# Two tennis balls

1. Nov 26, 2014

### hobbes1218

I joined the forum to hope for an answer to a question that has been bugging me for a while. If I hit a tennis ball in a vacuum with no other forces acting upon it and another chap hits a ball from the other end of the court - at the moment the balls cross if you could freeze time what properties would the balls have that could be examined to determine their continued direction? All I can find on the net is inertia - it's just a law of the universe? The balls must have different properties?

Thanks,
Chris

2. Nov 26, 2014

### Doug Huffman

Momentum is conserved. There will be some gravitational attraction unless that's your "no other forces acting."

3. Nov 26, 2014

### Danger

Do you mean that the balls collide, or just fly past each other?

4. Nov 26, 2014

### hobbes1218

As a thought experiment with no forces acting upon the balls. The balls don't collide. If you could examine them - what is different about them - that one ball would continue in a certain direction while the other continues on the opposite direction? You can't just say it's inertia? The balls have to have a different property?

5. Nov 26, 2014

### Danger

There would be an immeasurably small gravitational attraction between them. (Or is that one of the "no forces" that you include in the scenario?)
Really, I don't see how the question can be answered as proposed. If there are no forces, there is no reality and therefore no similarities or differences between the two balls.

6. Nov 26, 2014

### Doug Huffman

Yup, different, opposite, initial velocity component of their momentum.

7. Nov 27, 2014

### A.T.

The current direction?

8. Nov 27, 2014

### nasu

I think that he means, if you take a snapshot of the two balls when they are close to each other, can you tell which one will go left and which one will go right? But not necessarily a photographic snapshot, just examining the two balls somehow outside of time, in zero time interval. (frozen time)

It's kind of interesting to try to imagine this, assuming you can make observations "outside of time".:)

9. Nov 27, 2014

### jh0

Tennis balls are made from rubber, so I think you could observe that some of the kinetic energy would have been converted to elastic energy deforming the balls. You could definitely observe the deformation. In order to determine in which direction will go the balls after the collision you would need to know their speed at the moment of the snapshot. It will be zero if the two balls collide exactly center to center with opposite speeds (assuming equal masses) and the snapshot is taken at the exact moment when the deformation of the balls is maximum.

10. Nov 27, 2014

### A.T.

11. Nov 27, 2014

### jh0

Upps, sorry!
Of course each ball has a velocity or momentum (magnitude and direction) which is not observable in a single moment in time (as in a snapshot). I can't see a way to measure a time-related property like velocity, outside of time (like looking at a perfectly still photograph).

12. Nov 27, 2014

### Doug Huffman

LOL As I am reading - slowly - The principle of relative locality (arXiv:1101.0931v2) with spacetime emergent from momenta-energy.

13. Nov 27, 2014

### hobbes1218

Thanks for the answers folks but I can't say I am any the wiser. Momentum may be conserved but what is the difference between the two balls as they pass that dictates one will continue to the left and the other to the right. It doesn't matter what they are made of - the same could be said for a smaller object that has no deformation. The ball that carries on moving to the left must have some quality that is different to that of the ball that carries on to the right. It's a property that can't be determined in an instant? Only by observing how an object moves in relation to other objects?

14. Nov 27, 2014

### nasu

I would say that previous history determines what happens next.
Most processes are like this, are'n they?

15. Nov 27, 2014

### hobbes1218

I suppose it doesn't matter if there are two balls. I just don't understand. Inertia is the resistance of any physical object to any change in its state of motion, including changes to its speed and direction. It is a power of resisting by which every body, as much as in it lies, endeavours to preserve its present state, whether it be of rest or of moving uniformly forward in a straight line (Wiki). What I don't understand is how it preserves its present state? One could argue that the object is just observing the laws of physics. Just as light travels at a set speed. But going back to two balls - what is the measurable difference they have that would determine their continued direction? I know little about physics - just wondering if there is an explanation for this that a layman could comprehend?

16. Nov 28, 2014

### A.T.

17. Nov 28, 2014

### hobbes1218

I still don't understand what property the ball traveling to the right has that is different to the one traveling to the left.
Or maybe there is no way this can be determined at a set moment.
Either there is no easy way this can be explained to a layman or it is unknown?
Please ignore this if I'm getting repetitive - maybe this isn't a forum for a layman :)

I'll take my coat.