Moments, levers & gears

A ‘number balance’ is used to help children understand addition.

A number balance has a beam that can rotate around a pivot.

Small tags can be hung from the pegs on the beam.

Each tag has the same weight.

Figure 11 shows a number balance. The balance is in equilibrium.

The weight of each tag on the beam in Figure 11 causes a moment about the pivot.

What is meant by the ‘moment’ of a force?

Tick (✓) one box.

A student holds the beam in a horizontal position and adds a tag to position 7 to the right of the pivot.

Figure 12 shows the new arrangement.

The student lets go of the beam.

Which of the following describes the resultant moment on the beam?

Tick (✓) one box.

Explain where the student should add another tag to make the beam in Figure 12 balance.

Use the Physics Equations Sheet to answer questions 07.4 and 07.5.

Write down the equation which links distance (d), force (F) and moment of a force (M

One tag causes a moment about the pivot of 1.8 N cm.

The weight of the tag is 0.15 N.

Calculate the distance between the pivot and the tag.

Give your answer in cm.

Figure 7 shows some apparatus used to demonstrate the motor effect.

When the switch is closed the copper rod accelerates.

What direction is the acceleration of the copper rod?

Tick () one box.

The copper rod has a mass of 5.0 g.

The length of the copper rod in the magnetic field is 0.070 m.

The magnetic flux density is 0.30 T.

Calculate the initial acceleration of the rod when the current in the rod is 2.0 A.

Use the Physics Equations Sheet.

Figure 8 shows a simple motor.

There is a magnetic field between the two permanent magnets.

When there is a current in the coil, a magnetic field is produced around the coil.

Explain how these magnetic fields cause the coil to continuously rotate.

In Figure 8 the coil of the motor is in a horizontal position.

Explain why the resultant moment on the coil is zero when the coil is in a vertical position.

Figure 17 shows a young child using a baby walker.

The child is standing still.

What is the resultant vertical force on the child?

Give a reason for your answer.

Use the Physics Equations Sheet to answer questions 09.2 and 09.3.

Write down the equation which links distance (s), force (F) and work done (W).

The child pushed the baby walker 2.8 m across a horizontal floor.

The work done by the child was 35 J.

Calculate the horizontal force the child applied to the baby walker.

The child pushed the baby walker from a carpet onto a hard floor.

The child applied the same horizontal force to the baby walker.

Explain why the speed of the baby walker increased.

There are some toy gears on the front of the baby walker.

Figure 18 shows the gears.

The child applies a force to gear A.

This causes a moment about the pivot, so gear A rotates.

Use the Physics Equations Sheet to answer questions 09.5 and 09.6.

Write down the equation which links distance (d), force (F) and moment of a force (M).

The child applies a force of 2.0 N on gear A.

The perpendicular distance between the force and the pivot is 7.5 cm.

Calculate the moment of the force about the pivot.

Explain what happens to gear B when the child applies the force to gear A.

Figure 5 shows a young child using a baby walker.

The child is standing still.

What is the resultant vertical force on the child?

Give a reason for your answer.

Use the Physics Equations Sheet to answer questions 02.2 and 02.3.

Write down the equation which links distance (s), force (F) and work done (W).

The child pushed the baby walker 2.8 m across a horizontal floor.

The work done by the child was 35 J.

Calculate the horizontal force the child applied to the baby walker.

The child pushed the baby walker from a carpet onto a hard floor.

The child applied the same horizontal force to the baby walker.

Explain why the speed of the baby walker increased.

There are some toy gears on the front of the baby walker.

Figure 6 shows the gears.

The child applies a force to gear A.

This causes a moment about the pivot, so gear A rotates.

Use the Physics Equations Sheet to answer questions 02.5 and 02.6.

Write down the equation which links distance (d), force (F) and moment of a force (M).

The child applies a force of 2.0 N on gear A.

The perpendicular distance between the force and the pivot is 7.5 cm.

Calculate the moment of the force about the pivot.

Explain what happens to gear B when the child applies the force to gear A.

Some cars have a lever that is used to apply the handbrake. Figure 8 shows the handbrake lever in a car.

The driver applies the force shown in Figure 8. The force produces a moment about the pivot. How could the driver increase the moment about the pivot without increasing the size of the force?

The driver releases the handbrake. Figure 9 shows how the velocity of the car changes during the first 5 seconds of a journey.

After 3 seconds, the momentum of the car is 24 000 kg m/s. Calculate the mass of the car. Use the Physics Equations Sheet.

Determine the distance travelled by the car during the first 5 seconds of the journey. Use Figure 9.

