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GCSE Physics · GCSEChapter 14 · Momentum, moments and pressure
GCSE · Chapter 14

Momentum, moments and pressure

By the end of this chapter you will be able to

  • Calculate a moment and use the principle of moments to find an unknown force or distance
  • Explain why a lever or gear makes a job easier
  • Calculate pressure from force and area, converting cm squared to m squared
  • [H] Calculate pressure in a liquid and momentum, and use conservation of momentum

You should already know

  • Forces and resultant forces (Chapter 13)
  • Density and mass (Chapter 9)

Where this leads

  • Work done and energy (Chapter 15)
  • Motion and acceleration (Chapter 16)

GCSE Physics. Content follows the GCSE (9-1) subject content for England (Edexcel and AQA). Content references: Calculate a moment as force x perpendicular distance from the pivot, and use the principle of moments for a body in equilibrium, Explain how levers and gears transmit a turning effect, Calculate pressure on a solid using p = F / A, with units of pascals and areas in m squared, [H] Calculate pressure in a liquid using p = h rho g, [H] Calculate momentum using p = m v, and apply conservation of momentum to collisions and explosions, Describe atmospheric pressure, as the weight of the air pressing down.

Exercise P  Do you remember?

1
FWhat is 20 x 0.5? Write the number.
2
FWhat is 0.3 x 40? Write the number.
3
FWhich is the force that acts on an object when it is pushed by a person: a contact force or a non-contact force? Write contact or non-contact.
4
FWhat is the area of a square of side 0.2 m? Write the number of m squared.
5
FWhat is the unit of force? Write the unit.
14.1

Moments and levers

GCSE
Calculate a moment as force x perpendicular distance from the pivot, Explain how levers and gears transmit a turning effect
Video lesson: Moments and leversComing soon
Context. Ruby uses a crowbar to lift a paving slab. The longer the crowbar, the easier it is to lift. “Why does the length matter?” she asks. A force that turns something about a point gives a moment, and the moment depends on the force and the distance from the pivot.

A moment is the turning effect of a force about a pivot. It is found from:

moment (N m) = force (N) x perpendicular distance from the pivot (m)

The distance must be measured at right angles (perpendicular) to the line of the force. A lever is a bar that turns about a pivot; a longer lever arm gives a bigger moment for the same force, so less force is needed. A gear works in a similar way: a larger gear transmits a turning effect with a different speed and force.

Key ideas

  • Moment = force x perpendicular distance from the pivot.
  • Units: newton metres (N m).
  • A longer lever arm gives a larger moment for the same force.
  • Gears: a bigger gear turns a smaller one faster but with less force (and the reverse).
Worked example 14.1.1
A force of 20 N acts at a perpendicular distance of 0.5 m from a pivot. Calculate the moment. Write the number of N m.

Method. Use moment = F x d.

  1. Take the numbers. F = 20 N and d = 0.5 m.
    F = 20, d = 0.5
  2. Multiply. Moment = 20 x 0.5 = 10 N m.
    20 x 0.5 = 10
  3. Write the answer.
    10 N m

Check. 10 / 20 = 0.5 m. ✓

Worked example 14.1.2
A spanner is 0.2 m long. A force of 50 N is applied at the end, at right angles. Calculate the moment. Write the number of N m.

Method. The distance is the spanner length, which is at right angles.

  1. Take the numbers. F = 50 N and d = 0.2 m.
    F = 50, d = 0.2
  2. Multiply. Moment = 50 x 0.2 = 10 N m.
    50 x 0.2 = 10
  3. Write the answer.
    10 N m

Check. 10 / 0.2 = 50 N. ✓

Worked example 14.1.3
A 30 N force is applied at a perpendicular distance of 0.4 m from a pivot. Calculate the moment. Write the number of N m.

Method. Use moment = F x d.

