# Will this force be zero ?

Tags:
1. Feb 6, 2016

### Anjum S Khan

1. The problem statement, all variables and given/known data
I am pushing a very heavy ball with full strength, but the ball is not moving at all. So, there is no change in momentum of the ball.
Is it appropriate to say that the Force applied to the ball is zero ?

2. Relevant equations

3. The attempt at a solution

2. Feb 6, 2016

### Staff: Mentor

What are your thoughts on this?

3. Feb 6, 2016

### Anjum S Khan

Force is rate of change of momentum. But here change of momentum is zero, so Force should be zero.

4. Feb 6, 2016

### SteamKing

Staff Emeritus
Look at it another way. While pushing on the ball, you slip and the ball rolls on top of you and stops. The ball is not moving, yet it is crushing you. Is the force still zero?

5. Feb 6, 2016

### Anjum S Khan

Change the ball to say some other rigid body which is incompressible anyhow, with very heavy mass. I am pushing it but can't make it move. Then ?

6. Feb 6, 2016

### SteamKing

Staff Emeritus
There's a building which you are pushing against. The building does not move, yet your muscles are straining to push it over. Are you exerting no force against the side of this building?

7. Feb 6, 2016

### Staff: Mentor

What is the difference between force and net force?

8. Feb 6, 2016

### Anjum S Khan

Of course I am exerting pressure. But in physics terms (Newton's 2nd Law), Force should be zero.

9. Feb 6, 2016

### SteamKing

Staff Emeritus
Since pressure is defined as force per unit area, if you are exerting pressure, ipso fatso, you are exerting a force.

10. Feb 6, 2016

### Staff: Mentor

Newton's 2nd law says that the NET force acting on a body is equal to its mass times acceleration. It doesn't say that any individual force on a body is equal to its mass times acceleration.

If you are pushing on a building (and the building isn't moving), what are the two horizontal forces acting on the building?

11. Feb 6, 2016

### Nidum

12. Feb 6, 2016

### UchihaClan13

If you are pushing a rigid body and the body doesn't move
It's only so because the "net" force acting on the body equals zero yielding zero acceleration
ΣFexternal = m×a
Where m is the mass and a is the acceleration produced in the body
This is newton's 2nd Law of motion (simplified)
Look at it in this way
Let's say you're pushing a block on a smooth table and your friend is pushing the block in the opposite direction
Let's say both of you keep on pushing with equal forces but the block doesn't accelerate
Does this mean that you aren't applying a force on the ball?
You are right as you're pushing the ball
However the net force "applied"to the ball is zero as the forces of equal magnitude in opposite directions cancel each other out
Hence it's net force, not an individual force acting on a body which yields the acceleration!!

UchihaClan13