why do things bounce?

Question

this seems like a simple question...most people would answer "by newton's third law"...however, consider this:
an object is traveling at a velocity of 5m/s at a solid brick wall. The acceleration of the ball is 0 and thus, the ball is moving w/ a force of 0 N (F = ma = 0m = 0). obviously, the ball will bounce back off of the brick wall, but this cannot be caused by newtons third law can it??--there is no action force to cause a reaction force in the opposite direction...when the ball collides into the wall, there is no force pushing it, so how can there be a force pushing the ball backwards?

forget gravity--pretend that this thought experiment is conducted in space

many people have been talking about elasticity...however, this ignores the fact that there is no FORCE to push the ball back...if there is no force, how can the ball bounce back?

to michael c -- 1/2 mv^2 is an experession of energy, not force...also, wat causes the ball to decelerate to 0m/s when it reaches the ball?

Answer 1

The acceleration on the ball is zero _until_ the ball "touches" the wall. By "touches," I mean the ball is so close to the wall that the negative electric charge on the outside of the wall's atoms begin to repel the negative charge on the outside of the ball's atoms. (This happens because like charges repel each other.)

At this point, because of the mutual repulsion of the negative charges against each other, the wall is exerting a force on the ball. By Newton's 2nd Law, this causes the ball to decelerate. Within a fraction of a second, it decelerates to zero speed.

The force acting on the ball (during that fraction of a second) causes the ball to deform. If the ball is made of something elastic, the deformation will not break the ball's molecular bonds, but will just squeeze them together like a spring. (If the ball is made of something hard, this deformation may be microscopically small, but it still exists).

In the next fraction of a second, the ball springs back to its original shape. This causes it to exert a force against the wall. By Newton's 3rd Law, this means the wall automatically exerts an identical force against the ball. This force on the ball causes it to accelerate away from the wall until the two are no longer in contact. At that point, the ball stops accelerating (assuming no other forces are acting on it), and continues outward at a constant speed.

Answer 2

It is Newton's third law you just seem to have misunderstood a big part of it. I will try my best to explain. Let's say your ball is travelling at a constant speed of 5 m/s before it hits your wall. So you are right that at a constant speed the ball is not experiencing any acceleration. But think of the point of contact of the ball and the wall. At that point the loses all of it's velocity in the direction it was traveling (negative acceleration.) There goes your "equal and opposite" reaction force.

So if you want to know how to calculate it, the instantaneous change of velocity of the ball is the acceleration. If you have ever taken a calculs course, you would know how to find the acceleration of the object by taking a time derivitive of the velocity. That would be 1/2 velocity squared. So the reaction force on the ball would be 1/2 mass of the ball * velocity squared. (1/2)mv^2

Try not to think that just because f=ma you need to be speeding up or slowing down for moving objects to have a force. You have to understand the motion of the object over given time periods, and that the force will not be experienced by anything until it actually hits something. Hope that all makes sense, have fun learning about physics.

Answer 3

of course there is a force on the ball. it is a contact force, you are sat in a chair now. There are 2 main forces acting on you.

One is the earth pulling you down and the other is a contact force from the chair pushing up on you. This contact force is the same type as the forces between the wall and the ball when they collide. (NB the pull of earth on you and the force of the earth up on you are NOT a third law pair. Third law pairs are always applied by one object to the object applying the force on it. IE i push you, you push me. The earth pushes up on you, you push down on the earth. The earth pulls you, you pull the earth. (its weird to think, but just as the earth attracts you with a force calles gravity, you do the same to it! only the effect you have is tiny cos the earth is huge)

You seem to have no problem with the physical reason a ball bounces (compressing etc), only the calculations behind it so i will try to solve your problem this way :D

In your calculation, you calculate the force on the ball as it is moving freely not as it is colliding with the wall and you are correct there are no forces acting on it (ignoring air resistance blah blah). One fatal mistake you make is "the ball is moving with a force of 0 N" a force is applied, something does not move with a force of something. The ball has a force applied to it only during the impact. You are sat happily now without any resultant force, but i can apply a force to you and you will move. Just because no force was applied to you before, doesnt mean there isnt any applied later.

You are confusing this with momentum (= mass * velocity), the ball does have momentum as it moves. The force of the wall on the ball is calculated by (change in momentum)/(impact time). (which makes sense if you think about it: f=ma, a=v/t therefore f = mv/t)

In response to your question: "but this cannot be caused by newtons third law can it?" Yes it is. The ball has momentum (because it is moving) and so when it hits the wall it applies a force to the wall, equally the wall applies a force to it, causing it to move off in the opposite direction.

To approach the question in another way, you seem happy using f=ma. So lets look at the acceleration of the ball. It starts of travelling (say) to the left as 5m/s. It hits and bounces back, now travelling to the right at 5m/s, which i hope you will agree is the same as travelling to the left at -5m/s.
Acceleration is (change in velocity)/(time to for change to take place). I hope you can see now the change in velocity is 10m/s. Now lets say the ball is in contact with the wall for 1 second (which is a huge amount of time :D) So the acceleration is 10m/s/s. and lets say the ball weighs 1kg so the calculations is f = 10 * 1. Which of course comes to 10N. So you can see that there will always be a force applied to the ball in this situation.

i hope there is plenty for you to play with there. The important thing to realise is there is a force pushing the ball backwards and that is the third law contact force of the wall on the ball.

let me know if u need any more explanation

edit: just saw "wat causes the ball to decelerate to 0m/s when it reaches the ball?"

that is because it hits the wall and has a phsyical interaction with it. Just as the chair you sit on supports you. Matter cant go through other matter, they react. so the ball must experience a force from the wall to have it's motion changed

Answer 4

2

Answer 5

Why Do Things Bounce

Answer 6

This goes completely beyond my expertise, as I am not familiar with these scientific theories. Therefore, the only way I can answer your question in the simplest of terms is that some objects would be more likely to bounce back than others. You're talking about a ball, but what would happen if you threw a large rock at a solid brick wall? It would not bounce, but come crashing to the ground. Therefore, I must conclude that the weight, shape and material of the object being thrown would determine whether or not it would bounce or conform to Newton's Law. (Just a shot in the dark, I know! )

Answer 7

What you are trying to imply is that the ball is motionless, since no additional force is being applied to it. You over look the fact that force was required, initially, for the ball to obtain a velocity of 5m/s. This velocity will not change until resistance is applied to the ball and causing a drag effect or until more force is applied to the ball causing an acceleration effect.
The ball bounces off of the wall because of the initial force applied to the ball. This is not negated simply because no additional force is applied.

Answer 8

The force exerted by the wall is what pushes it back - the wall and what it is attached to has the force of inertia, which is exerted on the ball. According to Newton the wall (or street) moves too, just imperceptibly.

Answer 9

Use kinetic mass, not acceleration. Density, deflection, deformation and resilience will yield a better equation.ere

F = force acting on the surface
S = Size of object
E = modulus of elasticity
I = area moment of inertia

Answer 10

I think it has something to do with elasticity, like some things spring back to shape when you squish them.

I think it's a reaction on a much smaller level, but all together, you get a bounce.

If something is solid and not particularly elastic, like, say, a boulder, the force of the object falling or being thrown or whatever goes into smashing whatever it hits.

It's all about the squish factor.