  1. Take the numbers. F = 30 N and d = 0.4 m.
    F = 30, d = 0.4
  2. Multiply. Moment = 30 x 0.4 = 12 N m.
    30 x 0.4 = 12
  3. Write the answer.
    12 N m

Check. 12 / 30 = 0.4 m. ✓

Watch out

Watch out: the distance is measured from the pivot, at right angles to the force. A force pushed along the lever, through the pivot, has no turning effect.

Exam tip

Exam tip. A moment is always in newton metres (N m). Do not write N only.

Exercise 14A  Moments and levers

1
FWhat is the unit of a moment? Write the unit.
2
FA force of 8 N acts at a distance of 0.5 m from a pivot. Calculate the moment. Write the number of N m.
3
FA force of 15 N acts at 0.2 m from a pivot. Calculate the moment. Write the number of N m.
4
FA moment of 12 N m is produced by a force at 3 m from the pivot. Find the force. Write the number of newtons.
5
FA moment of 6 N m is produced by a 20 N force. Find the perpendicular distance. Write the number of metres.
6
PSWhich gives the larger moment: 10 N at 0.6 m, or 20 N at 0.4 m? Write 1 or 2 (1 for the first).
7
PSA child sits 2 m from a seesaw pivot. Her moment is 200 N m. What is her weight (the force)? Write the number of newtons.
8
RWhy is a longer spanner easier to turn a nut with? Write one reason in one sentence. Accept any sentence that says a longer distance gives a larger moment, so less force is needed.
9
RA pupil says a 50 N force pushed through the pivot of a lever has a moment of 50 N m. Explain why this is wrong. Accept any answer that says the perpendicular distance from the pivot is zero, so the moment is zero.
10
FA large gear turns a small gear. Which gear turns faster: the large or the small? Write large or small.
11
PSA lever is 1.5 m long and a 40 N load is at the far end. Calculate the moment about the pivot at one end. Write the number of N m.
12
RExplain in one sentence why the handle of a door is placed far from the hinge. Accept any sentence that says a larger distance gives a bigger moment, so the door is easier to open.
14.2

The principle of moments

GCSE
Use the principle of moments for a body in equilibrium
Video lesson: The principle of momentsComing soon
Context. A metre rule is balanced on a pivot at its centre. Two masses hang from it, one on each side. The teacher asks: “Where should the second mass go to balance it?” The principle of moments gives the answer.

The principle of moments says that for an object in equilibrium (balanced), the total clockwise moment about any point equals the total anticlockwise moment:

sum of clockwise moments = sum of anticlockwise moments

This is how seesaws, balances and levers are designed. It lets us find an unknown force or distance.

a balanced metre rule (example)40 N20 Npivot0.3 m0.6 m
A balanced metre rule: 40 N x 0.3 m = 20 N x 0.6 m = 12 N m on each side

Key ideas

  • For balance: clockwise moments = anticlockwise moments.
  • Find the moment on each side, then set them equal.
  • Rearrange to find an unknown force or distance.
  • The pivot is the point the object turns about.
Worked example 14.2.1
A 40 N force acts 0.3 m to the left of a pivot. A 20 N force acts to the right. How far from the pivot must the 20 N force act for balance? Write the distance in metres.

Method. Set the anticlockwise moment equal to the clockwise moment.

  1. Left. Anticlockwise moment = 40 x 0.3 = 12 N m.
    12 N m
  2. Right. Clockwise moment = 20 x d. Set equal: 20 x d = 12.
    20 d = 12
  3. Solve. d = 12 / 20 = 0.6 m.
    12 / 20 = 0.6
  4. Write the answer.
    0.6 m

Check. 20 x 0.6 = 12 N m. ✓

Worked example 14.2.2
A seesaw has a child of weight 300 N sitting 1.2 m from the pivot. The other child is 1.5 m from the pivot. Find the weight of the other child for balance. Write the number of newtons.

Method. Set the two moments equal.

