Velocity of Light vs Train: Why the Difference?

[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 can't 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

 

[Doc Al]

relativistic addition of velocities

The ball would appear to move at a velocity of 180 mph to someone on the ground right?

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.

 

[Doc Al]

I think if you had head lights on a spacecraft that shown forwards as you theoretically approached light speed then the light emitting from your head lights would (appear) to slow down from the observer in the space craft,
If Light emitting from the head lights was traveling 186,282 miles a second and the spacecraft was traveling at 186,281 miles a second then the light emitting from the head lights would appear to be traveling at 1 mile a second to the observer in the space craft.

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

 

[ranger]

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

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

 

[stmoe]

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

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.