Here is a Q that I can't wrap my brain around.

1. If you are on top of a train, running from back to front, does that mean that you're going faster than the train?

2. If you were to jump off the train after the initial run, will your distance be farther than if you jumped without running?

3. If you dropped a rock from the train to hit an oncoming signboard. will the impact be the same as if you were to throw that same rock at that sign board (while on that train)?

Explanation and links appreciated.

This post has been edited by TheDoer: Sep 16 2010, 10:12 AM

1. Yes. You have a higher velocity compared to the train. So if the train is travelling at 60 km/h, you will be travelling at a speed higher than 60 km/h.

2. Yes. Since you are travelling at a higher velocity if you're running, you would cover a longer distance while you're airborne before reaching the ground

3. Yes. Assuming no wind resistance, the rock would hit the signboard at 60 km/h when dropped from the train. It would produce the same force if you were to throw the rock at the signboard at 60 km/h.

Hope this helps..,

Sorry, edit, Q3. Throwing the rock, while on the train, which means that you're already travelling at 60km/h in addition there's the force from you throwing the rock.

Thanks for the reply. Keep them coming.

Q4. Can you think of an easy & safe experiment we can do to prove this?

A suggestion: post this somewhere else where more physics people around.

1. question 1 is ill defined. Notice velocity (speed of train) is always relative to a frame of references (ie where you make the measurement). For example, I can measure the speed of train from ground (60km/h, for example) or I can measure speed of train while I'm standing on the train itself (which makes 0km/h measurement). In the later case, I'll measure the ground move with speed -60km/h.

assume ground as reference, yes.

2. Assuming you measure relatively to the ground, yes. Lets say I stand on ground, looking at train running on 60km/h and YOU run on the train at speed 10km/h (you, relative to train), I'll see you moving away from me by 70km/h. This basically mean you move faster! *notice I assumed gallelian relativity, not special relativity*

3. Err. i don't understand. XP

im not really a physics guy,
but for the 1st question is this the right answer?

TS asked if a person is running from back to front, so does this mean the person is faster than the train?
what if the person is slowly walking?
he's still moving faster than the train as well?

im confused

the military did this.. known as "ramjet"

the technology is used for missle propulsion in Jet fighter...

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Added on September 16, 2010, 3:00 pmI give you a key word, you google la.. the cyber teacher are better.........

"doppler effects"

This post has been edited by Awakened_Angel: Sep 16 2010, 03:00 PM

It depends on the observer.

Under the assumption that there is no resistance and we're just talking purely velocity here.

1) From your POV, the train is going at 0 kmph. But from the POV of a person standing on the ground, your speed is Train + Your running velocity.

2) Under the assumption there's no friction and no resistance, you would go faster than the train. Under the assumption you don't lose velocity and that the initial velocity is higher than not jumping, yes, your distance would be further. Plus it would keep increasing.

3) No, it wouldn't be the same. If you dropped the rock the velocity relative to the train would 0, velocity relative to the ground would be the train's speed. If you threw it, the velocity relative to the ground would be speed of train + speed of throw.

everything in this universe in relative.
so is velocity.
so they stack.
and please refer carefully to frame of reference.
get this idea clearly in ur head while working on relative velocity.
however, the speed of light is absolute.
it will no change no matter what's the relative velocity.

This post has been edited by KeNGZ: Sep 16 2010, 10:28 PM

Read this, new kid in town.

http://en.wikipedia.org/wiki/Variable_speed_of_light

holy shit?
wow OMG
read it through already.
I've read about the different fine structure constant observed lately.
which could mean law of physics is not necessarily the same at every point in the universe.
and now I'd just learnt speed of light is not absolute?

so Einstein's special relativity postulates on light still hold true?
and does our different observers in different inertial frames of reference record different speed of light?

looks like things I've learnt are quite out-of-date.

thank you very much by the way for letting me know my mistake.

and this VSL,
where and when do we get to learn bout it?
or it is beyond the university syllabus?

Here's my question..

If i jumped inside a train... will i land on the exact point i stand or i will be behind by a little bit? ( assume that the train travel very very very very fast)

Exact point.

explanation please? When we leave the surface, arent we will decelerate?

Why would you? Under the assumption there is no air resistance and the velocity of the train is constant (a=0), there's no deceleration.

If you are moving through a perfect vacuum at 120 kmph with no other forces acting on you, you will forever travel at that speed.

assuming the train is running at constant speed....

exact point

Doppler effect is about the shift in a wavelength because of the change in wave velocity.

The answers here are all using newtonian physics. We are not using relativity here. And you DO NOT want to. Relativity will twist your mind into a knot.

1. pick two known points on the track beside the train. Say the start of a tunnel, bridge, etc. Two experimenters with synhronized watches. One sites in the 1st car, the second sits in the second car. Note the time the first point appears at your windows. The second person then gets up and walks to the front of the train. Note the time the 2nd point appears at your windows. Measure the distance between the two points. Work out the speed.

2. Don't jump. Throw a rock out the window. It might be easier to do this on a car. Drop a rock from your left hand, throw the one on the right. Release at the same time. Measure with a tape.

3. Do the same experiment as (2). Mark out distances on the road. Have some one record it with a camcorder. Examining the video frame by frame. You should be able to work out the speed.

Have i just done your homework for you?

This post has been edited by dkk: Sep 17 2010, 08:46 AM

I mean in real life though... so there is air resistance and its not vacuum ....

That mean if i have a cannon inside the train ( without roof), i shoot a ball up.. will the ball land at exact point o.0?

Ah... thanks.

haha... no it's not my homework. I'm seriously pondering on this. I'm hesistant to throw rocks from a moving vehicle I don't think that's safe.

It will land back on the canon itself. At point 0.0. You can test this yourself with a little toy canon that shoots the ball only 2m up.

Or even simpler. Hold a ball over a spot on the floor of the train. Let it go. It will fall straight down and hit the spot.

Air resistance does not enter the equation because the train is sealed. Close all the windows and doors before doing your experiment.