Turning forces
Turning forces
- Unit 1: Particles & Pressure
- Unit 2: Forces & Motion
- Unit 3: Energy & Heat
- Unit 4: Electricity
- Unit 5: Magnetism & Electromagnetism
- Unit 6: Waves: Sound & Light
- Unit 7: Scientific Investigations
In this topic you will:
- recognise when a force causes something to turn
- know how to use the term moment
- be able to calculate the moment caused by a force.
Key words
- lever
- moment
- newton metres
- pivot
- turn
Turning effects of forces
When you push down on a door handle, the handle turns.
When you push down on the pedal of a bicycle, the crank arm turns.
When you pull on a door, the door turns toward you.
These are all examples of forces that cause an object to turn.
The object that turns is called a lever.
The point around which the lever turns is called the pivot.
The lever and pivot are shown in the picture of the bicycle pedals.
When you bend your arm, the arm acts as a lever. Your elbow is the pivot.
Calculating moments
The moment of a force describes its turning effect.
The moment of a force depends on:
- the size of the force (the bigger the force, the bigger the moment)
- the distance between the position where the force acts and the pivot (the greater the distance, the greater the moment)
You can calculate a moment from this equation:
moment = force × distance
Distance in the equation is the distance from the pivot to the position where the force acts.
The unit of force is the newton. The unit of distance is the metre.
Therefore, the unit of moment is newton × metre, which is written as newton metre or N·m.
Remember to use an upper case N and a lower case m when writing N·m.
Worked example
Question
A pulling force of 35 N is needed to open a door. The distance from the door handle to the door hinges (the pivot) is 0.8 m. What is the moment caused by the pull on the door?
Answer
moment = force × distance
= 35 × 0.8
= 28 N·m
Worked example
Question
Look at this diagram.
What is the moment caused by the weight on the arm?
Answer
moment = force × distance
= 20 × 0.25
= 5 N·m
Balancing
A seesaw is a type of lever.
People sit on either side of the pivot of a seesaw and make the lever turn one way and then the other.
The result is that each person moves up and down. A seesaw will be balanced when the moments on both sides of the pivot are equal and opposite.
Worked example
Question
Marcus weighs 600 N and sits at a distance of 2 m from the pivot of a seesaw. Arun weighs 800 N. Where should Arun sit to make sure the seesaw is balanced?
Answer
Marcus will exert a moment of 600 × 2 = 1200 N·m
For the seesaw to be balanced, the moment on the other side must also be 1200 N·m.
moment = force × distance
So, distance = moment ÷ force
distance = 1200 ÷ 800
= 1.5 m
Questions
b Write the equation that links moment, force and distance.
c Write the unit of moment.
Show Answer
a. A moment is the turning effect of a force.
b. moment = force × distance
c. newton metre or N·m
Write the letter.
b Explain your answer to part a.
Show Answer
a. A
b. Arrow A applies the force farthest from the pivot and in the correct direction to maximize the moment.
Sofia pushes on the door handle with a force of 4 N at the position shown in the drawing.
Calculate the moment caused by this force.
Show Answer
moment = force × distance = 4 × 0.12 = 0.48 N·m
A moment of 1.8 N·m is needed to turn this brake lever.
Calculate the force F needed to produce a moment of 1.8 N·m.
Show Answer
force = moment ÷ distance = 1.8 ÷ 0.09 = 20 N
Sofia weighs 500 N.
Sofia sits on the seesaw on the other side of the pivot from Zara.
Calculate the distance from the pivot that Sofia must sit to balance the seesaw.
Show Answer
Zara’s moment = 450 × 1.5 = 675 N·m
Sofia’s distance = 675 ÷ 500 = 1.35 m
Think Like a Scientist
Calculating moments
In this investigation, you will investigate how the force needed to turn an object varies with distance from the pivot.
- metre rule
- forcemeter
- two clamp stands
- elastic (rubber) band
- ruler
- string
- sticky tape
- G-clamp
- Move the loop of string with the forcemeter as far from the pivot as you can.
- Record the distance between the pivot and the forcemeter.
- Raise the forcemeter so it is not pulling down on the metre rule.
- Use the forcemeter to pull down on the metre rule. The distance that you pull depends on the strength of the elastic band. The metre rule needs to be pulled down far enough to get a reading of about 1 N at the furthest point from the pivot.
- Use the ruler to record the distance that the metre rule moves. This will be the distance the metre rule should be pulled down each time.
- Record the force.
- Repeat this, pulling the metre rule down the same distance each time. Each time, use the loop of string to move the forcemeter closer to the pivot.
- Your results should be a set of distances and forces.
- Decide whether you need to repeat any of your measurements.
Show Answer
We used a G-clamp to secure the stand, pulled slowly to avoid snapping the elastic, and ensured no one was beneath the setup.
Show Answer
Example:
| Distance (m) | Force (N) |
|--------------|-----------|
| 0.10 | 4.0 |
| 0.20 | 2.0 |
| 0.30 | 1.3 |
Show Answer
Plot force on the y-axis and distance on the x-axis. The graph should show an inverse relationship: as distance increases, force decreases.
Show Answer
The greater the distance from the pivot, the less force is needed. This matches the equation: moment = force × distance.
Show Answer
Yes, the force increased as the distance from the pivot decreased.
Show Answer
Use a consistent pulling technique, measure distances more precisely, and take more repeats for each position.
- Describe anything you did during the investigation to help get more accurate results.
- a. Did you repeat any of your measurements? Explain why, or why not.
- b. Explain your answer to part a.