1. Feb 14, 2004

### jhirlo

Hypothetically, when I give acceleration to rock in outer space (nogravity, no air), e.g. 10m/s^2 will it continue to increase the speed forever or it'll (after some time) continue moving without acceleration, at constant speed (like 1.Newtons says, if you let them on their own, they'll be moving in constant speed or standing …) ?

Tnx!

2. Feb 14, 2004

### HallsofIvy

Staff Emeritus
What do you mean by "give acceleration to rock in outer space"?
An acceleration only applies for as long as you apply it. You can't "give acceleration to rock" so that it permanently has that acceleration unless you attach a motor to it (even then it will eventually run down!). You have to apply the acceleration for a specific time. You could then calculate the increase in speed for that time to find the rock's new speed. The rock will then continue to move at that new speed.

3. Feb 14, 2004

### deltabourne

I think he means if you apply a constant force on a rock, what will be the speed at which the rock maxes out at, knowing it can't make it to the speed of light.. I guess like it's "terminal" speed

4. Feb 14, 2004

### Staff: Mentor

The rock will think its constantly accelerating according to Netwon's formula. An outside observer will see it asymptotically approaching C.

5. Feb 14, 2004

### pmb_phy

No. It's only possible for the acceleration to be constant, as measured in your frame of reference, while the speed is less than the speed of light. The faster the particle goes the harder it is to accelerate it. This is due to the fact that the (relativistic) mass increases with speed and will approach infinity as the speed approaches the speed of light.