Moments and pressure
By the end of this chapter you will be able to
- Say that a force can turn an object about a pivot
- Calculate a moment: force x perpendicular distance from the pivot, in newton metres
- Use balanced moments to find a missing force or distance
- Explain why a longer lever needs a smaller force
- Calculate pressure = force / area, in pascals (Pa), and explain examples such as knives and snow shoes
You should already know
- Forces, pushes and pulls, and the newton as the unit of force (Chapter 11 of this book)
- Area of a rectangle, length x width (primary school maths)
Where this leads
- Forces and their effects in the later KS3 and GCSE physics courses
National Curriculum for England, Key Stage 3 science, Year 7. Content: P3 a force can turn an object about a pivot; moment = force x perpendicular distance from the pivot (N m); levers and balance, P4 pressure = force / area; simple examples such as knives and snow shoes (basic only).
Exercise P Do you remember?
Method. Recall the word.
- Recall. The point a lever turns about is the pivot.pivot
- Write the answer. pivot
Check. A seesaw turns about its pivot. ✓
Method. Recall the unit.
- Recall. Force is measured in newtons, written N.N
- Write the answer. N
Check. Weight is a force, so it is also in N. ✓
Method. Divide.
- Divide. Forty shared into five equal parts gives 8 each.40 / 5 = 8
- Write the answer. 8
Check. Check: 8 x 5 = 40. ✓
Method. Multiply the length by the width.
- Multiply. Area = 4 x 5.4 x 5 = 20
- Write the answer. 20
Check. Area is length times width. ✓
Method. Recall the force.
- Recall. The force of gravity on an object is its weight.weight
- Write the answer. weight
Check. Weight is measured in newtons. ✓
A force can turn an object about a pivot
A force can make an object turn, not just move. The point the object turns about is the pivot. A door turns about its hinges; a seesaw turns about its middle; a spanner turns about the bolt.
The turning effect of a force depends on two things: how big the force is, and how far it acts from the pivot.
Key ideas
- A force can turn an object about a pivot.
- The pivot is the point the object turns about.
- A bigger force has a bigger turning effect.
- A force applied further from the pivot has a bigger turning effect.
Method. Find the point the door turns about.
- Look at the door. A door turns about its hinges.hinges
- Name it. The hinges are the pivot of the door.hinges
- Write the answer. the hinges
Check. The door opens by turning about the hinges. ✓
Method. Compare the distances from the pivot.
- Compare. The handle is further from the hinges than a point near the hinges.further
- Decide. A force further from the pivot turns the door more easily, so the handle.handle
- Write the answer. handle
Check. The further the force from the pivot, the bigger the turning effect. ✓
Method. Find the point the spanner turns about.
- Look. The spanner turns the bolt, so it turns about the centre of the bolt.centre of the bolt
- Name it. The pivot is the centre of the bolt.centre of the bolt
- Write the answer. the centre of the bolt
Check. The turning happens around the bolt. ✓
Watch out
Watch out: a force does not always move an object; it can turn it instead. The turning effect depends on the distance from the pivot.
Exam tip
Exam tip. In a turning question, name the pivot first, then say how far the force is from it.
Try it (with your teacher, or a grown-up at home). With a grown-up, push a door near its handle, then near its hinges, using the same push each time. Say which was easier, and why. Name the pivot in each case.
Exercise 12A A force can turn an object about a pivot
Method. Recall the word.
- Recall. The point an object turns about is the pivot.pivot
- Write the answer. pivot
Check. A pivot does not move. ✓
Method. Recall the pivot.
- Recall. A seesaw turns about its middle, which is the pivot.middle
- Write the answer. middle
Check. The seesaw rocks about the middle. ✓
Method. Think about turning.
- Think. A force can turn an object about a pivot without moving it along a line.turn
- Decide. So the answer is no.no
- Write the answer. no
Check. Turning is a different effect from pushing. ✓
Method. Think about the distance from the pivot.
- Think. A longer spanner puts the force further from the bolt, the pivot.further
- Decide. So a long spanner is easier to turn.long
- Write the answer. long
Check. Distance from the pivot matters. ✓
Method. Recall the rule.
