# A question about special relativity? Car reference vs Earth reference?

1. Jul 23, 2009

### graphicer89

A question about special relativity??? Car reference vs Earth reference??

Hi im prepping for a upcoming exam and im trying to understand some concepts of physics and special relativity....which my exam will be on and i need to understand these problems so that i can know how to do the rest being that i was left in the dark without anything to guide my self by and i looked up information but i just dont understand it...and if you could help me out i would really appreciate it alot.....I gave my 2 cents...which i hope can clear any doubts that i have...thanks again...

Lets say there are 2 pictures of someone flipping a coin and letting it drop in a car moving at a constant speed of 30 km/h. One picture is from the reference frame of the car and the other is from the reference frame of the earth.The path followed by the dropped coin is different in each frame of reference.

A) What force is acting on the dropped coin in the 2 frames of reference?

My guess that Gravity is the force that acts on both cause ..both are still on earth(obvious?)

B) Is the acceleration expererienced by the dropped coin the same in both frames of reference?If not how does it differ?

I think that in both cases it is the same speed because though the references are different they both in up in the same spot...just like a rock...if you throw a rock down it falls and it takes time...throw a rock and it falls but takes the same amount of time....

C) Is the initial speed of the coin the same in both frames of reference? (That is,is the initial speed of the coin compared to the car the same as the initial speed of the coin compared to objects outside the car?) If not how does it differ?

I think the initial speed of both coins are different mostly because of earth's speed ....(thats where im confused ...earth moves 30 km/h right? so wouldnt both initial speeds be the same??)....

D) According to Newton's Law, what path should the dropped coin follow in the frame of the reference of the car?

According to Newtons law ,the path of the coin should move in a straight line .....

E) According to Newtons's Law what path should the dropped coin follow in the frame of reference of the earth?

F) Does the example of the 2 references support or contradict the first postulate of special relativity? Why or Why not?

Yes because according to the first postulate we cant conduct an experiment that tells us where our frame of reference or another frame of reference is at rest or moving at constant velocity...(my gut isnt agreeing with me)

G) If you were inside the car, from the motion of the coin could you tell if you were at rest or moving at a constant speed relative to the ground?

I think i could tell that im moving at a constant speed relative to the ground..(obvious?)

H) If you were standing as an observer outside the car, from the motion of the coin could you tell which was at rest, you or the car?

My guess is that i am at rest being that the coin travels faster as I am observer...

Last edited: Jul 23, 2009
2. Jul 23, 2009

### Hootenanny

Staff Emeritus
Re: A question about special relativity??? Car reference vs Earth reference??

Hello again graphicer89!

According to the rules in the homework forums, you need to show us you work - either an attempt at solving the problems yourself or, at the very least, you should detail your thoughts on the problem.

3. Jul 23, 2009

### Hootenanny

Staff Emeritus
Re: A question about special relativity??? Car reference vs Earth reference??

Thank you for taking the time to detail your thoughts. Let's take a look
Absolutely correct, once the coin has been dropped the only force acting upon it (neglecting drag) is of course it's weight.

Put yourself in the shoes of the two observers. If you are sat in the car, what do you see the coin do? Now, if you are stood watching the car go past, what do you see the coin do?
You are indeed correct.
You don't seem to have answered this question.
I think its best if we leave discussion of this question until you have answered all the previous parts correctly.
Could you? How would your observations differ if the car were at rest?
That wasn't the question was it?

4. Jul 23, 2009

### graphicer89

Re: A question about special relativity??? Car reference vs Earth reference??

Originally Posted by graphicer89
B) Is the acceleration expererienced by the dropped coin the same in both frames of reference?If not how does it differ?

I think that in both cases it is the same speed because though the references are different they both in up in the same spot...just like a rock...if you throw a rock down it falls and it takes time...throw a rock and it falls but takes the same amount of time....

.......So the acceleration on both coins are the same mostly because both end up at the same place? .....Hm can i get a little bit of more insight??

Originally Posted by graphicer89
C) Is the initial speed of the coin the same in both frames of reference? (That is,is the initial speed of the coin compared to the car the same as the initial speed of the coin compared to objects outside the car?) If not how does it differ?

I think the initial speed of both coins are different mostly because of earth's speed ....(thats where im confused ...earth moves 30 km/h right? so wouldnt both initial speeds be the same??)....

Put yourself in the shoes of the two observers. If you are sat in the car, what do you see the coin do? Now, if you are stood watching the car go past, what do you see the coin do?

........If i was inside the car i would see the coin go down and maybe hit the floor of the car at normal speed...(my guess that comes to mind).... Now if i saw the car pass by then i would see the coin take alot more time to come down...in other words it would look slow (my little light bulb just lit up )

E) According to Newtons's Law what path should the dropped coin follow in the frame of reference of the earth?

You don't seem to have answered this question.

........Well it follows a curved path but the path should be a straight path????

G) If you were inside the car, from the motion of the coin could you tell if you were at rest or moving at a constant speed relative to the ground?

I think i could tell that im moving at a constant speed relative to the ground..(obvious?)

Could you? How would your observations differ if the car were at rest?

........Hmm you made me think about the question again....So basically i wouldnt be able to tell that i am moving? ..if i was inside the car and im looking out and see a car moving back wards its because the car im in is moving forward.....

H) If you were standing as an observer outside the car, from the motion of the coin could you tell which was at rest, you or the car?

My guess is that i am at rest being that the coin travels faster as I am observer...

That wasn't the question was it?

.......This one really had to make me think ...hmm but i think its the car...i dont know why but an example....in a high speed jetliner if we flip a coin and catch it just like it was at rest....

I can attach a picture of what i am working with so that you can take a look at it....

5. Jul 23, 2009

### Hootenanny

Staff Emeritus
Re: A question about special relativity??? Car reference vs Earth reference??

Try using Newton's second law
No, you were correct the first time: the coin would hit the floor at time same time in both reference frames - but that doesn't mean that the initial speeds were the same in both reference frames.
It does indeed follow a parabolic path, but why do you think it should follow a straight path. (Once you answer the previous question correctly, you should be able to do this one).

Correct, the only way you could tell you were moving would be to look outside.
What would happen if instead of standing on the earth, you were also in a car. How could you tell which car was moving?

P.S. Could you please try and use quotes in your posts (using the [ quote ] tags), otherwise, this is going to get rather confusing rather quickly.