompressionThe force on a material

when it is being stretched.

The force on a material when it is being

squashed.


Tension or compression?

Your triceps muscle when you bend your arm.

A fishing line holding a fish.

A football being kicked.

A string holding a conker.

Pushing in a syringe.

An arrow in a bow.

Your biceps muscle when you bend your arm.

T – Tension C - Compression

T

T

T

T

T

T

T

C

C

C

C

C

C

C


Standing on a plank:

Where is the plank being compressed?

Where is the plank being stretched?

What force pulls down on the man?

Tension Compression

Gravity


Stretching springs

1. Attach a spring to a stand.

2. Record the length of the spring using a ruler.

3. Add a 50g mass to the spring.

4. Record the new length of the spring.

5. Continue to add masses to the spring up to 500g in total.

6. Plot a graph of load against extension.

7. What do your results tell you?

What do you predict will happen as masses are attached to a spring?

Mass (g)

Load(N)

Length(cm)

Extension(cm)

0 0 2.4 0

50 0.5


Load and extension for a metal spring

Load (N)

Extension (cm)

Where is load proportional to extension?Where the graph is a straight line.

Load extension

Elastic limit

Plastic region

What do we call the point after which the spring will not return to its original shape?

The Elastic Limit


How are mass, force and acceleration related?

Two masses of 1 kg and 4 kg are both subject to the same force, which one will accelerate the fastest?

The lower the mass, the greater the acceleration for a specific force. This is why Grand Prix cars get faster as they use up their fuel. Less fuel, means less mass, less mass means greater acceleration.


F=ma

We can express the relationship between force, mass and acceleration using the equation:

Force = mass x acceleration

F =ma

Force measured in Newtons (N)

Mass measured in kilograms (kg)

Acceleration measured in metres per second per second (ms-2)


Formula triangles

F

m a

x

Formula triangles help you to rearrange formula. The triangle for the f=ma formula is shown below:

Whatever quantity you are trying to find, cover it up and it will leave you with the calculation required.

So if you were trying to find mass, m…..

…you would cover m up…

…and you are left with the sum…

m = F

a


Force calculations:

1. What force will make a mass of 4 kg accelerate at 5 ms-2?

2. A force of 50N acts on a 2 kg mass. What will be the acceleration of the mass?

3. A force of 2 000N accelerates a car at 2.5 ms-2. What is the mass of the car?

Force = 20 N

Acceleration = 25 ms-2

Mass = 800 kg


Force questions:

1. Label the forces acting upon a moving bike.

2. Fill in the table:

A > (C+D)

A =(C+D)

A < (C+D)

Bike accelerating

Bike at steady speed

Bike decelerating

A

B

C

D

E

A - Thrust

B - Reaction

C – Air resistance

D - Friction

E - Gravity


Turning forces

Two masses on a see-saw.

What force acts on the masses?

Which way will the see-saw go?

pivotgravity gravity

The see-saw turns around the pivot.

What factors effect the size of a turning force?


Moments

The size of the turning force or moment depends upon:

1. The distance of the force from the pivot.

2. The size of the force.

Moment = Force x perpendicular distance from pivot

Moments measured in Newton metres (Nm)

Force measured in Newtons (N)

Distance measured in metres (m)


Principle of moments

Anticlockwise moments = Clockwise moments

Where should a force of 50N be positioned to balance the see-saw?

Anti-clockwise moments = 25 N x 2 m

= 50 Nm

Clockwise moments = 50 N x ? mAnti-clockwise moments = Clockwise moments

50 Nm = 50 N x ? M

distance = 1 m

25N

2 m 1 m

50N


Principle of moments

Drag and drop any of the masses onto the “see – saw” and try to get it to balance. The masses are in kilograms and the distance in metres.


Moments questions

1. Where should a force of 60N be positioned to balance the ruler below?

2. What size force should be positioned on the left to balance the ruler shown?

15N

4 m 1 m

60N

15N

4 m1 m

? NForce = 60 N


Pressure

Forces are pushes, pulls and twist.

Pressure is a measure of how spread out a force is.

As you in increase the size of the force you increase the size of the pressure.

As you increase the size of the area over which a force acts, you decrease the pressure.

Less pressure

More pressure

Less pressure

More pressure


Pressure questions

1. Why do tractors have such large tyres?

2. Why would a lady in stiletto heels standing on your foot hurt you more than a elephant standing on your foot?

The tyres have a large area so that the weight of the tractor is spread over a large area.

This reduces the pressure the tractor puts on the ground, so that it doesn’t sink into the mud.

The elephant has a greater weight than the lady and it would have a greater pressure if the area the weight acted upon was the same.

However, the lady’s weight is concentrated into a smaller area than that of the elephant’s. This means that the stiletto heel exerts a greater pressure than the foot of the elephant, and might hurt more.


P=F/A

We can express the pressure formula using the equation:

Pressure = Force ÷ Area

P =F/A

Pressure measured in pascals (Pa)

Force measured in newtons (N)

Area measured in metres squared (m2)


HydraulicsWe can use the fact that the pressure in a liquid is the same at any point to make a machine that magnifies force.

Comment on the area of the plungers, the relative distance travelled and the energy input/output.


Formula triangles

F

P A

x

Formula triangles help you to rearrange formula.

The triangle for the pressure formula is shown below:

Whatever quantity you are trying to find, cover it up and it will leave you with the calculation required.

So if you were trying to find pressure, P…..

…you would cover P up…

…and you are left with the sum…

P = F

A


The pressure you put on the ground during a handstand:

1. Put your hand on a piece of graph paper.

2. Count the number of 1cm2 squares your hand covers.

3. Multiply this number by two (you have two hands).

4. Measure your weight in Newtons on some scales.

5. Use the pressure formula to calculate the pressure you put on the ground.

P=F/A


Pressure calculations:

1. What pressure will a force of 20N acting over an area of 4 ms-2 put upon the ground?

2. A force of 50N acts on an area and creates a pressure of 1.0Pa. Over what area is the force acting?

3. A pressure of 6 000 Pa is created by a force acting on an area of at 5 m2. What is the size of the force?

Pressure = 5 Pa

Area = 50 m2

Force = 30 000 N


Pressure experiment

A fluid is a liquid or a gas. Get a tin can and drill three holes in the side, then fill it with water.

What do you observe?

What does this tell you about the pressure in fluids?

The water is forced out a greater distance as you move down the tin

can. This is because the pressure is greatest at the bottom - pressure in

fluids increases with depth.


Why are the walls of a dam wider at the bottom than at the top?

A. It is cheaper to build

B. Gravity increases with depth

C. Pressure increases with depth

D. Looks nice


A pressure of 600 Pa acts over an area of 2m2. What force creates this pressure?

A. 1 200 N

B. 300 N

C. 1 200 Pa

D. 300 Pa


Why are racing cars able to travel faster as a race progresses?

A. The engine has cooled down

B. The petrol has warmed up

C. There is less petrol to accelerate

D. There is more petrol to accelerate

© Boardworks Ltd 2003 KS4 Forces – Further Forces.

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