What is V of the small rocket as observed from inside it?

• Axidecimal
In summary, if a rocket is launched from a larger rocket, the smaller rocket's velocity is always zero.
Axidecimal

Homework Statement

If the total relativistic velocity of small rocket is 0.75c after being launched from a larger rocket traveling at 0.60c, what is the velocity of the smaller rocket as observed from inside it?

Can someone put this in layman's for me please. I solved many relativistic velocity question with no problem then i get thrown stuff like this.

Homework Equations

V= (V1+V2) / {1+(V1V2)/ c^2}

The Attempt at a Solution

.75c -.60c = .15c

Axidecimal said:
what is the velocity of the smaller rocket as observed from inside it?
Zero. Always zero, regardless of how it looks to anyone not at rest in the frame of reference of that rocket.

If you are asking what is the V of the small rocket as observed by the larger rocket, then it's .6c

If you are asking what is the V of the small rocket from the FOR in which the larger rocket is moving at .75c, THEN you need to bring in the equation for relativistic addition and the answer will be some 90%+ of c

EDIT: As orodruin points out below, I reversed the two Vs (the .6 and the .75)

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phinds said:
Zero. Always zero, regardless of how it looks to anyone not at rest in the frame of reference of that rocket.

If you are asking what is the V of the small rocket as observed by the larger rocket, then it's .6c

If you are asking what is the V of the small rocket from the FOR in which the larger rocket is moving at .75c, THEN you need to bring in the equation for relativistic addition and the answer will be some 90%+ of c

Axidecimal said:

phinds said:
Thanks but i think i will google "Physics"

Axidecimal said:
Thanks but i think i will google "Physics"
Don't know if you are being sarcastic or just silly.

VTOTAL= V1 + V2/ (1 + V1 x V2/c2)

0.75 = 0.60 + V / (1 + 0.60V)

0.75 + 0.45V = 0.60 + V

0.15 = 0.55 V

V = 0.27c

phinds said:
Don't know if you are being sarcastic or just silly.
Actually, "physics" is a better search term. It would be true in classical mechanics as well.

phinds said:
If you are asking what is the V of the small rocket as observed by the larger rocket, then it's .6c
No it isn't. The large rocket is moving at 0.6c and the small at 0.75c in the initial frame.

Axidecimal said:
VTOTAL= V1 + V2/ (1 + V1 x V2/c2)

0.75 = 0.60 + V / (1 + 0.60V)

0.75 + 0.45V = 0.60 + V

0.15 = 0.55 V

V = 0.27c

Depending on the frame where you want to compute the velocity of the small rocket, you will get different results (naturally). You need to specify this much clearer.

If it is the speed in the rest frame of the small rocket you want, it is always zero by definition as phinds has pointed out.

If it is the speed in the rest frame of the large rocket you can do what you did, but this does not seem to be what the question is asking.

The question should be a lot clearer but this was not of my exact making. Its lack of clearity is why i posted it here. I always seem to get questions like these on my tests to make sure i cannot receive 100%. Non of my lessons have practice problems as such. I believe you should be trained for any question and if you get it wrong you should be sole to blame. I received the solution. Thanks for the help

Orodruin said:
No it isn't. The large rocket is moving at 0.6c and the small at 0.75c in the initial frame.
ARRRGGGHHH. I need to pay closer attention. Obviously you're right, I got them backwards. Thanks for the correction.

Orodruin said:
Actually, "physics" is a better search term. It would be true in classical mechanics as well.
Well, yeah, but "physics" includes just a whole TON of stuff that has nothing to do with his question. He could spend months studying electricity, optics, thermodynamics, and on and on, and what would that have to do with his question?

Pursuant to your point however, it would have been better if I had pointed him to Galilean relativity rather than Special Relativity as a starting point.

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What is V of the small rocket as observed from inside it?

The velocity of a small rocket as observed from inside it is dependent on several factors such as the speed at which the rocket is launched, the direction in which it is traveling, and the forces acting upon it. Therefore, the velocity of a small rocket as observed from inside it can vary greatly and cannot be determined without specific information about the rocket's launch and trajectory.

How is the velocity of a small rocket measured from inside it?

The velocity of a small rocket can be measured from inside it using specialized instruments such as accelerometers or speedometers. These instruments can measure the acceleration and speed of the rocket and provide an accurate measurement of its velocity at any given moment.

Does the velocity of a small rocket change as it travels through space?

Yes, the velocity of a small rocket can change as it travels through space due to the influence of external forces such as gravitational pull, air resistance, and thrust. These forces can cause fluctuations in the rocket's speed and direction, resulting in changes in its velocity.

Why is the velocity of a small rocket important to know from inside it?

The velocity of a small rocket is important to know from inside it as it can affect the safety and success of the mission. Knowing the velocity can help the crew make necessary adjustments to the rocket's trajectory and ensure that it reaches its intended destination.

Can the velocity of a small rocket be controlled from inside it?

Yes, the velocity of a small rocket can be controlled from inside it through the use of propulsion systems and navigation systems. These systems allow the crew to adjust the rocket's speed and direction to maintain a desired velocity and reach its destination safely.

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