- Recall. A bigger force has a bigger turning effect.bigger force
- Write the answer. size
Check. Colour has no effect on turning. ✓
Method. Recall the rule.
- Recall. The further the force is from the pivot, the bigger the turning effect.further
- Decide. So the answer is yes.yes
- Write the answer. yes
Check. Distance matters as well as size. ✓
Method. Think about the distance.
- Think. The handle is furthest from the hinges.furthest
- Decide. So pushing near the handle is easiest.handle
- Write the answer. handle
Check. Furthest from the pivot is easiest. ✓
Method. Compare the distances.
- Compare. The force further from the pivot has a bigger turning effect.bigger
- Decide. So Ben is not right.no
- Write the answer. no
Check. Further from the pivot means more turning. ✓
Method. Compare the distances, for the same force.
- Compare. The same force at the bigger distance gives the bigger moment.bigger distance
- Decide. So the 0.4 m push gives the bigger moment.0.4 m
- Write the answer. 0.4 m
Check. Moment = force x distance, so 20 x 0.4 = 8 N m is bigger than 4 N m. ✓
Method. Check the pivot.
- Check. The pivot is the point the rule balances on, which is usually near the middle.balances
- Decide. So the pivot is not always at the end, and the answer is no.no
- Write the answer. no
Check. The pivot is where the object turns. ✓
Method. Think about the distance.
- Think about the distance. The handle is further from the hinges.further
- Think about the turning effect. So the force has a bigger turning effect about the pivot.bigger turning effect
- Write the answer. Accept any sentence that says the handle is further from the hinges, so the force has a bigger turning effect.
Check. Turning depends on distance and size. ✓
Method. Think about the two turning effects.
- Think about the lighter person. The lighter person sits further from the pivot.further
- Think about the balance. So their turning effects are equal and the seesaw balances.equal
- Write the answer. Accept any sentence that says the lighter person sits further from the pivot, so the turning effects balance.
Check. Turning effects must match to balance. ✓
Moments: force x distance from the pivot
The moment of a force is its turning effect about a pivot. It is found using:
moment = force x perpendicular distance from the pivot
The perpendicular distance is measured at a right angle to the line of the force, from the pivot. The unit of moment is the newton metre (N m).
The table gives three moments: a force, the distance from the pivot, and the moment.
| Force (N) | Distance from pivot (m) | Moment (N m) |
|---|---|---|
| 40 | 3 | 120 |
| 60 | 2 | 120 |
| 20 | 0.3 | 6.0 |
Key ideas
- moment = force x perpendicular distance from the pivot
- The distance is measured at a right angle to the force, from the pivot.
- The unit of moment is the newton metre, N m.
- A bigger force or a bigger distance gives a bigger moment.
Method. Use moment = force x distance.
- Write the values. The force is 40 N and the distance is 3 m.40 and 3
- Multiply. Moment = 40 x 3.40 x 3 = 120
- Write the answer. 120 N m
Check. 120 divided by 3 is 40. ✓
Method. Use moment = force x distance.
- Write the values. The force is 20 N and the distance is 0.3 m.20 and 0.3
- Multiply. Moment = 20 x 0.3 = 6.20 x 0.3 = 6
- Write the answer. 6 N m
Check. 6 divided by 0.3 is 20. ✓
Method. Use moment = force x distance.
- Write the values. The force is 60 N and the distance is 2 m.60 and 2
- Multiply. Moment = 60 x 2.60 x 2 = 120
- Write the answer. 120 N m
Check. 120 divided by 60 is 2 m. ✓
Watch out
Watch out: use the perpendicular distance, which is at a right angle to the force. If the force is at an angle, the distance must still be measured at right angles.
Exam tip
Exam tip. Write the moment formula, put in the force and distance with their units, multiply, and give the unit N m.
Try it (with your teacher, or a grown-up at home). With a grown-up, hang a small mass from a metre rule balanced on a pivot at one end. Measure the distance from the pivot to the mass. Multiply by the weight in newtons, using the mass in kg x 10, to find the moment in N m.
