# Mechanics & Relativity

1. Apr 17, 2006

### Pseudo Statistic

I'm having 2 problems.... with two questions...

1) A spaceship travels with a constant speed 0.8c as it passes over Earth; some time after passint Earth, the pilor shoots a laser pulse backward at a speed of 3 x 10^8 meters per second with respect to the spaceship. Determine the speed of the laser pulse as measured by a person on Earth.

What does relativity say about stuff like this involving things travelling at the speed of light? Would it be 0.8 c - 3 x 10 ^8? Or is that incorrect?

2) There's a question with a diagram like this:
http://www.brokendream.net/xh4/diagphys2.jpg
Blocks 1 and 2 of masses m1 and m2 respectively are connected by a light string, as shown above. These blocks are further connected to a block of mass M by another light string that passes over a pulley of negligible mass and friction. Blocks 1 and 2 move with a constant velocity v down the inclined plane, which makes an angle theta with the horizontal. The kinetic frictional force on block 1 is f and that on block 2 is 2f.

Normally I know how to do these questions but I'm having one little problem- how do I analyze the forces on m1 and m2? I mean, since they're moving down the incline at constant velocity, they're in dynamic equilibrium which means the sum of all forces = 0 and in m2's case I'm thinking the forces would be:
m2 g sin theta - 2f - T = 0 ??? And on m1, m1 g sin theta - f = 0 and T - Mg = 0? But does m1 or m2 exert a force on the other or something?
I don't know, I guess I never really understood Newton's third law...

Can someone please clarify? Thanks a lot.

Last edited by a moderator: Apr 22, 2017
2. Apr 17, 2006

### nrqed

What is the fundamental principle of special relativity? That in *any* inertial frame the speed of light is always equal to c, no?

Pat

3. Apr 17, 2006

### nrqed

There are *two* ropes in the problem and the tension in each rope is not (necessarily) the same. Your equation for m2 should be m2 g sin theta - 2f - T_1 +T_2 = 0 and so on.

Last edited by a moderator: Apr 22, 2017
4. Apr 17, 2006

### Pseudo Statistic

So the answer is 3 x 10^8 in this case?