In an emergency the driver needs to apply the brakes suddenly to stop the car quickly. The driver of the car is distracted. Explain why the distraction will increase the stopping distance.

Explain why the temperature of the brakes increases as they are used.

Figure 3 shows a cyclist on a bicycle. The cyclist is moving at a constant velocity. Arrows A and B represent the horizontal forces acting on the bicycle and cyclist.

What is force A ? Tick (✓) one box.

What is force B ? Tick (✓) one box.

What is the relationship between force A and force B when the cyclist travels at a constant velocity? Tick (✓) one box.

The cyclist applies a force of 150 N to one of the bicycle pedals. Figure 4 shows the distance between the force applied and the pivot.

Calculate the moment about the pivot caused by the force applied to the pedal in Figure 4. Use the equation: moment of a force = force × distance

Figure 5 shows how the pedal is connected to the back wheel of the bicycle.

Complete the sentence. Choose the answer from the box.

axle chain cog

The force from the cyclist pushing down on the pedal is transmitted to the back wheel

Figure 6 shows how the velocity of the cyclist changes during a journey.

What is the change in velocity of the cyclist in the first 20 seconds of the journey? Tick (✓) one box.

Determine the acceleration of the cyclist during the first 20 seconds of the journey. Use your answer from Question 02.6 Use the equation:

Complete the sentence. Choose the answer from the box.

deceleration speed velocity

Between 30 and 40 seconds the cyclist moves with

The cyclist travels from home to school. Figure 7 shows the route the cyclist followed.

Draw an arrow on Figure 7 to show the displacement of the cyclist.

Figure 11 shows a cyclist riding a bicycle. Force A causes the bicycle to accelerate forwards.

What name is given to force A ?

Figure 12 shows how the velocity of the cyclist changes during a short journey.

Determine the distance travelled by the cyclist between Y and Z.

Figure 13 shows the gears on the bicycle.

Describe how the force on the pedal causes a moment about the rear axle.

Figure 14 shows a different cyclist towing a trailer.

The speed of the cyclist and trailer increased uniformly from 0 m/s to 2.4 m/s. The cyclist travelled 0.018 km while accelerating. Calculate the initial acceleration of the cyclist.

The resultant force of the towbar on the trailer has a horizontal component and a vertical component. horizontal force = 200 N vertical force = 75 N

Determine the magnitude and direction of the resultant force of the towbar on the trailer by drawing a vector diagram.

Figure 17 shows four examples of a force causing an object to move.

Which object is not likely to rotate? Tick (✓) one box.

Figure 18 shows a simple device that can be used as a weighing scale. Figure 19 shows the device being used to measure a quantity of rice. The weight of the device is balanced by the weight of the rice and basket. Figure 18 Figure 19

The weight of the device acts through the point labelled X. What is point X called? Tick (✓) one box.

How does Figure 19 show that the weight of the device is balanced by the weight of the rice and basket?

The basket can hang from different points on the device. Where should the basket hang to measure the largest quantity of rice?

Tick (✓) one box.

Write down the equation which links distance, force and moment of a force.

In Figure 19, the weight of the device causes an anticlockwise moment of 0.15 Nm about the pivot. The weight of the rice and basket acts 0.06 m from the pivot.

Calculate the weight of the rice and basket.

Write down the equation which links gravitational field strength, mass and weight.

The basket has a mass of 0.04 kg gravitational field strength = 9.8 N/kg

Calculate the mass of rice in the basket.

Figure 19 shows the back of a lorry. The lorry is used to carry horses.

The ramp is lowered by pulling on the rope or by pulling on the handle. The hinge acts as a pivot.

Explain why it is easier to lower the ramp by pulling on the rope rather than pulling on the handle.

When the ramp is lowered, work is done to stretch a spring on the side of the ramp. Elastic potential energy is stored in the stretched spring. Figure 20 shows the ramp part way down in a balanced horizontal position.

With the ramp horizontal: the moment caused by the weight of the ramp = 924 Nm the spring is stretched by 0.250 m

Calculate the elastic potential energy stored in the stretched spring. Use data from Figure 20.

Figure 9 shows a fork-lift truck lifting a heavy crate.

The crate weighs 11 500 N and is lifted vertically 2.60 m. Calculate the work done to lift the crate. Use the equation: work done = force × distance

The weight of the crate causes a clockwise moment of 13 800 Nm about the centre of the front wheel of the fork-lift truck.

The weight of the fork-lift truck and driver cause an anticlockwise moment. What is the minimum size of the anticlockwise moment needed so that the fork-lift truck does not topple over?

Write down the equation which links distance, force and moment of a force.

Calculate the distance ‘ d ’ marked on Figure 9.