  1. Moment 1. Moment of the first child = 300 x 1.2 = 360 N m.
    360 N m
  2. Solve. Weight of the second child = 360 / 1.5 = 240 N.
    360 / 1.5 = 240
  3. Write the answer.
    240 N

Check. 240 x 1.5 = 360 N m. ✓

Worked example 14.2.3
A 5 N weight is hung 0.8 m from a pivot on a balanced rule. The other side has a 10 N weight. How far from the pivot is the 10 N weight? Write the distance in metres.

Method. Set the moments equal.

  1. Moment 1. Moment of the 5 N weight = 5 x 0.8 = 4 N m.
    4 N m
  2. Solve. Distance = 4 / 10 = 0.4 m.
    4 / 10 = 0.4
  3. Write the answer.
    0.4 m

Check. 10 x 0.4 = 4 N m. ✓

Watch out

Watch out: include every force on each side. If the rule has its own weight, it counts too (its weight acts at its centre).

Exam tip

Practical safety. Use a metre rule and small masses with a teacher. Keep the masses on the bench, and do not let them fall.

Exercise 14B  The principle of moments

1
FA clockwise moment of 12 N m balances an anticlockwise moment. What is the anticlockwise moment? Write the number of N m.
2
FA 10 N force is 0.5 m from a pivot on the left. A 10 N force is on the right. Where must it be for balance? Write the distance in metres.
3
PSA 6 N weight is 0.5 m from a pivot. What weight at 1.5 m balances it? Write the number of newtons.
4
PSA 4 N weight is 2 m to the left of a pivot. A 16 N weight is on the right. How far is the 16 N weight from the pivot? Write the distance in metres.
5
PSA 25 N force acts 0.4 m from a pivot. What force at 1 m balances it? Write the number of newtons.
6
PSA 60 N child sits 1.5 m from a pivot. Another child sits 1 m on the other side. Calculate the weight of the second child. Write the number of newtons.
7
RExplain why a seesaw balances when the heavier child sits closer to the pivot, in one sentence. Accept any sentence that says a bigger force needs a smaller distance to give the same moment.
8
RA pupil says “a balanced object has no forces acting on it.” Explain why this is wrong. Accept any answer that says the forces are balanced, so the resultant is zero, but the forces still act.
9
PSA uniform metre rule of weight 1 N is pivoted at its centre. A 3 N weight hangs at 0.2 m from the pivot on the left. Calculate the distance on the right, from the pivot, for a 1 N extra weight to balance. Write the distance in metres.
10
PSA 9 N weight hangs 0.4 m from a pivot. A 3 N weight on the other side balances it. How far from the pivot is the 3 N weight? Write the distance in metres.
14.3

Pressure on solids and atmospheric pressure

GCSE
Calculate pressure on a solid using p = F / A, Describe atmospheric pressure, as the weight of the air pressing down
Video lesson: Pressure on solids and atmospheric pressureComing soon
Context. A girl walks on soft snow in her boots, and sinks in. Her friend uses snowshoes, which spread her weight, and does not sink. The force is the same, but the area is larger, so the pressure is smaller.

Pressure is the force acting per unit area, at right angles to the surface:

pressure p (Pa) = force F (N) / area A (m2)

The unit of pressure is the pascal (Pa), which is one newton per square metre. Areas in cm2 must be converted: 1 cm2 = 0.0001 m2. The air around us has a weight, and it presses on everything. This atmospheric pressure is about 100 000 Pa at sea level.

the same force, different areas10 Nlarge area: lower pressure10 Nsmall area: higher pressure
The same force on a large area gives a lower pressure than on a small area
QuantitySymbolUnit
pressureppascal (Pa), which is N/m squared
forceFnewton (N)
areaAmetre squared (m squared); 1 cm squared = 0.0001 m squared
momentMnewton metre (N m)
momentump (or the momentum)kg m/s
densityrhokg/m cubed
depthhmetre (m)
The quantities and units in this section

Key ideas

  • p = F / A; the pressure is the force per unit area.
  • Unit: pascal (Pa) = N/m squared.
  • 1 cm squared = 0.0001 m squared; convert before you calculate.
  • A larger area gives a smaller pressure for the same force.
Worked example 14.3.1
A force of 600 N acts on an area of 0.02 m squared. Calculate the pressure. Write the number of pascals.

