# Velocity + train

1. Oct 11, 2005

### ranger

Lets say that I'm on a trian moving a velocity of 100 mph, and I release a ball with a velocity of 80 mph in the same direction the train is moving. The ball would appear to move at a velocity of 180 mph to someone on the ground right? Well what would happen if I shoot a beam of light. The light beam cant move at a speed of C + 100 mph, because nothing can go faster than C. To someone on the ground, the light beam would still move a constant speed C.
Why does this happen?

--thanks

2. Oct 12, 2005

### Staff: Mentor

That's just an approximation good at low speeds. The velocities actually add like this:
$$v = \frac{v_1 + v_2}{1 + v_1 v_2/c^2}$$
To understand better why this is so, read up on special relativity.

3. Oct 12, 2005

### Staff: Mentor

Not true at all! The light would travel at the same speed (c) whether observed from the space craft or from the Earth.

4. Oct 12, 2005

### ranger

Yea, that it because c is a constant. Its value never changes.

5. Oct 12, 2005

### stmoe

in actuality the value does change

.. but it takes a few billion years.

I won't say that it doesn't change because there has been 'proof' somehow that it does.. and the proof is over my head so I can't really dispell it.. but considering I'm not going to be here in a few billion years to see that light did in fact change by a decimal of a meter per second... so I think its safe to consider it a constant .. even if skeptics will badger it to the bitter end.