# Infinite force and mass

1. Apr 14, 2005

### misogynisticfeminist

I've received a hypothetical question. What happens if a force with magnitude collides upon an object with infinte mass? Is this a physically possible situation? If yes, state why, if no, state why as well.

I don't know much physics so what i did to answer it was to say that since $$F= \frac {dp}{dt}$$, we can draw a momentum time graph. And the only time when $$\frac {dp}{dt}$$ is infinity is when we have a vertical line t= something. Which means, that, momentum is undefined elsewhere, since momentum always have defined values, we say that this situation is not physically possible.

Is it also possible to say that momentum takes on complex values outside the line and since momentum is always real, this situation is not physically possible (something like faster than light travel).

Right?

I hope to hear some other alternatives......

2. Apr 14, 2005

### shyboy

First of all, if you are speaking about force, you need to state what body exerts the force. Even if we are talkin about the field, the field is provided by some object, or we need to consider the field itself as an object with mass.

Second, if you have a body with infinite mass, then any force, no matter how large it is , will not change the body state:
dv=F*dt/m==0. You may get the same result if you consider a collision between two balls with different mass, and look for a limit when one of the balls has an infinite mass.

Third, there is no problem to have an imaginary momentum. As a matter of fact, any decaying wave can be considered as a propagation with an imaginary momentum.

3. Apr 14, 2005

### Tom Something

I don't want to sound like a smart alek in saying this, but an object of infinite mass would have a gravitational field of infinite magnitude, regardless of the acting body's distance. Therefore the acting body would move to the infinite mass at an infinite acceleration. Instantly, the acting body would be gone just from having existed in the same universe as the infinite mass. Do you think that's right? Maybe the infinite mass would tear a hole in the universe and just sort of...fall out of it. Then we wouldn't have to worry about that mass anymore, but we'd still have a big hole in the universe. That would suck.

How about two bodies of infinite mass? Then you have a party!