Method. Use p = F / A.

  1. Take the numbers. F = 600 N and A = 0.02 m squared.
    F = 600, A = 0.02
  2. Divide. p = 600 / 0.02 = 30 000 Pa.
    600 / 0.02 = 30000
  3. Write the answer.
    30000 Pa

Check. 30000 x 0.02 = 600 N. ✓

Worked example 14.3.2
A force of 10 N acts on an area of 4 cm squared. Calculate the pressure. Write the number of pascals.

Method. Convert the area to m squared first.

  1. Convert. 4 cm2 = 4 x 0.0001 = 0.0004 m2.
    0.0004 m2
  2. Divide. p = 10 / 0.0004 = 25 000 Pa.
    10 / 0.0004 = 25000
  3. Write the answer.
    25000 Pa

Check. 25000 x 0.0004 = 10 N. ✓

Worked example 14.3.3
A pressure of 5000 Pa is produced by a force of 100 N. Find the area. Write the number of m squared.

Method. Rearrange to A = F / p.

  1. Take the numbers. F = 100 N and p = 5000 Pa.
    F = 100, p = 5000
  2. Rearrange. A = 100 / 5000 = 0.02 m squared.
    100 / 5000 = 0.02
  3. Write the answer.
    0.02 m squared

Check. 5000 x 0.02 = 100 N. ✓

Watch out

Watch out: convert cm2 to m2 by multiplying by 0.0001, not by 100. A smaller area gives a larger pressure.

Exam tip

Exam tip. Write p = F / A, then the numbers with their units (m squared), then calculate. Give the unit Pa.

Exercise 14C  Pressure on solids

1
FWhat is the unit of pressure? Write the unit.
2
FA force of 20 N acts on an area of 2 m squared. Calculate the pressure. Write the number of Pa.
3
FA force of 80 N acts on an area of 0.5 m squared. Calculate the pressure. Write the number of Pa.
4
FA pressure of 300 Pa is produced by a force of 150 N. Find the area. Write the number of m squared.
5
FConvert 2 cm squared into m squared. Write the number in standard form.
6
PSA force of 12 N acts on an area of 3 cm squared. Calculate the pressure. Write the number of Pa.
7
PSA 60 kg person stands on one foot of area 0.02 m squared. Calculate the pressure from the weight, using g = 9.8 N/kg. Write the number of Pa.
8
PSA person stands on both feet, each of area 0.02 m squared, with the same weight of 588 N. What is the pressure on the ground? Write the number of Pa.
9
RExplain why snowshoes stop a person sinking into snow, in one sentence. Accept any sentence that says a larger area spreads the weight and reduces the pressure.
10
RA pupil says “a sharp knife has a larger pressure because it has a smaller area.” Is the pupil right? Write yes or no.
11
RExplain why the pressure of the air is about 100 000 Pa at sea level, in one sentence. Accept any sentence that says the weight of the air above pushes down on each square metre.
12
PSA block of area 0.05 m squared exerts a pressure of 2000 Pa on a table. Calculate the force on the table. Write the number of newtons.
14.4

[H] Pressure in liquids

GCSE
[H] Calculate pressure in a liquid using p = h rho g
Video lesson: [H] Pressure in liquidsComing soon

[H] Higher tier only. The pressure in a liquid increases with depth, because there is more liquid pressing down. The pressure depends on the depth h (m), the density of the liquid rho (kg/m3) and the gravitational field strength g (N/kg): p = h x rho x g. The pressure acts equally in all directions at the same depth.

