Turning effects
Key idea: For the same perpendicular force, a greater distance from the pivot gives a greater turning effect.
Continue where you stopped
The core idea
Effects of Force · Lesson 5 of 5 · about 25–35 min
What you need to understand
For the same perpendicular force, a greater distance from the pivot gives a greater turning effect.
Definitions
- moment
- the turning effect of a forceFor example: A door turns more readily when pushed far from its hinges.
Explanation
A pivot is the point or axis about which an object turns. Door hinges form a pivot. A force can turn the door clockwise or anticlockwise when viewed from a stated side. Always identify the pivot and the viewing direction before naming the turn.
The turning effect of a force is called its moment. Compare pushing near the hinges with pushing at the handle, using the same force perpendicular to the door. The handle position gives a greater turning effect because the perpendicular distance from the pivot to the force’s line of action is larger.
Both force and position matter. To investigate distance fairly, keep the force and its direction the same. A force directed towards the pivot does not turn the object about that pivot. Here you need qualitative comparisons and observations, not a moment equation.
Pause and say it: For the same perpendicular force, a greater distance from the pivot gives a greater turning effect.
Common mistake
Tempting wrong idea: The point where the force is applied is always the pivot.
Why it fails: The handle is where the force is applied, but the door turns about its hinges. Find what stays fixed as the object turns to identify the pivot.
Use this instead: The pivot is the point or axis about which the object turns.
Worked examples
Modelled example 1
Compare the same force at two positions
Problem
This is a top view of a door. In separate trials, the same force acts down the page at A or B, perpendicular to the door.
Keep the force and its direction the same; change its position.
Try it. Identify the pivot and turning direction as viewed here. Which position gives the greater turning effect? Explain using distance from the pivot.
View complete solution
Reason from the observations
Method
Identify the fixed hinge line and compare the two push positions.Reason
With the same perpendicular force, increasing distance from the pivot increases the turning effect.Working
The hinge is the pivot. Both pushes turn the door clockwise in this view. B gives the greater turning effect because the perpendicular distance from the hinge is greater. A moment equation is not needed for this comparison.
Find the first error
A student uses a heavier load farther from a ruler’s pivot and concludes: “Distance alone caused the greater turning effect.” Explain the problem and improve the comparison.
Check the investigation repair
Two factors changed: the load changed the force, and its position changed the distance. Use the same load and force direction at different distances. Then the comparison isolates the effect of distance within this setup.
Practical work
What to show: Identify the pivot and turning direction and compare positions without using a moment equation.
Before you finish: No numerical moment calculation is required.
Practical: Turning effect and distance from the pivot
For the same force, how does the distance from the pivot affect the turning effect?
Safety: Use only light loads such as coins, and work over the desk so a sliding ruler cannot fall far.
- Change
- distance of the right-hand load from the pivot: 4, 8, 10 and 12 cm
- Observe
- what the ruler does after it is released: turns clockwise, turns anticlockwise or stays level
- Keep the same
- two identical loads, so each exerts the same downward force, left-hand load fixed 8 cm from the pivot, ruler balanced on its own before any load is added
View the apparatus, method and observation record, then use the record
Apparatus
30 cm ruler, round pencil taped to the desk as a pivot, four identical coins and sticky tape.
Method
- Rest the ruler across the pencil and slide it until it balances on its own. The pencil is the pivot.
- Tape the coins together in pairs to make two identical loads. Each load's weight acts straight down, so with the ruler level its perpendicular distance from the pivot is the distance measured along the ruler.
- Hold the ruler level and place one load with its centre 8 cm to the left of the pivot. Leave it there for every trial.
- Place the other load with its centre 4 cm to the right of the pivot. Release the ruler gently and record, as seen from the front, whether it turns clockwise, turns anticlockwise or stays level.
- Hold the ruler level again and repeat with the right-hand load at 8, 10 and 12 cm from the pivot.
- Repeat the whole sequence and check that the second set of observations agrees with the first.
| Right-hand load distance from pivot / cm | First run | Second run |
|---|---|---|
| 4 | turns anticlockwise | turns anticlockwise |
| 8 | stays level | stays level |
| 10 | turns clockwise | turns clockwise |
| 12 | turns clockwise, faster than at 10 cm | turns clockwise, faster than at 10 cm |
Use the record
What stays the same about the right-hand load in every trial, and what changes?
Check your answer
Its weight stays the same, so the downward force is the same in every trial. Only its distance from the pivot changes.
Use the record to compare the turning effect of the right-hand load at 4 cm and at 10 cm.
Check your answer
At 4 cm the ruler turns anticlockwise, so the right-hand load's turning effect is smaller than the left-hand load's. At 10 cm it turns clockwise, so the right-hand load's turning effect is now larger. The same force had a larger turning effect when it acted farther from the pivot.
Write a conclusion that answers the question.
Check your answer
For the same force, the farther the force acts from the pivot, the greater its turning effect. This is why a door opens most easily when you push at the handle, far from the hinges.
Limitation: Friction at the pencil, or a ruler that does not quite balance on its own, can make a nearly balanced arrangement turn slowly one way or not at all.
Improvement: Balance the ruler on a narrow edge instead of a round pencil, check it balances before each trial, and test extra positions such as 6 cm and 9 cm to see where the turning direction changes.
Guided practice
Try it with support
A ruler is viewed from the front with a pivot at its centre. A downward force acts to the right. Which way does it turn?
- Follow the movement of the right-hand end.
Check the guided answer
Answer: Clockwise: the right-hand end moves down.
Check: State the viewing direction when describing rotation.
Practise and continue
Practise this
The same downward force is applied 4 cm and then 12 cm to the right of the pivot. Compare turning direction and effect.
Need a hint?
- Keep force and direction fixed.
Check your answer
Answer: Both turn clockwise. The 12 cm position gives the larger turning effect because the perpendicular distance is greater.
Check: No numerical moment calculation is required.
Think like a scientist
A student changes both the load and its distance from the pivot. Can a different turn be attributed to distance alone?
Check the reasoning
No. The load changes the force too. Use the same load and direction at different distances.
Remember: A fair comparison isolates the effect being investigated.
One-minute check
- Hide the page and explain turning effects in your own words.
- Give a new example that is different from the worked example.
- Correct this common mistake: “The point where the force is applied is always the pivot.”
Continue with the next resource in this course.
Course and syllabus information
- Course
- SEC G1 Science
- Edition
- SEC G1 Science 2027