Exercise 12B Moments: force x distance from the pivot
Method. Recall the formula.
- Recall. Moment = force x perpendicular distance from the pivot.force x perpendicular distance
- Write the answer. force x perpendicular distance from the pivot
Check. The distance is measured at right angles. ✓
Method. Recall the unit.
- Recall. Moment is measured in newton metres, written N m.N m
- Write the answer. N m
Check. Force in N times distance in m gives N m. ✓
Method. Multiply the force by the distance.
- Multiply. Moment = 10 x 2.10 x 2 = 20
- Write the answer. 20 N m
Check. 20 divided by 2 is 10 N. ✓
Method. Multiply the force by the distance.
- Multiply. Moment = 5 x 4.5 x 4 = 20
- Write the answer. 20 N m
Check. 20 divided by 4 is 5 N. ✓
Method. Multiply the force by the distance.
- Multiply. Moment = 40 x 3.40 x 3 = 120
- Write the answer. 120 N m
Check. 120 divided by 3 is 40 N. ✓
Method. Multiply the force by the distance.
- Multiply. Moment = 20 x 0.3 = 6.20 x 0.3 = 6
- Write the answer. 6 N m
Check. 6 divided by 0.3 is 20 N. ✓
Method. Multiply the force by the distance.
- Multiply. Moment = 8 x 5 = 40.8 x 5 = 40
- Write the answer. 40 N m
Check. 40 divided by 5 is 8 N. ✓
Method. Multiply the force by the distance.
- Multiply. Moment = 20 x 0.2 = 4.20 x 0.2 = 4
- Write the answer. 4 N m
Check. 4 divided by 0.2 is 20 N. ✓
Method. Multiply the force by the distance.
- Multiply. Moment = 20 x 0.4 = 8.20 x 0.4 = 8
- Write the answer. 8 N m
Check. 8 is twice 4, because the handle is twice as long. ✓
Method. Multiply the force by the distance.
- Multiply. Moment = 15 x 4 = 60.15 x 4 = 60
- Write the answer. 60 N m
Check. 60 divided by 4 is 15 N. ✓
Method. Multiply, not add.
- Multiply. Moment = 20 x 3 = 60 N m.20 x 3 = 60
- Decide. Ben added the numbers instead of multiplying, so he is not right.no
- Write the answer. no
Check. Moments are found by multiplying. ✓
Method. Think about the distance.
- Think about the distance. The distance must be at a right angle to the force.right angle
- Correct it. Measure the perpendicular distance from the pivot to the line of the force.perpendicular
- Write the answer. Accept any sentence that says the distance should be the perpendicular distance, at right angles to the force.
Check. Only the perpendicular part of the distance counts. ✓
Method. Think about the calculation.
- Think about the calculation. Moment = force x distance.force x distance
- Think about the unit. So the unit is newtons times metres, not newtons alone.newtons times metres
- Write the answer. Accept any sentence that says the moment is a force multiplied by a distance, so its unit is newtons times metres.
Check. Units multiply as well. ✓
Method. Divide the moment by the force.
- Rearrange. Distance = moment / force = 36 / 12.36 / 12 = 3
- Check. Check: 12 x 3 = 36.12 x 3 = 36
- Write the answer. 3 m
Check. The formula can be rearranged. ✓
Balanced moments
An object is in balance about a pivot when the moments on each side are equal. A moment that turns the object clockwise is balanced by an equal moment that turns it anticlockwise.
total clockwise moment = total anticlockwise moment
Key ideas
- A balanced object has equal clockwise and anticlockwise moments.
- A heavier force must be closer to the pivot to balance a lighter one.
- The moments on each side can be compared by multiplying force and distance.
- If the moments are not equal, the object turns towards the bigger moment.
Method. Use moment = force x distance.
- Write the values. The left force is 200 N at 2 m from the pivot.200 and 2
- Multiply. Moment = 200 x 2 = 400.200 x 2 = 400
- Write the answer. 400 N m
Check. The right side also gives 400 N m. ✓
Method. Use moment = force x distance.
