Relativity QUESTION

[homedog]

Hey, i'm stuck on this relativity question. Anyone mind helping???

As you travel away from Mars at 4.5 x 10^6 m/s you notice another spaceship moving towards you at 5 x 10^5 m/s. Is the other craft heading toward or away from mars and at what speed???

I keep getting towards Mars at 5 x 10^6 m/s but the correct answer says "away from Mars at 4 x 10^6m/s"

[Lasercookie]

I initially got the same answer as you, but looking into further, the given answer is correct.

Take Towards Mars as a positive direction
Take Away from Mars as a negative direction
is you, is the other ship.

It's because the given velocity was measured by A, not B.

Initial values:












Therefore is moving away from Mars at

[Shark 774]

I initially got the same answer as you, but looking into further, the given answer is correct.

Take Towards Mars as a positive direction
Take Away from Mars as a negative direction
is you, is the other ship.

It's because the given velocity was measured by A, not B.

Initial values:












Therefore is moving away from Mars at

Wouldn't it involve using this kind of stuff: http://en.wikipedia.org/wiki/Velocity-addition_formula ??

Which is not on the course.

[homedog]

I initially got the same answer as you, but looking into further, the given answer is correct.

Take Towards Mars as a positive direction
Take Away from Mars as a negative direction
is you, is the other ship.

It's because the given velocity was measured by A, not B.

Initial values:












Therefore is moving away from Mars at

Thanks a lot, that makes sense. Thankyou

[Lasercookie]

Wouldn't it involve using this kind of stuff: http://en.wikipedia.org/wiki/Velocity-addition_formula ??

Which is not on the course.

I can see why that stuff isn't on the course...

I haven't seen any of these types of questions on any trial exams, but they are in the textbooks.
The textbook teaches the   method as well.

[Shark 774]

Wouldn't it involve using this kind of stuff: http://en.wikipedia.org/wiki/Velocity-addition_formula ??

Which is not on the course.

I can see why that stuff isn't on the course...

I haven't seen any of these types of questions on any trial exams, but they are in the textbooks.
The textbook teaches the  method as well.

Ok, well my text book didn't mention it at all, I've never seen it in a practice/real exam and it's not on the study design, so I assume we don't have to know it?

[Lasercookie]

Actually that is that same thing. It's the "Galilean addition of velocities" bit:

http://en.wikipedia.org/wiki/Velocity-addition_formula#Galilean_addition_of_velocities


It's in the Nelson textbook. It was kind of implied in the Heinemann book.

But yeah, pretty sure we don't have to know it.

Where did you find this question, "homedog"?

[Shark 774]

Actually that is that same thing. It's the "Galilean addition of velocities" bit:

http://en.wikipedia.org/wiki/Velocity-addition_formula#Galilean_addition_of_velocities


It's in the Nelson textbook. It was kind of implied in the Heinemann book.

But yeah, pretty sure we don't have to know it.

Where did you find this question, "homedog"?

Yeah that doesn't work for relativistic speeds.

[xZero]

Actually that is that same thing. It's the "Galilean addition of velocities" bit:

http://en.wikipedia.org/wiki/Velocity-addition_formula#Galilean_addition_of_velocities


It's in the Nelson textbook. It was kind of implied in the Heinemann book.

But yeah, pretty sure we don't have to know it.

Where did you find this question, "homedog"?

Yeah that doesn't work for relativistic speeds.

it works as long as the speed is not something like 0.5c, its like newtonian physics, it works for slow speed but once it shoots up to something like 0.1c we need to use relativity to work it out

[tony3272]

I initially got the same answer as you, but looking into further, the given answer is correct.

Take Towards Mars as a positive direction
Take Away from Mars as a negative direction
is you, is the other ship.

It's because the given velocity was measured by A, not B.

Initial values:












Therefore is moving away from Mars at

Wouldn't it involve using this kind of stuff: http://en.wikipedia.org/wiki/Velocity-addition_formula ??

Which is not on the course.

Although the numbers look big, the speed is only 0.015c, so relativistic effects don't apply.

[Shark 774]

I initially got the same answer as you, but looking into further, the given answer is correct.

Take Towards Mars as a positive direction
Take Away from Mars as a negative direction
is you, is the other ship.

It's because the given velocity was measured by A, not B.

Initial values:












Therefore is moving away from Mars at

Wouldn't it involve using this kind of stuff: http://en.wikipedia.org/wiki/Velocity-addition_formula ??

Which is not on the course.

Although the numbers look big, the speed is only 0.015c, so relativistic effects don't apply.

A right, I didn't bother thinking about that. But anyway, they won't ask those kind of questions anyway because the topic is special relativity and that's just basic newtonian mechanics.