KS3 · Physics
Changing speed (acceleration intro)
Pull a trolley with a steady force and it doesn't settle at one speed. It keeps getting faster all along the bench. So what does a force really do?
Physics · Forces and motion
Faster, slower or no change?
Pick a situation, then put it where you think it belongs: what happens to the trolley's speed?
Still to sort
Speeds up (0)
The resultant force points the same way the trolley is moving.
Where the line is: Same trolley, same motion as 'Slows down'. The only thing that changes is which way the left-over force points.
Slows down (0)
The resultant force points against the way the trolley is moving.
Where the line is: A forward force can be acting and the trolley can still slow down. What counts is the direction of the resultant, not whether some force points forward.
Speed stays the same (0)
The forces are balanced: nothing is left over, so the resultant force is zero.
Where the line is: Balanced doesn't mean small. Two big forces that cancel leave nothing over, just like two small ones.
Unless a card says otherwise, the trolley is rolling to the right, so right is forward. For each situation, take the backward forces away from the forward ones. What's left over is the resultant force. Then decide what it does to the speed.
Predict, then check
Decide before you open it. Your first answer shows you what you really believe about forces.
A trolley sits on a bench. A string is tied to it and runs over the end of the bench, where some masses hang from it. Let go, and the falling masses pull the trolley along the bench with a constant force. While the masses are falling, what happens to the trolley's speed?
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
Add up the forces, taking their directions into account. The force left over is the resultant force. It changes the speed for as long as it acts: with the motion it's faster, against the motion it's slower, and if nothing is left over the speed stays the same.
What you need to know
- The resultant force is the single overall force left when you add up all the forces on an object, taking their directions into account.
- A resultant force in the same direction as the motion makes an object speed up. A resultant force against the motion makes it slow down, and it can stop it.
- The speed keeps changing for the whole time a resultant force acts. It doesn't jump to one new speed.
- For the same object, a bigger resultant force changes its speed more quickly than a smaller one.
- When the forces are balanced there is no resultant force, so the speed doesn't change: a still object stays still and a moving object keeps a steady speed.
- On a distance–time graph, a steeper line means faster movement. A straight line shows a steady speed, and a line that gets steeper shows an object speeding up.
The big picture
A resultant force changes an object's speed, and keeps changing it for as long as it acts. If it points the same way as the motion, the object speeds up; if it points against the motion, the object slows down. A bigger resultant force changes the speed more quickly. Balanced forces leave the speed unchanged. On a distance–time graph, a line that gets steeper shows an object speeding up.
Key points
Worked example
Problem
A trolley rolls to the right along a bench. A string pulls it 6 N to the right. Friction pulls it 2 N to the left, and a second string pulls it 1 N to the left. What is the resultant force, and what happens to the trolley's speed?
⚠ Watch out
Thinking a moving object needs a forward resultant force just to keep moving. It doesn't. With balanced forces it keeps rolling at a steady speed. A forward resultant force would make it speed up.
Memory hook
What's left over takes over. Add the forces, then look at what's left: with you, faster; against you, slower; nothing left, no change.
Check yourself
Cover the page. A rolling trolley has a 3 N pull forward and 5 N of friction backward. What is the resultant force, and what happens to its speed? What would keep its speed steady?
Flashcards
(13)What is a resultant force?
A 7 N force pushes an object forward and a 3 N force pushes it backward. What is the resultant force?
The resultant force acts in the direction of motion. What does the object's speed do?
The resultant force on a moving object points against its motion. What happens?
The forces on a still object are balanced. What happens to it?
The forces on a moving object are balanced. What happens to it?
For how long does a resultant force keep changing an object's speed?
A 1 N and a 5 N resultant force each act on the same trolley. Which changes its speed more quickly?
Falling masses pull a trolley along a bench with a constant force. What happens to its speed?
On a distance–time graph, what does a steeper line mean?
What does a straight line on a distance–time graph show?
What does a distance–time line that gets steeper over time show?
Does a moving object need a forward force to keep moving at a steady speed?
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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