- Write the values. The right force is 160 N at 2.5 m from the pivot.160 and 2.5
- Multiply. Moment = 160 x 2.5 = 400.160 x 2.5 = 400
- Write the answer. 400 N m
Check. The two moments are equal, so the seesaw balances. ✓
Method. Make the moments equal.
- Find the moment. The moment on the first side is 60 x 2 = 120 N m.120 N m
- Find the distance. Distance = 120 / 30 = 4 m, so the 30 N force must act 4 m from the pivot.120 / 30 = 4
- Write the answer. 4 m
Check. 30 x 4 = 120 N m, which balances. ✓
Watch out
Watch out: a balance needs the moments to be equal, not the forces. The forces can be different, if the distances are different.
Exam tip
Exam tip. For a balance question, work out one moment, then find the missing force or distance on the other side.
Try it (with your teacher, or a grown-up at home). With a grown-up, balance a metre rule on a pivot with two small masses. Move one mass until the rule balances. Measure the two distances from the pivot, and check that the moments are equal.
Exercise 12C Balanced moments
Method. Recall the rule.
- Recall. An object balances when the clockwise and anticlockwise moments are equal.equal
- Write the answer. when the clockwise moments equal the anticlockwise moments
Check. Equal moments on each side. ✓
Method. Multiply.
- Multiply. 200 x 2 = 400.200 x 2 = 400
- Write the answer. 400 N m
Check. The same as the right side. ✓
Method. Multiply.
- Multiply. 160 x 2.5 = 400.160 x 2.5 = 400
- Write the answer. 400 N m
Check. Equal to the left side. ✓
Method. Compare the moments.
- Compare. Both moments are 400 N m.400 and 400
- Decide. So the seesaw balances, and the answer is yes.yes
- Write the answer. yes
Check. Equal moments balance. ✓
Method. Multiply.
- Multiply. 60 x 2 = 120.60 x 2 = 120
- Write the answer. 120 N m
Check. 120 divided by 2 is 60 N. ✓
Method. Divide the moment by the force.
- Divide. Distance = 120 / 30 = 4.120 / 30 = 4
- Write the answer. 4 m
Check. 30 x 4 = 120. ✓
Method. Find the moment, then divide by the distance.
- Find the moment. The moment is 10 x 6 = 60 N m.60
- Divide. Force = 60 / 3 = 20.60 / 3 = 20
- Write the answer. 20 N
Check. 20 x 3 = 60. ✓
Method. Find the moment, then divide by the force.
- Find the moment. The moment is 300 x 1 = 300 N m.300
- Divide. Distance = 300 / 150 = 2.300 / 150 = 2
- Write the answer. 2 m
Check. 150 x 2 = 300. ✓
Method. Find the moment, then divide by the force.
- Find the moment. The moment is 40 x 0.5 = 20 N m.20
- Divide. Distance = 20 / 20 = 1.20 / 20 = 1
- Write the answer. 1 m
Check. 20 x 1 = 20. ✓
Method. Think about moments.
- Think. The heavier child must sit closer to the pivot, to balance the moments.closer
- Decide. So Ben is not right.no
- Write the answer. no
Check. Heavier means closer, for balance. ✓
Method. Check the moments.
- Check. Balance depends on equal moments, not equal forces, if the distances are different.moments
- Decide. So equal forces alone are not the reason, and the answer is no.no
- Write the answer. no
Check. Forces and distances both matter. ✓
Method. Think about the distances.
- Think about the forces. A larger force can be at a smaller distance.smaller distance
- Think about the turning. So the two turning effects are equal and the object balances.equal
- Write the answer. Accept any sentence that says the larger force is at a smaller distance, so the two turning effects are equal.
Check. Turning effects, not forces alone, decide balance. ✓
Method. Find the moment, then divide by the distance.
- Find the moment. The moment is 30 x 1 = 30 N m.30
- Divide. Force = 30 / 3 = 10.30 / 3 = 10
- Write the answer. 10 N
Check. 10 x 3 = 30. ✓
Levers and the size of the force
A lever is a bar that turns about a pivot. Levers are used to lift or turn things with less effort: a spanner, a wheelbarrow, a bottle opener, a crowbar.
