Basic science question...

Post by **Chupacabra** » Wed Oct 26, 2005 11:42 pm

...with a twist. Obviously very unrealistic, but a good theory question.

A train is moving 3/4ths the velocity of light (c = 300,000 km/sec) relative to a lamp post. Assume that a person is running 3/4ths the velocity of light on top of the train.

What is the velocity of the person relative to the lamp post?

edit: the person is running 3/4ths the velocity of light relative to the train.

Post by **bitWISE** » Wed Oct 26, 2005 11:48 pm

I'm going to take a guess and say its still 3/4 light since you didnt say the person's speed is relative to anything.

Post by **tnf** » Wed Oct 26, 2005 11:49 pm

person is running 3/4(c) relative to the train then?

just want to clarify the parameters first.

Post by **-Replicant-** » Wed Oct 26, 2005 11:49 pm

won't he run off the train?

but seriously, i'd imagine (and my concept of physics is quite rusty, so don't quote me on this) that he's traveling at:

2 * (3/4c)

Post by **tnf** » Wed Oct 26, 2005 11:50 pm

hypothetical question.

Post by **[xeno]Julios** » Thu Oct 27, 2005 12:00 am

my first intuition is that the question is impossibly framed. He'd never be able to run up to 1/4 the speed of light while the train is moving @ 3/4 the speed of light, since his mass would approach infinity as he did so.

So while he may approach 1/4 the speed, he'd never reach or exceed it.

Post by **Chupacabra** » Thu Oct 27, 2005 12:02 am

tnf wrote:person is running 3/4(c) relative to the train then?

just want to clarify the parameters first.

right, sorry about that tnf and bit. i editted the first message.

Post by **werldhed** » Thu Oct 27, 2005 12:03 am

Just to fuck around, he could be moving at 0 km/h if he's running in the opposite direction.

Hi-yah!

Post by **Chupacabra** » Thu Oct 27, 2005 12:04 am

some interesting ideas.

if you can solve the problem, try giving an exact number

Post by **Chupacabra** » Thu Oct 27, 2005 12:05 am

-Replicant- wrote:won't he run off the train?

but seriously, i'd imagine (and my concept of physics is quite rusty, so don't quote me on this) that he's traveling at:

2 * (3/4c)

so you're saying that relative to the lamp post, he'd be going faster than the speed of light?

Post by **shiznit** » Thu Oct 27, 2005 12:08 am

He is running 4/4 of the speed of light, which means he will fall off and die.

Post by **[xeno]Julios** » Thu Oct 27, 2005 12:16 am

chup - see my post - the question doesn't seem to make sense...

it's like asking, if someone is running @ 3/4 c, and then increases her speed by an additional 3/4 c, how fast is she running?

Guest · Post by **Guest** » Thu Oct 27, 2005 12:26 am

Julios is right, you can't move faster than the speed of light.

Post by **Chupacabra** » Thu Oct 27, 2005 12:28 am

ignore the acceleration bit (that he would have to increase his velocity from (1/4)c to (3/4)c. imagine that he's running (3/4)c relative to the lamppost when he is on the ground and he is placed ontop of the train so he is running (3/4)c relative to the train.

also its different than asking if the person increases their speed. when theres a train and a person running, vector addition theory comes into play, so you have to consider that.

Guest · Post by **Guest** » Thu Oct 27, 2005 12:31 am

I have a science question for you. You have a ring, a sphere, and a coin rolling down a smooth incline. If they start at the same time, which one would be the first one to reach the bottom?

Post by **tnf** » Thu Oct 27, 2005 12:32 am

yea, just look at it this way -

you are on a train travelling at 60 mph relative to a stationary lamp post. Now you begin running 20 mph relative to the train. You will be moving 80 relative to the lamp post.

The situation you describe couldn't happen.

Post by **tnf** » Thu Oct 27, 2005 12:32 am

ToxicBug wrote:I have a science question for you. You have a ring, a sphere, and a coin rolling down a smooth incline. If they start at the same time, which one would be the first one to reach the bottom?

ignoring friction, all at the same time.

Guest · Post by **Guest** » Thu Oct 27, 2005 12:34 am

tnf wrote:
ToxicBug wrote:I have a science question for you. You have a ring, a sphere, and a coin rolling down a smooth incline. If they start at the same time, which one would be the first one to reach the bottom?
ignoring friction, all at the same time.

Ignoring friction, but considering that they are rolling without slipping. Therefore there is friction that makes them roll, they are not sliding down the hill.

Oh yeah and they're all of the same radius and mass.

Post by **R00k** » Thu Oct 27, 2005 12:35 am

Does the speed of light change things somehow then?

Because a person running 60mph relative to the train he's on, if the train is running 60mph relative to the lamppost, would be running 120mph in relation to the lamppost, would he not?

Aside from Jules' point that approaching the speed of light would infinitely distort his mass.

Post by **[xeno]Julios** » Thu Oct 27, 2005 12:38 am

Chupacabra wrote: imagine that he's running (3/4)c relative to the lamppost when he is on the ground and he is placed ontop of the train so he is running (3/4)c relative to the train.

but don't you see - if he's moving 3/4 c relative to train, and train is moving 3/4 c, relative to lampost, then he's moving 6/4 c relative to lampost, which is an impossibility.

this IS vector addition. All you do is add the speeds since they're moving in same direction.

Guest · Post by **Guest** » Thu Oct 27, 2005 12:39 am

I don't get it, is the lampost going somewhere?

Post by **tnf** » Thu Oct 27, 2005 12:40 am

ToxicBug wrote:
tnf wrote:
ToxicBug wrote:I have a science question for you. You have a ring, a sphere, and a coin rolling down a smooth incline. If they start at the same time, which one would be the first one to reach the bottom?
ignoring friction, all at the same time.
Ignoring friction, but considering that they are rolling without slipping. Therefore there is friction that makes them roll, they are not sliding down the hill.

ignoring friction they still all get to the bottom at the same time.
if there is friction we can't answer the question with additional info.

Guest · Post by **Guest** » Thu Oct 27, 2005 12:40 am

tnf wrote:
ToxicBug wrote:
tnf wrote: ignoring friction, all at the same time.
Ignoring friction, but considering that they are rolling without slipping. Therefore there is friction that makes them roll, they are not sliding down the hill.

ignoring friction they still all get to the bottom at the same time.
if there is friction we can't answer the question with additional info.

Wrong. They will slide to the bottom at the same time, but not roll.

Post by **tnf** » Thu Oct 27, 2005 12:42 am

ToxicBug wrote:
tnf wrote:
ToxicBug wrote: Ignoring friction, but considering that they are rolling without slipping. Therefore there is friction that makes them roll, they are not sliding down the hill.

ignoring friction they still all get to the bottom at the same time.
if there is friction we can't answer the question with additional info.
Wrong. They will slide to the bottom at the same time, but not roll.

I didn't even see roll in the oringal question..i just saw get to the bottom. and ignoring friction, they all GET to the bottom at the same time.
that's why I said "GET".

but yes...no friction no roll.

Guest · Post by **Guest** » Thu Oct 27, 2005 12:43 am

tnf wrote:
ToxicBug wrote:
tnf wrote:
ignoring friction they still all get to the bottom at the same time.
if there is friction we can't answer the question with additional info.
Wrong. They will slide to the bottom at the same time, but not roll.
I didn't even see roll in the oringal question..i just saw get to the bottom. and ignoring friction, they all GET to the bottom at the same time.
that's why I said "GET".

Yes, but later I replied saying that they are rolling without slipping!