Effects of forces
A force can change shape, motion or turning; compare before and after.
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The core idea
Effects of Force · Lesson 2 of 5 · about 25–35 min
What you need to understand
A force can change shape, motion or turning; compare before and after.
Definitions
- force effect
- an observable change in an object's motion, shape, size or turningFor example: A push can bend a ruler without moving it forwards.
Explanation
A force is a push or pull arising from an interaction between objects. Compare what happens before and while the interaction acts. A hand can squeeze a sponge, a foot can start a ball moving, and a sideways push can change a moving trolley’s direction.
Describe the change precisely: deformation changes shape or size; a motion change can mean starting, stopping, speeding up, slowing down or changing direction. A door turns about its hinges when pushed away from them. These are different effects of forces, so “it moves” is often too vague.
Continued motion is not itself evidence of an unbalanced push. A trolley may continue rolling after your hand leaves it; friction gradually slows it. Forces may also balance while an object remains at rest. Look for the interaction and the change, rather than assuming that every force must make something go faster.
Pause and say it: A force can change shape, motion or turning; compare before and after.
Common mistake
Tempting wrong idea: A force affects an object only when its centre moves.
Why it fails: A squeezed sponge can change shape while its centre stays still. Motion of the whole object is not the only possible effect of a force.
Use this instead: A force can change shape or size even if the object's centre does not move.
Worked examples
Modelled example 1
Describe the observation
Problem
A hand compresses a sponge without moving its centre forwards. Comparing it before and during the squeeze shows a change in shape and size. The hand exerts a force even without forward motion.
| Before the interaction | During the interaction |
|---|---|
| A ball is at rest | A foot kicks it and it starts moving |
| A trolley moves straight ahead | A sideways push changes its direction |
| A door is still | A push near its handle turns it about the hinges |
Try it. Name the effect in each row. Which involves a pivot? Explain why “all forces make things go faster” is not an adequate rule.
View complete solution
Reason from the observations
Method
Name what changes between the two observations.Reason
Changes in rest, direction, shape and turning are different effects of a force; increased speed is only one possibility.Working
The kick changes rest into motion. The sideways push changes direction; an increase in speed is not stated. The door turns about the hinge line, its pivot. The sponge shows deformation. Describe the observation instead of assuming that every force increases speed.
Is a continuing push necessary?
A trolley keeps rolling after a hand releases it. Before checking: decide whether this proves that the hand is still pushing.
Check the reasoning
It does not. Contact with the hand has ended. Continued motion does not require a continuing forward resultant force. Friction and air resistance can gradually slow the trolley.
Guided practice
Try it with support
A sponge is squeezed without moving its centre forwards. Name the force effect.
- Compare shape before and during the squeeze.
Check the guided answer
Answer: The sponge changes shape and size: it is deformed.
Check: Motion is not the only possible force effect.
Practise and continue
Practise this
A sideways push makes a moving trolley turn left while its speed stays the same. What changes? What would a push on an open door do?
Need a hint?
- Speed and direction describe different features of motion.
Check your answer
Answer: The trolley’s direction changes. A push away from the door hinges can turn the door about them.
Check: State the observed change instead of saying only that an object moves.
Think like a scientist
A trolley rolls after a hand stops pushing it. Does its motion prove that the hand is still exerting a force?
Check the reasoning
No. The hand is no longer in contact. The trolley can keep moving while friction gradually slows it.
Remember: Continued motion does not require a continuing forward resultant force.
One-minute check
- Hide the page and explain effects of forces in your own words.
- Give a new example that is different from the worked example.
- Correct this common mistake: “A force affects an object only when its centre moves.”