For the same moment, a longer distance from the pivot needs a smaller force. A short spanner needs a bigger force, but a long spanner needs less force.
Key ideas
- A lever is a bar that turns about a pivot.
- The moment = force x distance from the pivot.
- For the same moment, a longer distance means a smaller force.
- A longer handle makes a job easier.
Method. Use distance = moment / force.
- Write the values. The moment is 4 N m and the force is 20 N.4 and 20
- Divide. Distance = 4 / 20 = 0.2 m.4 / 20 = 0.2
- Write the answer. 0.2 m
Check. 20 x 0.2 = 4. ✓
Method. Use distance = moment / force.
- Write the values. The moment is 8 N m and the force is 20 N.8 and 20
- Divide. Distance = 8 / 20 = 0.4 m.8 / 20 = 0.4
- Write the answer. 0.4 m
Check. 20 x 0.4 = 8. ✓
Method. Compare the distances.
- Compare. For the same moment, the longer distance needs a smaller force.smaller force
- Decide. So the 0.4 m handle needs the smaller force.0.4 m
- Write the answer. 0.4 m
Check. A longer handle is easier. ✓
Watch out
Watch out: a longer lever does not give more moment for the same force. It gives the same moment with a smaller force, or a bigger moment for the same force.
Exam tip
Exam tip. Say that a longer lever needs a smaller force to produce the same moment, and explain why.
Try it (with your teacher, or a grown-up at home). With a grown-up, use a spanner with a short handle and a long handle to turn the same bolt, or use a bottle opener and a lever such as a ruler. Describe which handle needed less force, and explain why, using the word moment.
Exercise 12D Levers and the size of the force
Method. Recall the definition.
- Recall. A lever is a bar that turns about a pivot.bar that turns
- Write the answer. a bar that turns about a pivot
Check. Spanners and wheelbarrows are levers. ✓
Method. Think of a tool.
- Think. A spanner turns a bolt about a pivot.spanner
- Name one. So a spanner is a lever.spanner
- Write the answer. Accept any one: spanner, wheelbarrow, crowbar, bottle opener.
Check. Many tools are levers. ✓
Method. Use moment = force x distance.
- Think. If the distance is bigger, the force must be smaller to keep the moment the same.smaller
- Decide. So a longer distance needs a smaller force.smaller
- Write the answer. smaller
Check. Force and distance trade off. ✓
Method. Compare the numbers.
- Compare. 0.4 is bigger than 0.2.0.4 is bigger
- Write the answer. 0.4 m
Check. The longer handle is 0.4 m. ✓
Method. Multiply the force by the distance.
- Multiply. 20 x 0.2 = 4.20 x 0.2 = 4
- Write the answer. 4 N m
Check. 4 divided by 20 is 0.2 m. ✓
Method. Divide the moment by the distance.
- Divide. Force = 4 / 0.4 = 10.4 / 0.4 = 10
- Write the answer. 10 N
Check. 10 x 0.4 = 4. ✓
Method. Multiply.
- Multiply. 5 x 6 = 30.5 x 6 = 30
- Write the answer. 30 N m
Check. 30 divided by 6 is 5 N. ✓
Method. Divide the moment by the distance.
- Divide. Force = 60 / 3 = 20.60 / 3 = 20
- Write the answer. 20 N
Check. 20 x 3 = 60. ✓
Method. Use the same moment.
- Find the moment. The moment is 20 x 0.2 = 4 N m.4
- Divide. Force = 4 / 0.4 = 10 N.4 / 0.4 = 10
- Write the answer. 10 N
Check. The longer handle halves the force. ✓
Method. Use the idea of moment.
- Think. A longer spanner needs a smaller force for the same moment.smaller
- Decide. So Ali is not right.no
- Write the answer. no
Check. Longer means easier. ✓
Method. Check with the formula.
