Physics riddle (relocate if necessary)

[atro]

Last night my friends and I had a discussion about a science riddle that one of them had brought up. The question triggered some debate and the four of us were split in half with our answers. The question/riddle is:

If you're standing on top of a train traveling at 50mph and you throw a baseball in the direction that you're moving which leaves your hand at 50mph, what will happen to the baseball?

This experiment would be done under controlled conditions i.e. no wind or anything other than the atmosphere acting as an opposite force.

 

[rcgldr]

The problem states the ball leave your hand at 50mph in the direction you're (and the train) is moving which is also 50mph, so relative to the ground, the ball initially is moving at 100mph.

If you ignore the train's effect on the air, then the situation is essentially identical to a ball thrown at 100mph by another person on a stationary platform at the same height of the train. Otherwise, you'd have to take the train's effect on the air into account.

 

[Borek]

which leaves your hand at 50mph

Could be my English fails me, but I have no idea what you mean. 50 mph relative to what - ground or train?

 

[russ_watters]

"Leaves your hand at 50 mph" means 50 mph relative to your hand.

 

[DaveC426913]

...the ball initially is moving at 100mph.

If you ignore the train's effect on the air, then the situation is essentially identical to a ball thrown at 100mph by another person on a stationary platform at the same height of the train. Otherwise, you'd have to take the train's effect on the air into account.

I think you're supposed to ignore the train's effect on the air, yes.

So, as you say, the pitcher is standing on the platform, throwing the ball at 100mph.
The observer is looking out the window and tracks the path of the ball in his FoR.

I can't be positive, but it does seem like the oberver will see the ball follow a path exactly as if its initial velocity were 50mph.

 

[atro]

lets put it this way: If you were to use a ball launcher programed to fire at 50mph for the experiment instead of a human.

yes, we would be ignoring the train's effect on the air but not the air's effect on train or the thrown ball.

I believe that if you were to trow the ball inside the train that yes, someone standing outside on the ground would clock the ball at 100mph.
On top of the train, however, the ball is already traveling at 50mph relative to the ground under the power of the train before the ball is thrown
if the ball is thrown at 50mph relative to the ground, I believe it would maintain it's speed of 50mph until it loses it's momentum almost immediately as it would be under it's own power.

 

[Doc Al]

So, as you say, the pitcher is standing on the platform, throwing the ball at 100mph.

The pitcher throws the ball at 50 mph with respect to the train.

The observer is looking out the window and tracks the path of the ball in his FoR.

I can't be positive, but it does seem like the oberver will see the ball follow a path exactly as if its initial velocity were 50mph.

From the viewpoint of a train observer, the initial speed of the ball is 50mph but there's a 50mph head wind to contend with.

From the viewpoint of a ground observer, the initial speed of the ball is 100 mph. (Exactly as Jeff Reid described.)

 

[Borek]

"Leaves your hand at 50 mph" means 50 mph relative to your hand.

Arghh, ball was a subject here, not hand. Sometimes I get lost

 

[Doc Al]

I believe that if you were to trow the ball inside the train that yes, someone standing outside on the ground would clock the ball at 100mph.

The speed of the ball is given as 50 mph with respect to the train. Thus the initial speed of the ball is 100 mph with respect to the ground, regardless of whether the ball is thrown inside or outside of the train.

On top of the train, however, the ball is already traveling at 50mph relative to the ground under the power of the train before the ball is thrown

True.

if the ball is thrown at 50mph relative to the ground, I believe it would maintain it's speed of 50mph until it loses it's momentum almost immediately as it would be under it's own power.

If the ball were thrown at 50 mph with respect to the ground that would mean that the pitcher just dropped it without throwing it.

 

[DaveC426913]

From the viewpoint of a train observer, the initial speed of the ball is 50mph but there's a 50mph head wind to contend with.

Perfect. That's what I was missing.

So, the end result is that the observer on the train sees a ball thrown at 50mph wrt to him, into a 50mph headwind.

This will skew the inverted parabolic trajectory in the -x direction.

That still leaves the question as to what exactly the ball does. Does it start to reverse its direction before it hits ground?

 

[Redbelly98]

Yes, the OP did say the atmosphere acts as an opposing force. It's a question of does the ball remain in the air long enough for you to see it reverse direction.

 

[schroder]

The headwind should not be a big factor as the ball will experience the same air resistance as any baseball traveling at 100 mph. Pitchers do throw baseballs at close to that velocity and air resistance does not seem to slow them appreciably. What should happen is the ball will follow a parabolic path and bounce on the roof of the train a few meters away from the pitcher without reversing direction. The exact distance it travels horizontally will depend on the height it is released and the angle of trajectory. Say it is released at a height of 2 meters (above the roof), then the gravitational acceleration will take about 0.6 seconds to pull it down to the roof. During that time, traveling at 50 mph ( 22 m/s) it will travel about 13 meters. The train will travel half that distance, so the pitcher should see the ball bounce on the roof about six meters in front of him.

Edit: It is traveling at 100 mph (44 m/s) so double the distances I gave. In 0.6 seconds it will go about 26 meters while the train goes 13 meters, so it will bounce on the roof about 13 meters in front of the pitcher, I think!

 

[Harrybarlow]

another riddle is if you were in a car traveling at the speed of light and your turned the headlights on, what would happen?

relativity states the speed of light is the same relative to all observers right?

so the light can't go at 2c because it can't travel faster than itself, so would you see the light rays beside you or what?

 

[Grogerian]

another riddle is if you were in a car traveling at the speed of light and your turned the headlights on, what would happen?

relativity states the speed of light is the same relative to all observers right?

so the light can't go at 2c because it can't travel faster than itself, so would you see the light rays beside you or what?

