Inaccurate physics questions in exam (1 Viewer)

[Cloesd]

For example, the multiple choice question in 2008, q5.

Space ship is travelling at 1.8 * 10^8, two light pulses .5 seconds apart are pointed at earth. To an observer on earth what is the gap between the pulses.

Now there's three ways to answer this question.

First, the basic randomly plug into formula way, which produces the correct answer. .63 seconds.

Then to the person who was thinking a little more way, he would have got it wrong.

, If you were reading the question with a proper understanding, you would have go which factors in the gap between the pulses. ie; if the gap between the first pulse, and the second pulse is .5 seconds, then IN those .5 seconds the ship has moved 90,000 kilometers, meaning the SECOND pulse needs to travel for a longer time, putting the answer to something near .92 seconds.


And finally, if you're interested in physics, and not just what the syllabus says, and you read ahead into relativistic Doppler effect. well.. heh.. no one would've got it wronger than you did.

 

[boxhunter91]

You do realise it is Time Dilation?
tv=to/root(1-v2/c2)
tv=0.5/root(1-0.6^2)
tv=0.5/root(0.64)
tv=0.5/0.8
tv=0.625

I don't see the problem?

 

[kaz1]

For example, the multiple choice question in 2008, q5.

Space ship is travelling at 1.8 * 10^8, two light pulses .5 seconds apart are pointed at earth. To an observer on earth what is the gap between the pulses.

Now there's three ways to answer this question.

First, the basic randomly plug into formula way, which produces the correct answer. .63 seconds.

Then to the person who was thinking a little more way, he would have got it wrong.

, If you were reading the question with a proper understanding, you would have go which factors in the gap between the pulses. ie; if the gap between the first pulse, and the second pulse is .5 seconds, then IN those .5 seconds the ship has moved 90,000 kilometers, meaning the SECOND pulse needs to travel for a longer time, putting the answer to something near .92 seconds.


And finally, if you're interested in physics, and not just what the syllabus says, and you read ahead into relativistic Doppler effect. well.. heh.. no one would've got it wronger than you did.

wtf?

 

[darkchild69]

What does doppler effect have to do with the observed time betwen pulses?

 

[boxhunter91]

He obviously misinterpreted the question. it's time dilation mate.

 

[Mattww]

I'm not sure if the Doppler effect applies here.

Does the relative velocity between source and observer stay the same because of a constant speed of 1.8*10^8 ?

And then in relation to the extra distance that is travelled, I'm pretty sure the formula takes into it into account..

 

[00iCon]

All of you quting time dilation are imbeciles. Sry, theres no less harsh way to say it.
I like the second [distance covered x speed] one, but did you also take into account length contraction???
But when calculating the speed of light, it the ship shines it out the back and it's travelling away from earth, the light would travel at C - 1.8*10^8 . That's assuming the ship is travelling way from from Earth, it could be moving at any angle to Earth.
WHAT A CRAPPY QUESITON!
Demand to have some marks!

 

[moo92]

I like the second [distance covered x speed] one, but did you also take into account length contraction???
But when calculating the speed of light, it the ship shines it out the back and it's travelling away from earth, the light would travel at C - 1.8*10^8 . That's assuming the ship is travelling way from from Earth, it could be moving at any angle to Earth.

Whilst I agree that your premise is correct - one should have to take into account the distance travelled between light pulses - must also realise that light IS CONSTANT FOR ALL OBSERVERS!!!!
Hence the light would be travelling at 3 x 10^8 m/s no matter where the spaceship is going!!
and you'd have to figure out the dilated length as well.

Basically for HSC purposes you'd just have to assume time dilation because as far as we are concerned there is no other way of working it out it's stupid...but I guess that's why we get the option of taking physics in Uni
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[helper]

Don't use the expression Light is constant for all observers. It is a big no, no and marked wrong. Be explicit, that the speed of light is constant for all observers.

 

[Cloesd]

You do realise it is Time Dilation?
tv=to/root(1-v2/c2)
tv=0.5/root(1-0.6^2)
tv=0.5/root(0.64)
tv=0.5/0.8
tv=0.625

I don't see the problem?

Hmm let me attempt to explain my premise.

We know the speed of light is constant.. and the spacecraft is moving AWAY from earth.

Lets say... to someone on the spaceship, the pulses were sent an hour apart. So.. one at 5:00am and one at 6:00. You can see clearly that IN that ONE hour between the pulses.. the spaceship has travelled astronomical distances (due to its near light speed), such that the SECOND beam of light (although traveling at the same speed as the first) has a MUCH further distance to travel before it reaches the earth.

| | (earth)
6:00 light 5:00 light.


You might now ask? kk, but where's doppler effect come into this. Consider the two pulses of light as two wavefronts. The object thats emitting the wavefronts is moving.

And finally no, regular time dialation does not take into account the doppler effect. If there was only a SINGLE pulse of light, you'd use time dialation. As there are multiple pulses, simple application of the time dialation formula would not model the situation accurately.

Thus we're at a state where the most correct answer to the question isn't even listed.

 

[tareksalman]

I agree totally with you, however the HSC is about knowing what's on your syllabus and a simple matter of knowing the type of questions they ask.
That's exam technique - if it was a written question they would of had it checked and since it is a correct answer you would probably get the marks.

Jason