- Check. Moment = force x distance, so a longer distance with the same force gives a bigger moment.bigger moment
- Decide. So the answer is yes.yes
- Write the answer. yes
Check. Distance multiplies the force. ✓
Method. Think about the distance.
- Think about the distance. A longer bar means the force is further from the pivot.further
- Think about the force. So a smaller force gives the same moment, and the stone is easier to lift.smaller force
- Write the answer. Accept any sentence that says a longer bar gives a bigger distance from the pivot, so less force is needed for the same moment.
Check. Distance trades off against force. ✓
Pressure: force divided by area
Pressure is the force acting on each square metre of a surface. It is found using:
pressure = force / area
Force is in newtons (N) and area is in square metres (m²). Pressure is measured in newtons per square metre (N/m²), which is also called the pascal (Pa).
A small area gives a high pressure, so a sharp knife cuts easily. A large area gives a low pressure, so snow shoes stop you sinking.
The table gives the force and area of three objects, and the pressure on the surface.
| Object | Force (N) | Area (m squared) | Pressure (Pa) |
|---|---|---|---|
| a block on the table | 600 | 0.5 | 1200 |
| a child's shoe, standing still | 500 | 0.02 | 25000 |
| a snow shoe, standing on snow | 500 | 0.5 | 1000 |
Key ideas
- pressure = force / area, in pascals (Pa) when force is in N and area in m².
- A small area gives a high pressure.
- A large area gives a low pressure, for the same force.
- Sharp knives cut with high pressure because the blade has a small area.
Method. Use pressure = force / area.
- Write the values. The force is 600 N and the area is 0.5 m².600 and 0.5
- Divide. Pressure = 600 / 0.5 = 1200.600 / 0.5 = 1200
- Write the answer. 1200 Pa
Check. 1200 x 0.5 = 600. ✓
Method. Use pressure = force / area.
- Write the values. The force is 500 N and the area is 0.02 m².500 and 0.02
- Divide. Pressure = 500 / 0.02 = 25000.500 / 0.02 = 25000
- Write the answer. 25000 Pa
Check. 25000 x 0.02 = 500. ✓
Method. Use pressure = force / area.
- Write the values. The force is 500 N and the area is 0.5 m².500 and 0.5
- Divide. Pressure = 500 / 0.5 = 1000.500 / 0.5 = 1000
- Write the answer. 1000 Pa
Check. The snow shoe gives a much lower pressure than the shoe. ✓
Watch out
Watch out: a larger area reduces the pressure, but does not reduce the force. Snow shoes spread the same weight over a bigger area.
Exam tip
Exam tip. In a pressure question, use the area in m², then give the unit Pa or N/m².
Try it (with your teacher, or a grown-up at home). With a teacher, look at a sharp knife and a blunt one, and say which has the smaller area at the edge. Compare the pressure they would make with the same force. Do not cut anything; use a piece of paper folded into a point and a flat edge to show the idea.
Exercise 12E Pressure: force divided by area
Method. Recall the formula.
- Recall. Pressure = force / area.force / area
- Write the answer. force divided by area
Check. Pressure is force per unit area. ✓
Method. Recall the unit.
- Recall. Pressure is measured in newtons per square metre, which is the pascal (Pa).pascal
- Write the answer. pascal
Check. The unit is written Pa. ✓
Method. Think about the formula.
- Think. Dividing the same force by a small area gives a bigger answer.bigger answer
- Decide. So a small area gives a high pressure.high
- Write the answer. high
Check. Sharp blades have small areas. ✓
Method. Think about the area.
- Think. A snow shoe is wider, so its area is bigger.wider
- Decide. A bigger area gives a lower pressure, so the snow shoe spreads the load.lower pressure
- Write the answer. wider
Check. Bigger area, lower pressure. ✓
Method. Divide the force by the area.
- Divide. Pressure = 600 / 0.5 = 1200.600 / 0.5 = 1200
- Write the answer. 1200 Pa
Check. 1200 x 0.5 = 600. ✓
Method. Divide the force by the area.
- Divide. Pressure = 500 / 0.5 = 1000.500 / 0.5 = 1000
- Write the answer. 1000 Pa
Check. 1000 x 0.5 = 500. ✓
Method. Divide the force by the area.
