Physical ScienceMotion and ForcesYear 8 Science

Mass, unbalanced forces and balancing a seesaw

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What this lesson covers

Why more mass needs a bigger unbalanced force for the same change
Levers, pivots and the seesaw
Turning effect: force multiplied by distance from the pivot
When a seesaw balances and when it tips
Unbalanced forces in everyday activities
Designing a fair test with mass or distance

Key idea

More mass, harder to change

Mass measures how much matter an object contains, in kilograms (kg).
Push an empty supermarket trolley and a full one with the same force: the empty one speeds up much more quickly.
The more mass an object has, the bigger the unbalanced force needed to change its motion by the same amount.
This works for slowing down too. A loaded truck needs a much bigger braking force than a car to stop in the same distance.

Worked example

Room 9 tests mass

Room 9 launched a toy cart along the floor with a rubber band stretched by the same 10 cm each time, adding 100 g weights to the cart. They measured how far it rolled.

Room 9's results
Mass added	Trial 1	Trial 2	Trial 3	Mean distance
0 g	182 cm	176 cm	179 cm	179 cm
100 g	121 cm	125 cm	123 cm	123 cm
200 g	90 cm	86 cm	88 cm	88 cm
300 g	70 cm	66 cm	68 cm	68 cm

Same stretch each timeso the launching force was kept the same
Mean distance falls from 179 cm to 68 cm as mass goes upread down the last column
The same force gave the heavier cart less speed, so it rolled less farlink to the idea
More mass: the same unbalanced force changes the motion less

Key idea

A seesaw is a lever

A lever is a rigid bar that can turn about a fixed point called the pivot (or fulcrum).
A seesaw is a lever with the pivot in the middle and a person pushing down on each side.
Each force tries to turn the seesaw: the force on the left tries to turn it one way, the force on the right the other way.
Scissors, spanners, wheelbarrows and bottle openers are levers too.

Key idea

Turning effect depends on force and distance

How hard a force turns something depends on two things: the size of the force and how far from the pivot it acts.
Turning effect = force × distance from the pivot. With force in newtons and distance in metres, the unit is newton metres (N m).
A small force far from the pivot can turn things as much as a large force close to it. That is why a door handle sits far from the hinges.
A seesaw balances when the turning effect on the left equals the turning effect on the right.

A seesaw on a central pivot with a 600 N force 1 m to the left and a 400 N force 1.5 m to the right; both sides have a turning effect of 600 N m, so it balances

Key idea

Try it: balance the seesaw

Worked example

Where should he sit?

Aroha weighs 360 N and sits 2 m from the pivot of a seesaw. Her brother weighs 480 N. How far from the pivot must he sit, on the other side, to balance it?

Seesaw: Aroha, 360 N, sits 2 m left of the pivot; her brother, 480 N, must sit at an unknown distance to the right for the seesaw to balance

Aroha's turning effect = 360 N × 2 m = 720 N mforce × distance
To balance, his side must also be 720 N mturning effects equal
480 N × distance = 720 N mhis force times the unknown distance
distance = 720 ÷ 480 = 1.5 mdivide to undo the multiply
He sits 1.5 m from the pivot, closer than Aroha because he is heavier

Key idea

When a seesaw tips

If one side has a bigger turning effect, the seesaw is unbalanced and turns towards that side.
This can happen because that person is heavier, sits further from the pivot, or both.
In the example above, if Aroha's brother moved back to 2 m, his side would have 480 × 2 = 960 N m against her 720 N m, so his end would go down.
A lighter child can tip a heavier adult by sitting right at the end while the adult sits near the middle.

Worked example

Designing a fair test with a ruler seesaw

Question: how does the distance from the pivot change how many coins it takes to balance a load?
Balance a 30 cm ruler on a pencil at its 15 cm mark. Put a stack of 4 coins 5 cm left of the pivot and leave it there for every trial.
Independent variable: the distance from the pivot on the right side. Dependent variable: the number of coins needed there to balance.
Controlled variables: the same ruler, the same pivot point, the same type of coin, and the same load on the left.

Room 9's results
Distance right of pivot	Coins needed to balance
5 cm	4
10 cm	2
20 cm	1

Left side: 4 coins × 5 cm = 20 coin cmturning effect, with coins as the force
Right side: 4 × 5 = 20, 2 × 10 = 20, 1 × 20 = 20each balanced row
Doubling the distance halves the coins needed

Key idea

Unbalanced forces all around you

Every time something speeds up, slows down or turns, an unbalanced force is at work.

Activity	Unbalanced force	Change in motion
Kicking a rugby ball off a tee	Boot pushes the ball forward	Speeds up from rest
Braking on a bike	Friction from the brakes and road, backward	Slows down
Heading a football	Head pushes the ball at an angle	Changes direction and speed
Tacking a yacht on the Waitematā	Wind on the sail from a new side	Changes direction

Key idea

Try it: what does the force do?

Common mistakes

A seesaw balances when the two people weigh the same.
It balances when the turning effects are equal, so a heavier person can balance a lighter one by sitting closer to the pivot.
Turning effect = force + distance.
Turning effect is force multiplied by distance from the pivot, in N m.
A heavy object needs a force to keep moving at a steady speed but a light one does not.
Mass decides how big an unbalanced force is needed to change motion, not whether a steady speed needs a force.
Measure the distance from the end of the seesaw.
Always measure from the pivot, because that is the point everything turns around.

Try it at home

Balance a ruler seesaw

You need: a 30 cm ruler, a round pencil, 10 identical coins, sticky tape

Tape the pencil to the table and balance the ruler across it at the 15 cm mark.
Place 2 coins stacked at the 10 cm mark, which is 5 cm left of the pivot.
Find where 1 coin on the right side balances them, and write down its distance from the pivot.
Repeat with 3 coins, then 4 coins on the left, each time finding where 2 coins on the right balance them.
Look for the pattern: coins × distance should come out the same on both sides whenever the ruler balances.

Safety: Coins are a choking hazard for babies and toddlers, so do this away from little children and count the coins back into a container afterwards.

Your turn

The same push is given to a 5 kg box and a 20 kg box on the same smooth floor. Which speeds up more?

A 300 N force acts 2 m from a pivot. What is its turning effect?

A 500 N person sits 1.2 m from a seesaw's pivot. How far from the pivot must a 400 N person sit to balance?

On a seesaw, the left side has 800 N m of turning effect and the right side 650 N m. Which way does it tip?

Why is it easier to open a door by pushing near the handle than near the hinges?

Show answer

1. The 5 kg box, because less mass needs less force for the same change. 2. 300 × 2 = 600 N m. 3. 500 × 1.2 = 600 N m, so 600 ÷ 400 = 1.5 m. 4. The left side goes down, because its turning effect is bigger. 5. The handle is further from the pivot (the hinges), so the same force gives a bigger turning effect.

Try it

Balancing Act

Full screen · PhET Interactive Simulations, University of Colorado Boulder, CC BY 4.0

Summary

Recap

The more mass an object has, the bigger the unbalanced force needed for the same change in motion.
A seesaw is a lever turning about a pivot, with forces on both sides.
Turning effect = force × distance from the pivot, in newton metres.
A seesaw balances when both turning effects are equal, and tips towards the side with the bigger one.
In a fair test, change one variable, measure one, and keep the rest the same.

Maths

The maths behind it

Year 8 MathsEquations, and what the letter is doing Year 8 MathsMean, median, mode and range

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