# A push to an object in complete vaccum.

1. Jan 7, 2010

### R Power

Hi guys
When we push an object i.e apply some force on it in complete vaccum, then due to absence of any friction; and inertia, what will happen, will the object accelerate forever because it has no reason to decelerate? Or will it move with constant speed?Why?
If the object will move forever it gives us infinite work for just a small push as input. How can this happen???

2. Jan 7, 2010

### ZapperZ

Staff Emeritus
I suspect you have a misunderstanding of Newton's laws here.

If you push and THEN stop your push, the object will move with contant velocity. If you continue to push, then the object will continue to accelerate (i.e. F=ma). So if you push on it forever, you will be doing work on it forever. If you stop pushing, and it moves with constant velocity, you are no longer doing work (work = force x distance. No force, no work!).

Zz.

3. Jan 7, 2010

### ponjavic

Agreed except for the limitation of relativity?

4. Jan 7, 2010

### R Power

imagine i throw a ball, won't it accelerate?

5. Jan 7, 2010

### Staff: Mentor

As long as you keep pushing it. But once it leaves your hand, you are no longer exerting a force on it.

6. Jan 7, 2010

### R Power

but when players on football field kick a football, we see it accelerating even after leaving player's foot

7. Jan 7, 2010

### ponjavic

It is not accelerating. It is accelerating relative to the player's foot due to the decelleration of the player's foot.

8. Jan 7, 2010

### R Power

you mean to say that if in vaccum the player kicks a ball, then after kicking, players foot along with player will also move in opposite direction to that of football and so then football will move with constant velocity w.r.t player.

9. Jan 7, 2010

### FoxCommander

That is because there is gravity on earth so it decelerates when it is going up and accelerates when it is going down. In a complete vacum IN SPACE (ie no gravity) when you throw a ball the ball will accelerate from the moment you start throwing the ball till you let go of it. Once you let go of the ball the ball will continue to go at the speed it reached will accelerating in you hand, with no external forces (wether physical, electrical, magnetic, or gravitational) the ball will continue at that velocity until something hits it or applies a force on it.

Sincerely,
FoxCommander

10. Jan 7, 2010

### ponjavic

I am saying that if a player kicks a ball in real life then it looks like the ball is accelerating away from the player due to him decellerating his foot. It is actually moving at constant velocity (ignoring gravity and friction).

If he kicks the ball in vaccum he would have no way of decelerating his own foot (without the help av gravity) so initially it would look like his foot moved with the ball but due to him having a body he would start rotating :D

It's a complicated consideration and not very intuitive. The point is as has been made in this thread an object is accelerating as long as you supply it with a force (this implies contact). Once released there can be no acceleration.

11. Jan 7, 2010