# Relative motion

1. Nov 30, 2007

### olga11

1. The problem statement, all variables and given/known data

An observer on Earth sees two spaceships moving in opposite directions and finally they collide. At t=0 the observer on Earth says that the spaceship 1 which moves to the right with Ua=0.8c is at the point A and the spaceship 2 which moves to the left with Ub=0.6c is at the point B. The distance AB=L=4,2.10^8 m.
When do the two spaceships collide to the earth frame of reference?
What is the velocity of the spaceship 2 to the frame reference of the spaceship 1?
What is the velocity of the spaceship 1 to the frame reference of the spaceship 2?
When does the collision happen to the frame reference of the spaceship 1 and to the frame reference of the spaceship 2?

2. Relevant equations

3. The attempt at a solution

Let D be the point of the collision and AD=x, so DB=L-x
The velocity is constant.
spaceship 1: DB=L-x=0,6c.t
L=(0,8c+0,6c).t
t=4,2.10^8/(1,4.3.10^8)
t=1sec
Is it ok so far?

A hint to go on, please?

2. Dec 1, 2007

### olga11

The velocity of the spaceship 2 to the frame reference of the spaceship 1 is Uba=Ua+Ub=1.4c
The velocity of the spaceship 1 to the frame reference of the spaceship 2 is also Uab=Ua+Ub=1.4c

When does the collision happen to the frame reference of the spaceship 1 and to the frame reference of the spaceship 2?

t=AB/Uab=4,2.10^8/(1,4.3.10^8)=1 sec

Something is wrong. It cannot be the same.

3. Dec 1, 2007

### fotisz

You don't consider the fact that the speeds are comparable to the speed of light.

4. Dec 3, 2007

### olga11

1. The problem statement, all variables and given/known data

An observer on Earth sees two spaceships moving in opposite directions and finally they collide. At t=0 the observer on Earth says that the spaceship 1 which moves to the right with Ua=0.8c is at the point A and the spaceship 2 which moves to the left with Ub=0.6c is at the point B. The distance AB=L=4,2.10^8 m.
When do the two spaceships collide to the earth frame of reference?
What is the velocity of the spaceship 2 to the frame reference of the spaceship 1?
What is the velocity of the spaceship 1 to the frame reference of the spaceship 2?
When does the collision happen to the frame reference of the spaceship 1 and to the frame reference of the spaceship 2?

2. Relevant equations

3. The attempt at a solution

Let D be the point of the collision and AD=x, so DB=L-x
The velocity is constant.
spaceship 1: DB=L-x=0,6c.t
L=(0,8c+0,6c).t
t=4,2.10^8/(1,4.3.10^8)
t=1sec
Is it ok so far?