Key ideas

  • [H] p = h rho g. Use depth in m, density in kg/m cubed, g = 9.8 N/kg.
  • [H] Pressure increases with depth.
  • [H] At the same depth, the pressure is the same in all directions.
  • [H] Density of water is about 1000 kg/m cubed.
Worked example 14.4.1
[H] Calculate the pressure at a depth of 2 m in water of density 1000 kg/m cubed. Write the number of pascals.

Method. Use p = h rho g.

  1. Take the numbers. h = 2 m, rho = 1000 kg/m3, g = 9.8 N/kg.
    h = 2, rho = 1000, g = 9.8
  2. Multiply. p = 2 x 1000 x 9.8 = 19 600 Pa.
    2 x 1000 x 9.8 = 19600
  3. Write the answer.
    19600 Pa

Check. 19600 / (1000 x 9.8) = 2 m. ✓

Worked example 14.4.2
[H] The pressure at depth in a liquid is 4900 Pa. The density is 1000 kg/m cubed. Find the depth. Write the number of metres.

Method. Rearrange to h = p / (rho g).

  1. Rearrange. h = 4900 / (1000 x 9.8) = 4900 / 9800.
    4900 / 9800
  2. Divide. h = 0.5 m.
    0.5
  3. Write the answer.
    0.5 m

Check. 0.5 x 1000 x 9.8 = 4900 Pa. ✓

Worked example 14.4.3
[H] Calculate the pressure at a depth of 1 m in water (rho = 1000 kg/m cubed, g = 9.8 N/kg). Write the number of Pa.

Method. Use p = h rho g.

  1. Take the numbers. h = 1 m, rho = 1000 kg/m3, g = 9.8 N/kg.
    h = 1, rho = 1000
  2. Multiply. p = 1 x 1000 x 9.8 = 9800 Pa.
    9800
  3. Write the answer.
    9800 Pa

Check. ✓

Watch out

Watch out: [H] the density must be in kg/m cubed, not g/cm cubed, for p = h rho g to give pascals.

Exam tip

Exam tip. [H] Convert the density first if it is in g/cm cubed: multiply by 1000 to get kg/m cubed.

Exercise 14D  [H] Pressure in liquids

1
F[H] What is the pressure at 3 m depth in water (rho = 1000 kg/m cubed, g = 9.8 N/kg)? Write the number of Pa.
2
F[H] What is the pressure at 10 m depth in water? Write the number of Pa.
3
PS[H] A liquid of density 800 kg/m cubed is at a depth of 5 m. Calculate the pressure. Write the number of Pa.
4
PS[H] Calculate the depth at which the pressure in water is 19 600 Pa. Write the number of metres.
5
PS[H] A diver goes from 2 m to 4 m depth in water. How many times bigger is the pressure at 4 m? Write the number.
6
R[H] Explain why a dam is thicker at the bottom than at the top, in one sentence. Accept any sentence that says the pressure increases with depth, so the bottom must be stronger.
7
PS[H] Calculate the pressure at a depth of 5 m in seawater of density 1025 kg/m cubed, with g = 9.8 N/kg. Write the number of Pa.
8
PS[H] A pressure of 29 400 Pa is measured in water. What is the depth? Write the number of metres.
9
F[H] Calculate the pressure at a depth of 4 m in water. Write the number of Pa.
10
R[H] Explain in one sentence why the pressure in a liquid is the same in all directions at the same depth. Accept any sentence that says the liquid pushes equally from all sides at that depth.
14.5

[H] Momentum and conservation of momentum

GCSE
[H] Calculate momentum using p = m v, and apply conservation of momentum to collisions and explosions
Video lesson: [H] Momentum and conservation of momentumComing soon
Context. Two trolleys on a track collide and stick together. The total momentum before the collision is the same as after, if no outside force acts. This is the principle of conservation of momentum.

