4.6 Turning forces
Summary: A tightrope walker uses a long pole to help them balance in Figure 1. The pole spreads the combined mass of the person and the pole far away from the pivot point the person’s feet, so they stay… Tags: science oxford-science student-book-7 physics Created: 2026-05-19 Last Updated: 2026-05-19
After this topic, you will be able to:
- define the moment of a force
- calculate the moment of a force
- apply the law of moments.
Think back
- State the equation for work.
- State where handles are in relation to hinges: close together or far apart.
- State the difference between Balanced and Unbalanced Forces.
Key idea A moment (Nm) is a turning force about a pivot. The law of moments explains this relationship.
Key word pivot, moment, newton-metre (Nm), law of moments, centre of Gravity, centre of mass
A tightrope walker uses a long pole to help them balance in Figure 1**. The pole spreads the combined mass of the person and the pole far away from the pivot point (the person’s feet), so they stay balanced. Without the pole, they would fall to one side and gravity would act on their body.**
Figure 1Figure 1 Tightrope walking over Niagara Falls.
What is a moment?
Whenever you open a door, you are using a turning force. A turning force acts a certain distance from a pivot. The turning effect of a force is called a moment. The moment depends on the force being applied and how far it is from the pivot.
moment (Nm) = force (N) x perpendicular distance from the pivot (m)
We measure force in newtons (N) and distance in metres (m).
We calculate a moment in newton-metres (Nm).
A Compare calculating work and calculating a moment.
Figure 2You need to apply a turning force to open a door.
What is the law of moments?
You sit on the left of a see-saw with your friend at the other end. It balances like the apples in Figure 3. The moment of your Weight acts anticlockwise (to the left). The moment of your friend’s weight acts clockwise (to the right).
Figure 3These apples are in Equilibrium.
When an object is in equilibrium (balanced), the sum of the clockwisemoments is equal to the sum of the anticlockwise moments. This is the law of moments.
B Look at Figure 3. Explain why the large apple is closer to thepivot than the small apple when they are in equilibrium.
You can work out if the see-saw in Figure 4 is going to be balanced by calculating the clockwise and the anticlockwise moments.
Figure 4The see-saw does not turn if it is in equilibrium.
The moments are the same. The see-saw balances.
What makes objects unbalanced?
When you push back and tip your chair slightly, there is a turning force that brings your chair back (see Figure 5a). That turning force is your weight acting about the point where the legs touch the floor. If you push back far enough, you will fall over (see Figure 5c).
All the weight of an object seems to act through a point called the centre of gravity (or centre of mass). If the centre of gravity is above the pivot, there is no turning force (see Figure 5b). If the centre of gravity is to the left or right of the pivot, there will be a turning force.
Figure 5There is a turning force in a and c, but not in b. Full text alternative to the content
C Explain why it is hard to balance a pencil on its point.
Maths skills A mother and child are on a see-saw 4 m long. The mother has a weight of 600N and the child has a weight of 150 N. Calculate where the mother must sit to balance the child who is sitting at the other end.
Stretch zone
- Copy and complete the sentences. The falling/turning effect of a force is called a moment. You can calculate the moment of a force by multiplying Energy/force by the direction/distance. If the anticlockwise moments equal the clockwise moments, the object will be in conservation/ equilibrium. This is the idea/ law of moments. The height/ weight of an object acts through a point called the centre of gravity/Pressure.
- A child applies a force of 5N to close a door. The handle is 0.75 m from the hinge. Calculate the moment of the force.
Stretch zone
- Look at Figure 4. Suggest and explain, using the law of moments, what the girl on the see-saw would need to do to balance the see-saw if: the boy moved closer to the pivotanother boy sat between the first boy and the pivot.