Forces /newtons law

After i drop a ball and gravity has made it hit the ground when it reaches the top of its maximum bounce height, what forces other than gravity are acting on it? To be suspended in air is it a balanced force of gravity and air resistance?

Reply 1

ThiagoBrigido

Do you mean at its higher position? Air resistance is negligible, can not on its own uphold the ball in the air.
Vaguely speaking, when the ball reaches its final position the force resultant is zero. It is related to Conservation of Energy = Energy is not created or destroyed, instead it is transferable. If you're studying towards the A-levels, I suppose you will study Kinetic energy = F((V)^2)/2a and Gravitational potential energy = mgh. As the ball falls vertically it gains kinetic energy, then the ball hits the ground, elastic deformation occurs and the energy stored is released as kinetic energy.

Reply 2

Pangol

Original post by Strach2k

When the ball reaches it's highest point, it isn't correct to say that it is "suspended" in air. That would imply that it was hanging there for some time, that we could look at it while it stays stationary. If it were doing that, then yes, there would be a need for an upwards force to balance it's downwards weight. But that's not what's happening.

All the time that the ball is in the air - whether on the way up, the way down, or momentarily stationary at the top of it's motion - it's weight is pulling it downwards. You've probably seen Newton's second law, F = ma, and this tells you that because of this downwards force, the ball will accelerate downwards. This doesn't mean that it is moving downwards, just that it's velocity will change in a downwards direction. So when it is on the way up, it's speed will get smaller (i.e. it slows down), and when it is on the way down, it's speed will get larger (i.e. it speeds up). There must be one insantaneous moment when the motion changes from up to down, and this is when it's speed is zero, but that is a direct result of the fact that there is just this one downwards force acting on it. So there's no need for another force to keep it up there. It isn't staying up there anyway!

There may be some air resistance, although it is usually taken as too small to care about in these problems, but there won't be any when the ball is at it's highest point. There needs to be some motion going on for the air resistance to oppose, and there isn't any at the highest point. This is similar to the way that you feel "the wind in your hair" when cycling on a day where there isn't actually any wind - what you are feeling is air resistance, and it gets bigger the faster you cycle, but if you come to a stop, then it's not there any more.

Reply 3

Squigman

Ok so the one downward force acting is gravity and nothing is keeping it up? Im starting to see why that would be the case thankyou.

Reply 4

Pangol

Original post by Strach2k

Ok so the one downward force acting is gravity and nothing is keeping it up? Im starting to see why that would be the case thankyou.

As you say, there is nothing keeping it up. It would stay up if there were!

Reply 5

Squigman

So its an unbalanced gravitational force pushing downwards in the ball?

Reply 6

Pangol

Original post by Strach2k

So its an unbalanced gravitational force pushing downwards in the ball?

I think I would say pulling rather than pushing, but that's just semantics. Yes, the gravitational force on the ball isn't being balanced by anything. If it were, then the resultant force on the ball would be zero, in which case it would either stay motionless or continue moving in a straight line at a constant speed (this is what Newton's first law tells us). It's clearly not going to do either of those things.