[H] Higher tier only. Momentum p is the product of mass and velocity, p = m x v, in kg m/s. Momentum is a vector, so direction matters. In a collision or explosion with no external force, the total momentum is conserved: the total before equals the total after. Do not confuse momentum p with pressure p: here p means momentum.

before and after a collision3 m/sbefore (2 kg moving, 1 kg at rest)after (stuck together)
A moving trolley hits a stationary one and they stick together

Key ideas

  • [H] Momentum p = m v, in kg m/s.
  • [H] Momentum is a vector: direction matters, so give a sign or direction.
  • [H] Total momentum before = total momentum after (no external force).
  • [H] An explosion from rest: the total momentum is zero, so the parts move in opposite directions.
Worked example 14.5.1
[H] A 0.5 kg ball moves at 4 m/s. Calculate its momentum. Write the number of kg m/s.

Method. Use p = m v.

  1. Take the numbers. m = 0.5 kg and v = 4 m/s.
    m = 0.5, v = 4
  2. Multiply. p = 0.5 x 4 = 2 kg m/s.
    0.5 x 4 = 2
  3. Write the answer.
    2 kg m/s

Check. 2 / 4 = 0.5 kg. ✓

Worked example 14.5.2
[H] A 2 kg trolley moving at 3 m/s hits a stationary 1 kg trolley, and they stick together. Find the velocity after the collision. Write the number of m/s.

Method. Conserve momentum: total before = total after.

  1. Before. Total momentum before = 2 x 3 + 1 x 0 = 6 kg m/s.
    6 kg m/s
  2. After. Total mass after = 3 kg, so v = 6 / 3 = 2 m/s.
    6 / 3 = 2
  3. Write the answer.
    2 m/s

Check. 3 x 2 = 6 kg m/s. ✓

Worked example 14.5.3
[H] Two trolleys at rest on a track push apart: a 1 kg trolley moves right at 4 m/s. Find the velocity of a 1 kg trolley moving left. Write the number of m/s with a minus sign for left.

Method. Total momentum is zero before, so it is zero after.

  1. Before. Total momentum before = 0.
    0
  2. After. 1 x 4 + 1 x v = 0, so v = -4 m/s, which is 4 m/s to the left.
    -4
  3. Write the answer.
    -4 m/s

Check. ✓

Watch out

Watch out: [H] momentum has direction. Use a minus sign for one direction and be consistent. Momentum is not conserved if an outside force acts, such as friction.

Exam tip

Exam tip. [H] Write the momentum before and after on separate lines. If they are equal, you have conserved momentum.

Exercise 14E  [H] Momentum calculations

1
F[H] Calculate the momentum of a 2 kg object moving at 5 m/s. Write the number of kg m/s.
2
F[H] A 0.2 kg ball moves at 15 m/s. Calculate its momentum. Write the number of kg m/s.
3
F[H] An object has a momentum of 12 kg m/s and a mass of 3 kg. Find its velocity. Write the number of m/s.
4
PS[H] Two objects of mass 1 kg each have momenta of +6 and -6 kg m/s. What is the total momentum? Write the number of kg m/s.
5
PS[H] A 1500 kg car travelling at 20 m/s collides with a stationary 1500 kg car, and they stick together. Find the speed after. Write the number of m/s.
6
PS[H] A gun of mass 2 kg at rest fires a bullet of mass 0.02 kg at 500 m/s. Find the recoil velocity of the gun. Write the number of m/s, with a minus sign for the recoil.
7
R[H] Explain why momentum is conserved in a collision with no external force, in one sentence. Accept any sentence that says the forces between the objects are equal and opposite, so the total momentum does not change.
8
F[H] A 2 kg object moves at 3 m/s. Calculate its momentum. Write the number of kg m/s.
9
PS[H] A 1 kg trolley moves right at 2 m/s and a 1 kg trolley moves left at 2 m/s. They stick together after a collision. What is their velocity afterwards? Write the number of m/s.
10
R[H] Explain in one sentence why the total momentum is zero after an explosion from rest. Accept any sentence that says the total momentum is zero before, so it must still be zero after.

