Which forces affect the ball in A, B, C and D?

[Jirya]

Homework Statement

I have attached a picture. I just want to know which forces affect the ball at different times: A, B, C and D. Could someone please sum it up really fast? : -)

Homework Equations

The Attempt at a Solution

 

[NascentOxygen]

What forces do you think are acting on the ball at A?

 

[Jirya]

Of course, the gravitation force and a force from the 'table'. The force from the table is greater; and that means that the ball jumps up.

But actually, I'm just interested to know which forces are acting between A and B. A friend of my claim that only the gravitation force is acting. Is that true?

 

[adjacent]

But actually, I'm just interested to know which forces are acting between A and B. A friend of mine claim that only the gravitation force is acting. Is that true?

What about the force when moving through air?

 

[Jirya]

What about the force when moving through air?

I don't see how that's relevant here? :)

 

[adjacent]

I don't see how that's relevant here? :)

Yes.If your question says air resistance is negligible,it's not relevant.

However,in the OP, you said that you want to know the forces acting on the ball.
You did not say that there's a vacuum there.

Still,there's another force.Think of why the ball is slowing down.Energy is transformed from kinetic to another form.We know that it's not potential energy,soo...

 

[Jirya]

Yes.If your question says air resistance is negligible,it's not relevant.

However,in the OP, you said that you want to know the forces acting on the ball.
You did not say that there's a vacuum there.

Still,there's another force.Think of why the ball is slowing down.Energy is transformed from kinetic to another form.We know that it's not potential energy,soo...

Yeah, it's in vacuum. My mistake. I should have informed that.

... What are you implying? :-D

 

[adjacent]

... What are you implying? :-D

Oh sorry.I forgot that you only want the forces acting between A and B.
If the place is a vacuum,then the force(s) acting between A and B is Gravity alone.You're right.

 

[Jirya]

Oh sorry.I forgot that you only want the forces acting between A and B.
If the place is a vacuum,then the force(s) acting between A and B is Gravity alone.You're right.

But why is the ball moving up then?


If only gravity works alone you could imagine that the ball should move downwards?

 

[adjacent]

But why is the ball moving up then?

If only gravity works alone you could imagine that the ball should move downwards?

Where is it moving up? It's moving down after some time.
It's actually accelerating downwards. $\vec F=m\vec a$
A force only accelerated an object in some direction.It does not mean that if you have a force downwards,it should move only downwards.

Think of this: If you throw a ball upwards,why does it move up for sometime instead of falling the moment you release it?

 

[adjacent]

(Think of this: thing)
At the moment you release the ball,the only force .i.e gravity, is acting downwards.It will then accelerate it downwards(Decreasing the upward velocity).When the upward velocity becomes zero,it stops moving up and starts to fall down.

 

[Govind_Balaji]

Think of Newton's laws

 

[Rellek]

Are we assuming no air resistance?

 

[adjacent]

Think of Newton's laws

How is Newton's laws going to help here?
-His first law is not useful here because there is an external force acting(Gravity)
-His second law will work but we don't need it here because a is 10m/s^2 or 9.81m/s^2(Depending on the amount of accuracy required)
-His third law will not work here because it's not touching anything.
Note that Jirya wants the forces acting between A and B.

Are we assuming no air resistance?

Yeah.Read the posts carefully.

Yeah, it's in vacuum.