• Support PF! Buy your school textbooks, materials and every day products Here!

Spaceship problem

dear friends, days ago on a physics forum I saw this interesting question regarding fixed power and speed of a spaceship. After thinking for a while I came up with my answer but I am not really sure its correct.So please tell me what you think the answer is.

The question is: imagine there are 2 identical spaceship(with 2 identical engines),1 in stationary state,other travels at constant speed in a straight direction.Now 2 engines starts, how do people in different spaceship feel about the force(or the acceleration)?

some people believe that the one with higher initial speed(lets call it spaceship B) "feels" more force acting on it, becoz of the formula p=fv(since p is fixed, f decreases while v increases).

Somehow I disagree with it.(tell me if i am wrong:))
here is my reasoning:
since the way spaceship's engine works is to utilise the reaction force of its emitted gas(i don't know how to describe it but i assume you understand:)), so the velocity in the formula has something to do with the gas.Since both spaceships A and B was in balanced state, after the engines starts(assuming they start at the same time), A and B would travel at the same accelerating speed at any given time(take their emitted gas as reference frame).since p=fv, f is the same.
so my conclusion is people in both spaceship would feel the same force when the engines start at the same time. If i am wrong about this, please tell me why.
 

Nathanael

Homework Helper
1,650
238
The "one with the higher initial speed" thinks that the other one is the one with the initial speed. Each spaceship thinks that they are the one that is stationary. And so each spaceship uses "v=0" in their own equation.

so my conclusion is people in both spaceship would feel the same force when the engines start at the same time. If i am wrong about this, please tell me why.
Yes, I believe this conclusion is correct. (I didn't completely follow your reasoning, though.)
 

HallsofIvy

Science Advisor
Homework Helper
41,728
881
"Force equals mass times acceleration". With the same acceleration, the same force will be felt. Initial speed is irrelevant. That goes back to "Gallilean relativity".
 

Related Threads for: Spaceship problem

  • Last Post
Replies
1
Views
3K
  • Last Post
Replies
15
Views
2K
  • Last Post
Replies
4
Views
6K
  • Last Post
Replies
5
Views
1K
  • Last Post
Replies
6
Views
2K
  • Last Post
Replies
1
Views
554
Replies
4
Views
7K
  • Last Post
Replies
9
Views
505
Top