- Divide. Pressure = 10 / 2 = 5.10 / 2 = 5
- Write the answer. 5 Pa
Check. 5 x 2 = 10. ✓
Method. Divide the force by the area.
- Divide. Pressure = 24 / 4 = 6.24 / 4 = 6
- Write the answer. 6 Pa
Check. 6 x 4 = 24. ✓
Method. Divide the force by the area.
- Divide. Pressure = 300 / 6 = 50.300 / 6 = 50
- Write the answer. 50 Pa
Check. 50 x 6 = 300. ✓
Method. Divide the force by the pressure.
- Divide. Area = force / pressure = 80 / 40.80 / 40 = 2
- Write the answer. 2 m²
Check. 40 x 2 = 80. ✓
Method. Check the area.
- Check. The edge of a sharp knife has a very small area.small area
- Decide. So a high pressure is made, and Ben is right.yes
- Write the answer. yes
Check. Small area, high pressure. ✓
Method. Check the formula.
- Check. Pressure depends on both the force and the area.both
- Decide. A bigger area can give a lower pressure even with a bigger force, so the answer is no.no
- Write the answer. no
Check. Pressure = force / area. ✓
Method. Think about the area.
- Think about the area. The snow shoe spreads the same force over a bigger area.bigger area
- Think about the pressure. So the pressure on the snow is lower, and the wearer sinks less.lower
- Write the answer. Accept any sentence that says the same force is spread over a bigger area, so the pressure on the snow is lower.
Check. The area changes the pressure, not the force. ✓
Method. Think about the tip.
- Think about the tip. The tip of a pin has a very small area.very small area
- Think about the pressure. A small force on a small area gives a high pressure, which pushes the pin in.high pressure
- Write the answer. Accept any sentence that says the tip of the pin has a very small area, so even a small force gives a high pressure.
Check. Small area, high pressure, even with small forces. ✓
Chapter summary
| Skill | Method |
|---|---|
| Pivot and turning | A force turns an object about a pivot. Its turning effect depends on size and distance. |
| Moment | moment = force x perpendicular distance from the pivot, in N m. |
| Balance | Balanced when the clockwise moments equal the anticlockwise moments. |
| Levers | For the same moment, a longer distance needs a smaller force. |
| Pressure | pressure = force / area, in Pa. A small area gives a high pressure; a large area a low pressure. |
Can you…?
- I can say that a force can turn an object about a pivot
- I can calculate a moment, in N m, from a force and its perpendicular distance
- I can use balanced moments to find a missing force or distance
- I can explain why a longer lever needs a smaller force
- I can calculate pressure = force / area, in pascals, and explain snow shoes and knives
Key terms
- pivot
- the point an object turns about
- moment
- the turning effect of a force about a pivot: force x perpendicular distance, in N m
- perpendicular distance
- the distance from the pivot measured at a right angle to the force
- balance
- an object is balanced when its clockwise and anticlockwise moments are equal
- lever
- a bar that turns about a pivot
- pressure
- force per unit area; pressure = force / area
- pascal (Pa)
- the unit of pressure, equal to one newton per square metre
Exercise 12Z Check-up
Method. Recall the word.
- Recall. The point an object turns about is the pivot.pivot
- Write the answer. pivot
Check. A pivot does not move. ✓
Method. Multiply the force by the distance.
- Multiply. Moment = 40 x 3 = 120.40 x 3 = 120
- Write the answer. 120 N m
Check. 120 divided by 3 is 40 N. ✓
Method. Recall the unit.
- Recall. A moment is in newton metres, written N m.N m
- Write the answer. N m
Check. Force in N times distance in m. ✓
Method. The seesaw balances, so the moments are equal.
- Balance. The seesaw is balanced, so the moments are equal.equal
- Read. The right moment is 160 x 2.5 = 400 N m.400
- Write the answer. 400 N m
Check. Equal moments balance. ✓
Method. Divide the moment by the force.
