# Force in Deep space

1. Dec 6, 2014

### lovethepirk

Thanks in advance.

An asteroid is moving through deep space with ZERO gravity and ZERO matter around it. It is moving at a perfectly constant 100 ft/s.

F=m*a

As I understand it this asteroid has zero acceleration and hence zero force. This confuses me b/c If another asteroid was not moving at all and got hit by this moving asteroid there would be force in that impact for sure.

Help this lost soul please :)

2. Dec 6, 2014

### Staff: Mentor

And therefore the asteroids would each accelerate.

3. Dec 6, 2014

### phinds

If there is nothing else in the universe, there is nothing to reference the motion to and so your "100f/s" is meaningless. All motion is relative so you have to say 100f/s relative to WHAT?

Now if you have two asteroids and they are moving at 100f/s relative to each other, you can choose a frame of reference in which one of them is at rest and assign the 100f/s to the other relative to the first.

How would this be any different that throwing a bowling ball, which then has a constant speed after you let it go, that then hits a pin at the end of the alley?

4. Dec 6, 2014

### DivergentSpectrum

the equation force=m*a means that if a force of N newtons acts on a body with K kilograms, it will accelerate to N/K meters per second^2
In other words, if the same force acts on 2 different bodies, and body A has 2 times as much mass as body B, then the acceleration of body A will be half that of body B

5. Dec 6, 2014

### A.T.

Motion is referenced to reference frames (hence the name). It doesn't require a real physical object as reference.

Relative to a reference frame that moves at -100f/s relative the asteroid.

6. Dec 6, 2014

### lovethepirk

I'm still confused so let me put some more details out there. Let's again assume the moving asteroid is going 100 ft/s and to satisfy the poster thinking there is no other matter for relative measurement, let's assume there are two non-moving/stationary, very small bananas that we can use to calculate this with the gravity of said bananas being so minor that it is futile to add as a calculation.

Question:
1) What is the force of this asteroid?
2) Is the acceleration zero b/c it is moving at a perfect constant rate?

7. Dec 6, 2014

### Bandersnatch

1) there is no such thing as a "force of the asteroid".
2)yes

Force is defined as the rate of change of momentum. You can act with a force on the asteroid to change its momentum(i.e., accelerate; change the state of motion), or you can have it act on something else to do the same. You can't have it "have a force".

Note that you're describing two different situations: the asteroid moving at constant speed in a straight line, where no force is acting on it to change its state of motion, and a collision with another asteroid, where there most certainly is a change of the state of motion.

How comfortable are you with Newton's laws of motion? It's all contained in those three.

8. Dec 6, 2014

### lovethepirk

I understand now by what you just stated thanks.

Let me pose this since it is a backended way to get the force. Say the asteroid hit a stationary one flat surface to flat surface they meet perfectly aligned with no deflection and both are 1000 grams only one is traveling at 100 ft/s the other is stationary.

Since they are perfect twin asteroids and impact perfectly aligned I would think you can calculate the force place on the stationary twin and hence know the 'power' that is built up in this moving asteroid. Does that make sense......I am trying to figure out how powerful this object is. I'm thinking measuring the force place on its twin is a good way to get this.

9. Dec 6, 2014

### phinds

Fair enough, but I think that's a bit subtle for the OP and I was trying to help him understand the relativity of motion in more direct terms.

10. Dec 6, 2014

### lovethepirk

This question may help me:

Someone throws a baseball in deep space with no gravity and the baseball is constant. It hits a 1000 gram object and the object moves all over the place from the amazing impact.... Now same baseball but it hits a 1million gram object and that object laughs at the baseball while only moving back 1 millimeter.

Is there more force in the first impact. I see this as the same force.

11. Dec 6, 2014

### Staff: Mentor

OK, power, force, energy, momentum, work, impulse are all different concepts. An asteroid moving at a constant 100 ft/s has energy and momentum, but not force. If it collides with another asteroid then there will be a force between the objects (note that the force is part of the interaction, not part of either object alone). That force times the time it is applied gives the impulse which is the change in momentum. That force time the distance it is applied gives the work which is the change in energy.

You seem to be mixing up these distinct concepts.

12. Dec 6, 2014

### lovethepirk

GOTCHA thanks.....much thanks. I was thinking force was part of the stored up energy of a moving object. Now I see it is something applied to that object to change its momentum.

Is the force the same for this asteroid hitting a massive object vs a small one?

13. Dec 6, 2014

### Staff: Mentor

No, the force is larger if it hits a big object (all other things equal).

14. Dec 6, 2014

### A.T.

No, you wrongly claimed that those "direct terms" (a physical reference) is necessary to quantify velocity. You are not helping anybody, by making such wrong claims.

15. Dec 6, 2014

### phinds

Well, I disagree. It seems to me that relative motion is best explained at first by talking about concrete objects instead of an abstract frame of reference. I DO see your point, though, that I should not have insisted that another object was absolutely necessary in order for the motion to be meaningful.

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