The Student Room Group

Unbalanced or balanced force

After a ball has fallen and bounced once - reaching its maximum rebound height - are the forces acting on it balanced or unbalanced and why?
Reply 1
After a ball has fallen and bounced once on the floor - reaching its maximum rebound height - are the forced acted on it balanced or unbalanced and why?
Original post by Simon556
After a ball has fallen and bounced once on the floor - reaching its maximum rebound height - are the forced acted on it balanced or unbalanced and why?

when the ball is in air (if upthrust and drag are ignored) the only force acting on the ball is its weight, so there is a resultant (unbalanced) force. the ball is in free fall and its acceleration is g.
Reply 3
Original post by Simon556
After a ball has fallen and bounced once on the floor - reaching its maximum rebound height - are the forced acted on it balanced or unbalanced and why?


Is the velocity changing at that point?
When it is stationary (when it stops for a short time mid-air), the forces are balanced. So, yes. When the ball is in motion, the forces are not, as it the ball is constantly accelerating and decelerating.
Reply 5
Original post by SagaciousSag
When it is stationary (when it stops for a short time mid-air), the forces are balanced. So, yes. When the ball is in motion, the forces are not, as it the ball is constantly accelerating and decelerating.


It is not the case that the forces on the ball are balanced just because it has stopped moving at the top of the bounce. It is still feeling its weight downwards because of gravity, but there is no corresponding upwards force acting on it.

Objects feel no net force when they are not accelerating, not when they are not moving.
Reply 6
Thanks so much for your answer.

But I am so torn between the answer of (in simple words) 1. its balanced because for a moment it is static and no forces are acting on it or 2. its balanced because the pull of gravity is still present but nothing is counteracting this force.

I think we can deffinitely agree on the fact that its balanced but I need to know which explanation is correct.

Thanks so much for your help!!
Reply 7
Original post by Sinnoh
Is the velocity changing at that point?


The question is 'indicate what forces (balanced or unbalanced) are acting on the ball at the top of its bounce (maximum height)

So technically no, there no velocity changing because the ball has already been dropped, it has bounced and NOW at its max height, whats the forces?

Thanks so much for your help!!
Reply 8
Original post by Simon556
Thanks so much for your answer.

But I am so torn between the answer of (in simple words) 1. its balanced because for a moment it is static and no forces are acting on it or 2. its balanced because the pull of gravity is still present but nothing is counteracting this force.

I think we can deffinitely agree on the fact that its balanced but I need to know which explanation is correct.

Thanks so much for your help!!

The second version is closer to the truth, but not completely right.

It is the case that gravity is still present and that nothing is counteracting it. But this means that there is an unbalanced force acting on the ball. There is nothing balancing its weight.
(edited 1 year ago)
Reply 9
Original post by Pangol
The second version is closer to the truth, but not completely right.

It is the case that gravity is still present and that nothing is counteracting it. But this means that there is an unbalanced force acting on the ball. There is nothing balancing its weight.

So the full and correct answer is: 'The forces are unbalanced due to the force of gravity pulling the ball down and the acceleration of the ball being zero as the bounce has caused it to lose energy' ? More or less?
Original post by Simon556
So the full and correct answer is: 'The forces are unbalanced due to the force of gravity pulling the ball down and the acceleration of the ball being zero as the bounce has caused it to lose energy' ? More or less?

You don't have to talk about acceleration or energy at all. You just have to think about the forces that are acting on the ball. Its weight (gravity) is pulling it down, and there is nothing pushing it up, so there is nothing balancing its weight.

Why do you think the ball's acceleration is zero at the top of the bounce?
Reply 11
Original post by Pangol
You don't have to talk about acceleration or energy at all. You just have to think about the forces that are acting on the ball. Its weight (gravity) is pulling it down, and there is nothing pushing it up, so there is nothing balancing its weight.

Why do you think the ball's acceleration is zero at the top of the bounce?



Thank you for all your help and fast answering.

My thought process of the acceleration being zero is that, when the ball was dropped at any given height, the force of gravity, apart from pulling it towards the floor, will cause it to accelerate (9.8 m s2) but since where talking about the ball at its maximum height having already bounced, there would be zero acceleration, in any case it would be slowing down. Am I right in thinking this?
Reply 12
Original post by Simon556
Thank you for all your help and fast answering.

My thought process of the acceleration being zero is that, when the ball was dropped at any given height, the force of gravity, apart from pulling it towards the floor, will cause it to accelerate (9.8 m s2) but since where talking about the ball at its maximum height having already bounced, there would be zero acceleration, in any case it would be slowing down. Am I right in thinking this?



So like, the bounce has caused the acceleration to equal out to the force of gravity. So in the exact moment that the ball is stopped at its maximum height, the two forces gravity and acceleration are equal, so theyre balanced...???
I keep going back and forward with this. How is something so simple, so confusing?
Original post by Simon556
Thank you for all your help and fast answering.