Chapter summary

SkillMethod
Momentmoment = F x d (perpendicular); unit N m
BalanceSum of clockwise moments = sum of anticlockwise moments
Pressure on a solidp = F / A; pascal (Pa); convert cm squared to m squared (x 0.0001)
[H] Pressure in a liquidp = h rho g
[H] Momentump = m v; total momentum conserved in collisions and explosions

Can you…?

  • I can calculate a moment and use the principle of moments
  • I can calculate pressure and convert cm squared to m squared
  • I can explain why a larger area reduces pressure
  • [H] I can calculate the pressure in a liquid
  • [H] I can use momentum and conservation of momentum

Key terms

moment
the turning effect of a force about a pivot, in N m
pivot
the point about which an object turns
pressure
force per unit area, in pascals (Pa)
pascal (Pa)
the unit of pressure: one newton per square metre
[H] momentum
the product of mass and velocity, in kg m/s
[H] conservation of momentum
the total momentum before a collision equals the total momentum after, with no external force

Exercise 14Z  Check-up

1
Calculate the moment of a 15 N force at 0.4 m from a pivot. Write the number of N m.
2
A balanced rule has 10 N at 0.6 m on the left. What force at 1.2 m on the right balances it? Write the number of newtons.
3
Calculate the pressure from a force of 200 N on an area of 0.5 m squared. Write the number of Pa.
4
A force of 12 N acts on 2 cm squared. Find the pressure. Write the number of Pa (use 0.0002 m squared).
5
What is the unit of a moment? Write the unit.
6
What is the unit of pressure? Write the unit.
7
Explain why a longer lever makes a job easier, in one sentence. Accept any sentence that says the longer distance gives a bigger moment for the same force.
8
[H] Calculate the pressure at a depth of 1.5 m in water. Write the number of Pa.
9
[H] A 4 kg object moves at 2.5 m/s. Calculate its momentum. Write the number of kg m/s.
10
Calculate the force needed to produce a pressure of 500 Pa on an area of 0.2 m squared. Write the number of newtons.

Total 29 marks. Allow about 29 minutes.

Exercise 14X  Exam-style questions

1
FA force of 25 N acts 0.6 m from a pivot. Calculate the moment. Write the number of N m.[2 marks]
2
FA force of 500 N acts on a block with an area of 0.25 m squared. Calculate the pressure. Write the number of Pa.[2 marks]
3
PSA 30 N force acts 0.5 m from a pivot on one side of a balanced lever. A 15 N force acts on the other side. Calculate the distance of the 15 N force from the pivot. Write the number of metres.[3 marks]
4
RDescribe how a student could show the principle of moments using a metre rule, a pivot and two masses. Accept any answer that says balance the rule, measure the distances, calculate each moment and show that they are equal.[4 marks]
5
RExplain why a person on snowshoes sinks less than a person in ordinary boots, using pressure. Accept any answer that says the snowshoes have a larger area, so the same weight gives a smaller pressure.[3 marks]
6
PS[H] Calculate the pressure at a depth of 6 m in a liquid of density 900 kg/m cubed, with g = 9.8 N/kg. Write the number of Pa.[3 marks]
7
PS[H] A 1 kg trolley moving at 6 m/s collides with a stationary 2 kg trolley, and they stick together. Calculate the velocity after the collision. Write the number of m/s.[3 marks]
8
RDescribe how atmospheric pressure acts on the body, and explain why we do not feel it crushing us. Accept any answer that says the air presses on all surfaces, and the pressure inside the body balances it.[3 marks]
9
RA 10 kg object is raised. Explain, using a moment, why it is easier to lift with a longer lever. Accept any answer that says the same force gives a bigger moment with a longer distance, so less force is needed to balance the load.[3 marks]
10
R[H] Explain why the total momentum of two trolleys is the same before and after a collision, in terms of forces. Accept any answer that says the forces between the trolleys are equal and opposite, and there is no external force.[3 marks]
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