- Find the moment. The moment is 60 x 2 = 120.120
- Divide. Distance = 120 / 30 = 4.120 / 30 = 4
- Write the answer. 4 m
Check. 30 x 4 = 120. ✓
Method. Use the formula.
- Use the formula. A longer distance needs a smaller force for the same moment.smaller
- Write the answer. smaller
Check. Longer spanners are easier. ✓
Method. Recall the formula.
- Recall. Pressure = force / area.force / area
- Write the answer. force divided by area
Check. Pressure is force per unit area. ✓
Method. Recall the unit.
- Recall. Pressure is measured in pascals, Pa.pascals
- Write the answer. pascal
Check. One pascal is one newton per square metre. ✓
Method. Divide the force by the area.
- Divide. Pressure = 600 / 0.5 = 1200.600 / 0.5 = 1200
- Write the answer. 1200 Pa
Check. 1200 x 0.5 = 600. ✓
Method. Think about the area.
- Think about the area. A sharp edge has a smaller area.smaller area
- Think about the pressure. So the pressure is higher for the same force.higher pressure
- Write the answer. Accept any sentence that says the sharp edge has a smaller area, so the pressure is higher for the same force.
Check. Small area, high pressure. ✓
Total 22 marks. Allow about 22 minutes.
Exercise 12X Practice test questions
Method. Multiply the force by the distance.
- Multiply. Moment = 12 x 3 = 36.12 x 3 = 36
- Write the answer. 36 N m
Check. 36 divided by 3 is 12 N. ✓
Method. Recall the word.
- Recall. The point an object turns about is the pivot.pivot
- Write the answer. pivot
Check. The pivot does not move. ✓
Method. The moments are equal when balanced.
- Balance. The moments are equal, so the other side is 200 x 2 = 400 N m.400
- Write the answer. 400 N m
Check. Equal moments balance. ✓
Method. Divide the moment by the force.
- Divide. Distance = 60 / 20 = 3.60 / 20 = 3
- Write the answer. 3 m
Check. 20 x 3 = 60. ✓
Method. Think about the distance.
- Think about the distance. A longer handle gives a bigger distance from the pivot.bigger distance
- Think about the force. So less force is needed for the same moment.less force
- Write the answer. Accept any sentence that says a longer handle gives a bigger distance from the pivot, so less force is needed for the same moment.
Check. Distance trades off against force. ✓
Method. Recall the formula.
- Recall. Pressure = force / area.force / area
- Write the answer. force divided by area
Check. Pressure is measured in Pa. ✓
Method. Divide the force by the area.
- Divide. Pressure = 500 / 0.5 = 1000.500 / 0.5 = 1000
- Write the answer. 1000 Pa
Check. 1000 x 0.5 = 500. ✓
Method. Think about the formula.
- Think. A smaller area gives a bigger answer when the force is divided by it.bigger answer
- Write the answer. a small area
Check. Sharp knives have small areas. ✓
Method. Think about the area.
- Think about the area. The snow shoe has a bigger area.bigger area
- Think about the pressure. So the pressure on the snow is lower.lower pressure
- Write the answer. Accept any sentence that says the wider area spreads the same force, so the pressure on the snow is lower.
Check. The area changes the pressure. ✓
Method. Divide the force by the area.
- Divide. Pressure = 300 / 6 = 50.300 / 6 = 50
- Write the answer. 50 Pa
Check. 50 x 6 = 300. ✓
Method. Find the moment, then divide by the distance.
- Find the moment. The moment is 30 x 1 = 30 N m.30
- Divide. Force = 30 / 3 = 10.30 / 3 = 10
- Write the answer. 10 N
Check. 10 x 3 = 30. ✓
Method. Think about the area.
- Think about the tip. The tip has a very small area.very small area
- Think about the pressure. So a small force gives a high pressure.high pressure
- Write the answer. Accept any sentence that says the tip has a very small area, so a small force gives a high pressure.
Check. Small area gives high pressure. ✓
Method. Recall the unit.
- Recall. Moment is in newton metres, written N m.N m
- Write the answer. N m
Check. Force times distance. ✓