Moments and Levers: KS3 Physics Explained

A moment is the turning effect of a force about a fixed point called a pivot. It depends on both the size of the force and how far from the pivot it acts. Levers are simple machines that use moments to multiply the effect of a force, making it possible to lift heavy loads with far less effort.

What is a moment and how do you calculate it?

The moment of a force is defined as:

Moment (N·m) = Force (N) × perpendicular distance from the pivot (m)

The unit of moment is the newton-metre (N·m).

The key word is perpendicular — the distance must be measured at a right angle (90°) to the direction of the force. If you push at an angle, the effective distance is shorter, so the moment is smaller.

Worked example: A force of 20 N is applied to a wrench at a distance of 0.3 m from the pivot (the nut).

Moment = 20 N × 0.3 m = 6 N·m

If you use a longer wrench (0.6 m), the same force produces:

Moment = 20 N × 0.6 m = 12 N·m — twice the turning effect.

This is exactly why mechanics use long-handled spanners for tight bolts.

What is the principle of moments?

The principle of moments states: for a body in rotational equilibrium (not rotating), the sum of the clockwise moments about any pivot equals the sum of the anticlockwise moments.

Clockwise moments = Anticlockwise moments

Worked example — a balanced see-saw:

A child of mass 30 kg sits 2 m from the pivot on the left side. How far from the pivot must a child of mass 40 kg sit on the right side to balance?

Left (anticlockwise moment) = 30 × 10 × 2 = 600 N·m (using g = 10 N/kg)

Right (clockwise moment) = 40 × 10 × d

For balance: 600 = 400 × d → d = 600 ÷ 400 = 1.5 m

The heavier child must sit closer to the pivot to balance the lighter child who is further away — exactly what you observe on a playground see-saw.

What is a lever and what are the three classes?

A lever is a rigid bar (or rod) that can rotate about a fixed pivot (also called a fulcrum). Levers are one of the six classical simple machines. They work by using moments: a small force (the effort) applied at a greater distance from the pivot can balance or overcome a larger force (the load) applied closer to the pivot.

There are three classes of lever, depending on the positions of the load, effort, and pivot:

Class | Arrangement | Example | Advantage
1st class | Pivot between load and effort | See-saw, crowbar, scissors | Can multiply force or distance
2nd class | Load between pivot and effort | Wheelbarrow, nutcracker | Always multiplies force (effort < load)
3rd class | Effort between pivot and load | Tweezers, fishing rod, forearm | Multiplies distance (moves faster)

First-class lever example — crowbar: The pivot is in the middle. Applying a small effort at the long end produces a large moment, lifting a heavy load at the short end with much less effort.

Second-class lever example — wheelbarrow: The wheel is the pivot at the front; the load (soil) sits in the middle; you lift the handles at the back. The load is always closer to the pivot than the effort, so the mechanical advantage is always greater than 1 (effort needed is less than the load).

Third-class lever example — forearm: Your elbow is the pivot. The biceps muscle (effort) attaches close to the elbow. The hand (load) is at the far end. You need more force from your biceps than the object's weight — but the hand moves through a much greater distance and speed than the muscle contracts. You trade force for speed and range of motion.

How does a lever provide a mechanical advantage?

Mechanical advantage (MA) = Load ÷ Effort

A lever with MA > 1 means the effort is less than the load — the lever multiplies your force. A wheelbarrow might let you lift 200 N of soil with only 100 N of effort (MA = 2).

A lever with MA < 1 means the effort is greater than the load — but the load moves through a greater distance per unit of effort (like tweezers). You trade force for increased range of movement.

No energy is created. A lever cannot give you something for nothing. If you use less force, you must apply it over a greater distance: Work = Force × Distance. The work done on the effort side equals the work done on the load side (assuming no friction or bending).

Where do moments and levers appear in everyday life?

Object | Class | Pivot | Load | Effort
Scissors | 1st | Screw in the middle | Material being cut | Finger pressure
Wheelbarrow | 2nd | Front wheel | Contents of the tray | Hands on the handles
Tweezers | 3rd | Held end | Object gripped | Squeezing fingers
Door handle | 1st | Hinge | Door weight | Hand on handle
Human forearm | 3rd | Elbow joint | Object in hand | Biceps muscle
Bottle opener | 2nd | Rim of bottle | Cap | Hand at far end

Frequently asked questions

What is a moment in KS3 physics?

A moment is the turning effect of a force around a pivot point. It is calculated as the force multiplied by the perpendicular distance from the pivot to where the force acts. The unit of moment is the newton-metre (N·m). A larger force or a greater distance produces a greater moment (more turning effect).

What is the principle of moments?

The principle of moments states that when an object is in equilibrium (balanced, not rotating), the total clockwise moment about any pivot is equal to the total anticlockwise moment about the same pivot. You use this principle to solve see-saw balance problems and to understand how levers work.

How do you increase the moment of a force?

You can increase a moment by increasing the force applied, or by increasing the perpendicular distance from the pivot to the point where the force acts — or both. This is why a longer spanner makes it easier to undo a tight nut, and why it is easier to push open a door near the edge (far from the hinge) than near the hinge.

What is the difference between a first-class and a second-class lever?

In a first-class lever, the pivot is between the effort and the load (like a see-saw or crowbar). In a second-class lever, the load is between the pivot and the effort (like a wheelbarrow). Second-class levers always give a mechanical advantage greater than 1 — the effort is always less than the load — making them ideal for lifting heavy objects.

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