A **moment** is a turning effect of a force and you come across them every day of your life. For the physics GCSE, you need to know some examples of forces that create turning effects, how **the principle of moments** can be utilised and how to calculate the magnitude of turning forces and moments.

If an object is fixed in place using a **pivot** (a shaft or other fixing that is designed to allow movement of the object), then you have the exact situation required for a turning force to arise. When an object is placed on something narrower, the narrow object can act as a pivot too, for example, a plank placed on a brick. Finally the edges or curved parts of objects can act as pivots too - take for example, a fork. Placed with the curved side uppermost, it is stable, however, placed with the curved side on the table, pressing downwards on the prongs will create a turning force that raises the handle.

1 .

What is a moment?

A turning effect of a force

The mass of an object at a point

The distance of a mass from a pivot

When a system is in equilibrium

Moments can be used to your advantage when using a lever

2 .

What is the formula for the size of a moment?

This is a direct proportionality so if the force is greater, the moment is larger. If the distance is longer, the moment will be larger too. The opposite is true for smaller values of both terms of the equation

3 .

What does d in the above equation stand for?

Parallel distance from line of action to the pivot

Perpendicular distance from line of action to the pivot

Distance of the plane

Distance between two masses

This is the shortest distance between the point at which the force is acting and the point around which an object pivots

4 .

If an object is not turning, the total clockwise moment, compared to the total anti-clockwise moment about any pivot, must be what?

Clockwise moment is twice as large as anti-clockwise moment

Clockwise moment is three times as large as anti-clockwise moment

Clockwise moment is half as large as anti-clockwise moment

Clockwise moment is exactly equal in magnitude to the anti-clockwise moment

For an object to be at rest on a pivot, all the forces acting on it must be in equilibrium

5 .

What is the size of a moment if F = 10 N and d = 125 cm?

10.125 N m

1250 N m

12.5 N m

1.25 N m

Did you remember to convert 125 cm into metres?

6 .

A see-saw is balanced on a pivot with two children on it. One child is sitting 1.5 m to the left of the pivot and has a mass of 50 kg. Another child of mass 30 kg is sitting on the right hand side of the pivot. What distance away from the pivot is the child on the right of the pivot?

30 cm

1.5 m

2.5 m

Impossible to say without knowing the length of the see-saw

The key word in the question is *balanced* so the principle of moments calculation can be applied

7 .

What is the force that creates a moment of 10 N m when it is applied 0.25 m from the pivot?

30 N

40 N

50 N

60 N

Rearrangement of the equation for calculating the size of a moment

8 .

If the point of application of the force was moved further away from the pivot, what would be the effect on the moment?

The moment would be greater because the distance is greater

The moment would remain the same because the force hasn't changed

The moment would be smaller because it is further away and therefore has less effect on the pivot

More information is needed to be able to answer this question

Changing either (or both) of the force and distance will alter the moment. An example of this would be using a spanner to turn a nut. A longer spanner enables you to apply the force from a greater distance, increasing the moment and magnifying the force that your muscles can apply

9 .

A plank of wood is balanced on a pivot. One mass of 10 kg is then placed 1 m to the left of the pivot on the wood. What weight needs to be placed 0.5 m to the right of the pivot for the wood to still be balanced?

10 kg

20 kg

10 N

200 N

Did you spot that the question asked for the weight? The weight of an object is the mass multiplied by the strength of the gravitational field - 10 N/kg is an acceptable approximation for the gravitational field strength at the surface of the Earth

10 .

Which of the following is an example of the principle of moments being utilised?

Lifting a book

A rock falling

A crowbar being used to lift a drain cover

A USB stick

The crowbar is being used as a lever. Levers are a favourite of the examiners for testing your knowledge of moments

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