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

1. Apr 30, 2014

Jirya

1. The problem statement, all variables and given/known data

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? : -)

2. Relevant equations

3. The attempt at a solution

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2. Apr 30, 2014

Staff: Mentor

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

3. Apr 30, 2014

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?

4. Apr 30, 2014

adjacent

What about the force when moving through air?

5. Apr 30, 2014

Jirya

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

6. Apr 30, 2014

adjacent

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......

7. Apr 30, 2014

Jirya

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

... What are you implying? :-D

8. Apr 30, 2014

adjacent

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.

9. Apr 30, 2014

Jirya

But why is the ball moving up then?

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

10. Apr 30, 2014

adjacent

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?

11. Apr 30, 2014

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.

Last edited: Apr 30, 2014
12. Apr 30, 2014

Govind_Balaji

Think of newton's laws

13. Apr 30, 2014

Rellek

Are we assuming no air resistance?

14. Apr 30, 2014

adjacent

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.

Yeah.Read the posts carefully.

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