Physics Question

B_Man · Post by **B_Man** » 17 Jun 2005 22:04

This was on my final and I had a debate about it with a friend. I don't even know if I answered it correctly so I'm just going to see what the more physic-ly able persons say. The concept is the same but the numbers are made up.

A spaceship traveling 2.5 x 10^8 m/s towards Earth sends out a microwave to it's headquarters 6.5 x 10^8 meters away (on Earth). How long would it take for HQ to receive the signal?

A) 2.6 seconds
B) 2.2 seconds
C) 1.2 seconds

wolfpac444 · Post by **wolfpac444** » 17 Jun 2005 22:38

Do you want to take special relativity into account?

bigdirtyfoot · Post by **bigdirtyfoot** » 18 Jun 2005 07:24

It's multiple choice, so the answer is C right?

God I don't understand this stuff at ALL.

Jeremy · Post by **Jeremy** » 18 Jun 2005 11:33

Very long time since I've done anything like that (3 years) but I'd say the answer is b.

Microwaves are electro magnetic radiation - so they travel at light speed 3 x 10^8 ms^-1. 6.5/3 = 2.2

But it depends if you're talking about the time taken from the perspective of the space ship or the headquaters (or an outside obvserver) because a spaceship travelling at 2.5 x 10^8 ms^-1 would be faced with some noticable time dilation so for the people in the space ship the time taken for the message would be a lot quicker. I guess it's all relative

But as I said it's years since I've done any physics of this nature - I could be wrong.

Jeremy · Post by **Jeremy** » 18 Jun 2005 11:35

sorry for the double post.

The notable thing to remember is that electro magnetic radiation (ie. light) can't travel any faster than 3 x 10^8 ms^-1 so it's not a vector equation.

SandWraith · Post by **SandWraith** » 18 Jun 2005 15:13

Well, as Jeremy said, it would take 2.2 seconds from the headquarters perspective, but it would take 2.2 divided by the relativistic constant from the spaceship's perspective. That constant turns out to be about 1.8 in this case, and so from the ship's perspective it would take 2.2/1.8 = 1.2 seconds for the microwave to reach headquarters. I voted for 2.2 because it seems like what the question is asking for, but I don't think it'd be on the test if it were supposed to be that easy. Still, I never liked questions about relativity for that very reason: many teachers don't really specify the perspective for which you're supposed to find the answer.

ShredOnLawn · Post by **ShredOnLawn** » 18 Jun 2005 23:19

This kind of question is why I dropped out of Physics in High School.

Sorry, Can't help ya...

B_Man · Post by **B_Man** » 19 Jun 2005 07:34

Thanks for the responces (you too Lon

).

The prespective was my argument and the answer that I chose was C. The way I saw it, the source of the EMR was traveling so fast so it would be added to the speed of light for someone spectating the event.

I mean, it's just like if you were standing on a vehicle's rooftop going 100 km/h. If you threw a ball at 10 km/h ahead of you, the velocity of that ball is 110 km/h now. I just applied that concept to the question given.

And yes, it was on our physics final

.

Jeremy · Post by **Jeremy** » 19 Jun 2005 07:42

You're wrong. The speed of light is a specific number - Light never travels faster than 3 x 10^8 ms^-1.

There is the famous question of if you're travelling in a train at the speed of light (or a smidgen less) and you walk from the back of the train to the front - are you travelling faster than the speed of light. The correct answer is no because it's impossible (for a number of reasons which I don't have time to go into - ask your teacher!) - but it's about relativity.

It's like with the time travel topic. Even though you can't travel faster than the speed of light - it's still theoretically possible to travel to a distant star in a time that logically means that you've travelled faster than the speed of light. This is because of the time distortion effect of movement - which is greater the faster you're travelling.

Having given the question some thought - I think the correct answer was to put both b. and c. and explain the two situations. Simply putting answer b or c is too easy. But putting both demonstrates that you understand special reletivity - which presumably is what the question is testing you on. Dividing 6.5 by 3 and 6.5 by (2.5+3) are both very easy equations - it's hard to imagine a question in a physics exam which would be so easy unless more was expect of you.

FlexThis · Post by **FlexThis** » 20 Jun 2005 16:02

I guessed 1.2, but I guess I was thinking wrong as well.

B_Man · Post by **B_Man** » 20 Jun 2005 22:59

Alrighty, thanks for the insight! Who knows, maybe you guy's will save my glutes on the big diploma on Thursday

Wish me luck

SandWraith · Post by **SandWraith** » 21 Jul 2005 17:54

LUCK

Err... how'd it go? What was the 'right' answer? (i.e. what your teacher wanted)

SlashC · Post by **SlashC** » 23 Jul 2005 12:00

The answer they'de be looking for in the Alberta Phys30 program is 2.2 seconds. I don't think that's much help now, but... whatever

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