My thought process of the acceleration being zero is that, when the ball was dropped at any given height, the force of gravity, apart from pulling it towards the floor, will cause it to accelerate (9.8 m s2) but since where talking about the ball at its maximum height having already bounced, there would be zero acceleration, in any case it would be slowing down. Am I right in thinking this?

It can't both have zero acceleration and be slowing down. Slowing down is a form of acceleration!

Just because something is not moving, it does not mean that it is not accelerating. If the ball had zero acceleration when it is at its maximum height, then it would stay there. It obviously isn't going to do that!

If an object is moving under gravity, then it is always accelerating downwards at 9.8 m s-2. It doesn't matter if it is falling down, moving up, or is still at the top of this motion, it will always be accelerating downwards. If it is already falling down, this will make it fall faster. If it is moving up, this will slow it down. And if it is at the top of the motion, this will make it start to fall back down.
(edited 1 year ago)
Original post by Simon556
So like, the bounce has caused the acceleration to equal out to the force of gravity. So in the exact moment that the ball is stopped at its maximum height, the two forces gravity and acceleration are equal, so theyre balanced...???
I keep going back and forward with this. How is something so simple, so confusing?

You need to be sure what are forces and what aren't. Acceleration is not a force. If it were, it would be measured in newtons. Acceleration is a measure of how something's velocity is changing in a given time.

The ball may have bounced off the ground to get to its maximum height, but you don't need to care about that. All you need to do is think about what the ball is "feeling" at the moment that you are considering. And at the moment you are interested in, the only force it feels is its weight (a bit better to say this than just "gravity", although I am sure you could get away with that). There is no upwards force, so there is nothing to balance this weight.
Reply 15
Original post by Pangol
You need to be sure what are forces and what aren't. Acceleration is not a force. If it were, it would be measured in newtons. Acceleration is a measure of how something's velocity is changing in a given time.

The ball may have bounced off the ground to get to its maximum height, but you don't need to care about that. All you need to do is think about what the ball is "feeling" at the moment that you are considering. And at the moment you are interested in, the only force it feels is its weight (a bit better to say this than just "gravity", although I am sure you could get away with that). There is no upwards force, so there is nothing to balance this weight.

Ok, I think I've got it.

You have been incredibly helpful, I can't thank you enough!! Honestly, everything you said makes total sense and your words are being added right now to my notebooks haha Thank you thank you thank you!!
Original post by Simon556
The question is 'indicate what forces (balanced or unbalanced) are acting on the ball at the top of its bounce (maximum height)

So technically no, there no velocity changing because the ball has already been dropped, it has bounced and NOW at its max height, whats the forces?

Thanks so much for your help!!

I know we're talking about this on the other thread you made for this question, but it's worth pointing out that there is a change in velocity going on even when it stops at the top of the bounce. If there was no change in velocity, it would stay there!
Reply 17
Original post by Pangol
You need to be sure what are forces and what aren't. Acceleration is not a force. If it were, it would be measured in newtons. Acceleration is a measure of how something's velocity is changing in a given time.

The ball may have bounced off the ground to get to its maximum height, but you don't need to care about that. All you need to do is think about what the ball is "feeling" at the moment that you are considering. And at the moment you are interested in, the only force it feels is its weight (a bit better to say this than just "gravity", although I am sure you could get away with that). There is no upwards force, so there is nothing to balance this weight.

Just one last thing before we close this. If I were to draw a picture and point out the forces of this same situation, the only force I would have to point out would be the force of gravity then. Yes?
Original post by Simon556
Just one last thing before we close this. If I were to draw a picture and point out the forces of this same situation, the only force I would have to point out would be the force of gravity then. Yes?

Yes. One thing you could do that might help in this situation is to imagine yourself as the ball, but you've got a blindfold on so you can't see what is happening, you can only feel the forces involved. So, if you were lying on the floor, you would "feel" your weight, but you would also "feel" the ground pushing up on you. If you were falling through the air, the only thing you would "feel" is your weight. There would be nothing pushing you up. (This is all assuming we can ignore air resistance, which you may or not be able to depending on the question, but at the top of the bounce it wouldn't have any effect anyway.)
Reply 19
Original post by Pangol
Yes. One thing you could do that might help in this situation is to imagine yourself as the ball, but you've got a blindfold on so you can't see what is happening, you can only feel the forces involved. So, if you were lying on the floor, you would "feel" your weight, but you would also "feel" the ground pushing up on you. If you were falling through the air, the only thing you would "feel" is your weight. There would be nothing pushing you up. (This is all assuming we can ignore air resistance, which you may or not be able to depending on the question, but at the top of the bounce it wouldn't have any effect anyway.)

This makes so much sense. Thank you for this great exaplanation!!

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