KS3 · Physics

Levers and turning forces

Push a door right next to its hinges and it barely budges. Push it at the handle and a fingertip will do. Same door, same you. So what changed?

One mass on peg 8. How many on the other side?

23.556.5801.252.53.755Peg on the other side (distance from the pivot)Masses needed to balance(5, 1.6)Masses × peg number: 8

Peg on the other side (distance from the pivot): 5. Masses needed to balance: 1.6. Masses × peg number: 8

Drag the dot along the curve and multiply the two numbers it shows.

Watch out: The curve falls as the pegs move further out: a far peg needs fewer masses. It shows what the balance rule predicts at every distance. Real masses come in whole numbers, so only some points can actually be hung, such as 4 masses on peg 2.

Predict, then check

A door turns around its hinges, just as a roundabout turns around its centre.

You want to open a door with the smallest push you can manage. Where on the door should you push?

Find the parts

Pivot, effort and load: scissors and a steering wheel
ScissorsSteering wheel

Showing 1 layer: Scissors and steering wheel

Explore

Tap a part to light it up.

Tap a part to see what it does.

Physics · Levers

What is in the middle?

There are different types of lever, each with the pivot in a different position. For each one, decide what sits in the middle: the pivot, the load or the effort.

Still to sort

Pivot in the middle (0)

The pivot is between the effort and the load.

Where the line is: Check the pivot first: if it sits between the effort and the load, the lever belongs here.

Load in the middle (0)

The load is between the effort and the pivot.

Where the line is: The pivot is at one end and the effort at the other, with the load between them.

Effort in the middle (0)

The effort is between the load and the pivot.

Where the line is: The pivot is at one end and the load at the other, with the effort between them.

4 of 4 still to sort.

Relationship matrix

Tap any cell to reveal it. Tap a column header to read one property down every item.

What happens to the load forceWhere you meet it
A bigger effort
Effort further from the pivot
Load closer to the pivot
Load force bigger than the effort

Each cell hides a short answer and the reason behind it. Predict before you tap.

Physics · Moments

Your turn: moments on a beam

Your turn. Work out moments, choose the right unit, then decide whether a beam balances. Each time the method stops, pick the step that comes next.

  1. Moment = force × distance from the pivot. Start with a force of 5 N acting 1.5 m from a pivot.
  2. missing step
Which line is step 2?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

  • A force can make an object rotate as well as push it. The turning effect of a force is called a moment.
  • A smaller force is needed to cause a turn when the force acts further from the turning point, the pivot.
  • Moment = force × distance from the pivot. It is measured in newton-metres (N m) when the distance is in metres, and newton-centimetres (N cm) when the distance is in centimetres.
  • A lever is a simple machine that rotates around a pivot. The force you apply is the effort and the force the lever produces is the load.
  • A beam is balanced when its clockwise moments equal its anticlockwise moments. If they are not equal, it turns in the direction of the greater moment.

The big picture

A force can turn things as well as push them. The turning effect of a force is called a moment, and it depends on two things: how big the force is and how far from the pivot it acts. Multiply them (moment = force × distance) and you can predict whether a beam balances, and why one mass far out can hold up four masses close in.

Key points

1A moment is the turning effect of a force: moment = force × (perpendicular) distance from the pivot, in N m or N cm to match the distance.
2Where the force acts matters as well as its size. One mass on peg 8 balances four masses on peg 2, because 1 × 8 = 4 × 2: unequal weights can balance if their distances are right.
3Every lever has a pivot, an effort and a load. The pivot can be hinges, an axle or a simple point. Types of lever differ in what sits in the middle: crowbar (pivot), wheelbarrow (load), tweezers (effort).
4A lever whose load force is larger than its effort force is a force multiplier, such as a crowbar. Levers can also multiply a small movement into a larger movement.
5A bigger effort, or an effort further from the pivot, increases the load force. A load closer to the pivot has a larger force on it.
6Clockwise and anticlockwise describe the direction of a turning effect. A beam balances only when the clockwise moments equal the anticlockwise moments.

Worked example

Problem

Ella pushes a door with a force of 20 N at the handle, 0.8 m from the hinges. Mo pushes the same door with the same 20 N, but only 0.2 m from the hinges. Whose push has the greater moment, and how many times greater is it?

⚠ Watch out

Thinking the bigger force always wins. A beam turns according to force × distance, so a smaller force that acts further from the pivot can balance a bigger force close in. And say "turning effect" (or moment), not "turning force".

🧠

Memory hook

How hard × how far. A moment needs both, so a small force far out can match a big force close in.

✓

Check yourself

A 3 N force acts 20 cm from a pivot. Find its moment, with the unit. What happens if the force moves to 40 cm? (60 N cm; it doubles.)

Flashcards

(13)
What is a moment?
The turning effect of a force.
How do you calculate a moment, and in what units?
Moment = force × (perpendicular) distance from the pivot. It is in N m when the distance is in metres and N cm when the distance is in centimetres.
Why is a door handle on the opposite side to the hinges?
A smaller force is needed to turn the door when the force acts further from the turning point.
What is a lever?
A simple machine designed to use the turning effect of a force. It rotates around a pivot.
Effort or load: which is the force you apply, and which is the force the lever produces?
The effort is the force applied to the lever. The load is the force the lever produces.
What can the pivot of a lever be?
A set of hinges (a door), an axle (a door handle) or a simple point (a seesaw).
What are the three types of lever?
Pivot between effort and load (crowbar), load between effort and pivot (wheelbarrow), effort between load and pivot (tweezers).
What is a force multiplier?
A lever that produces a larger load force than the effort force. A crowbar is one.
Apart from multiplying a force, what else can a lever do?
Multiply a small movement into a larger movement.
Along the blades of a pair of scissors, where is the load force largest?
Close to the pivot. It is smaller further from the pivot.
When is a beam balanced?
When the clockwise moments are equal to the anticlockwise moments. If they are unequal, it turns in the direction of the greater moment.
Can two unequal weights balance a beam?
Yes, if their distances from the pivot are right. One mass on peg 8 balances four masses on peg 2 because 1 × 8 = 4 × 2.
What force does a 10 g mass produce?
0.1 N.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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