If your'e moving at the speed of light, what good are the headlight's going to do you? your pretty much a goner, regardless, if you turn the headlights on while driving at the speed of light, the headlights will turn on but you won't see them as you will be moving the same speed as them :)

 

[Harrybarlow]

If your'e moving at the speed of light, what good are the headlight's going to do you? your pretty much a goner, regardless, if you turn the headlights on while driving at the speed of light, the headlights will turn on but you won't see them as you will be moving the same speed as them :)

ah but relative to an observer, what would you see?
assuming you could see it before its gone from sight :P

 

[Grogerian]

I assume that if you are traveling at the speed of light you yourself will become light and if not, i am sure you will combust through friction, etc. ^.^, since an observer can not see a beam *(or track a beam of light)* then i am assuming that no matter what happens they will see nothing, but since there will be no proof of this for a long time (if there ever will be) i don't think we need to worry about it :D

of course i skipped your question sort of, i assume that light is a particle/wave well its not like light is being sent out at this speed if you have an emitter and you throw it in the air, i don't think the waves nor the particles will change speed, showing that light is unaffected by the gravity of Earth I'm assuming it is MOSTLY waves so, the waves would be moving the same speed as the car? it's not fun to think about :( just gives me a brain-ache

 

[Harrybarlow]

I assume that if you are traveling at the speed of light you yourself will become light and if not, i am sure you will combust through friction, etc. ^.^, since an observer can not see a beam *(or track a beam of light)* then i am assuming that no matter what happens they will see nothing, but since there will be no proof of this for a long time (if there ever will be) i don't think we need to worry about it :D

of course i skipped your question sort of, i assume that light is a particle/wave well its not like light is being sent out at this speed if you have an emitter and you throw it in the air, i don't think the waves nor the particles will change speed, showing that light is unaffected by the gravity of Earth I'm assuming it is MOSTLY waves so, the waves would be moving the same speed as the car? it's not fun to think about :( just gives me a brain-ache

ouch i feel the brain ache aswell, i still don't really know what waves are, since I've not gotten onto them at school yet :(

you still answered the question with some reasoning and logic (Y)
rekon you could take a look at my thread, its about two below this one, cheers

 

[rcgldr]

A bunch of photons in a beam of light in space would all be traveling at the same speed, the speed of light, and none of them would "see" each other.

 

[Doc Al]

another riddle is if you were in a car traveling at the speed of light and your turned the headlights on, what would happen?

A car--or any massive body--cannot attain the speed of light. But you can make the same point with the car moving at 0.99c.

relativity states the speed of light is the same relative to all observers right?

Right.

so the light can't go at 2c because it can't travel faster than itself, so would you see the light rays beside you or what?

A person in the car would see the headlights working normally. A person on the road will see the car moving at 0.99c and the light moving at c. Nothing moves at 2c (or 1.99c).

 

[Grogerian]

Nothing moves at 2c (or 1.99c).

That we know of =O, what about that theory of particles moving at times multiple of that in c (wow, that is bad grammar, but I'm positive you know what I'm talking about) I don't know what that theory is lol, but it does exist doesn't it? i didn't dream it right?

 

[Integral]

another riddle is if you were in a car traveling at the speed of light and your turned the headlights on, what would happen?

relativity states the speed of light is the same relative to all observers right?

so the light can't go at 2c because it can't travel faster than itself, so would you see the light rays beside you or what?

Since this is a fantasy question you can make up your answer.

In the real world you cannot travel at the speed of light. At any speed that the car can travel at you would see your headlights out in front of you just as you always do.

 

[Doc Al]

That we know of =O, what about that theory of particles moving at times multiple of that in c (wow, that is bad grammar, but I'm positive you know what I'm talking about) I don't know what that theory is lol, but it does exist doesn't it? i didn't dream it right?

You are speaking of tachyons, very hypothetical particles for which there is no evidence. Not relevant to this thread.

 

[Harrybarlow]

Doc Al, i was talking theoretically, but since your going to be like that

whos going to stand ontop of a train traveling at 50mph anyway, and then pitch a ball at exactly the same speed as the train, not to mention the headwind and everything else that is wrong with the question

so let's say, theoretically, what would happen, relative to an observer, if the headlights were switched on?

 

[Doc Al]

Doc Al, i was talking theoretically, but since your going to be like that

Theoretical or not, you were talking about something that had nothing whatsoever to do with the problem stated in this thread. As I stated already nothing in this problem is moving faster than light speed: not the light, not the car.

so let's say, theoretically, what would happen, relative to an observer, if the headlights were switched on?

I already answered that one.

Assuming that you want a physics-based answer, not just some made-up nonsense, then your theoretical scenario must be consistent with known physics.

 

[Harrybarlow]

i wouldn't say it has nothing whatsoever to do with the problem stated

two bodies, the car and the train, both moving with a constant speed, two things being launched off the object, the ball, and the light (if were treating light an a particle)

and when you answered it you answered it as if the car were at .99c, now treat it as c
just strictly theoretically, ofcourse

(not trying to get into a flame war here or anything) i saw the problem somewhere else and it got me thinking :D

 

[ZapperZ]

i wouldn't say it has nothing whatsoever to do with the problem stated

two bodies, the car and the train, both moving with a constant speed, two things being launched off the object, the ball, and the light (if were treating light an a particle)

and when you answered it you answered it as if the car were at .99c, now treat it as c
just strictly theoretically, ofcourse

(not trying to get into a flame war here or anything) i saw the problem somewhere else and it got me thinking :D

Actually, it doesn't. The original problem was purely classical, Newtonian mechanics. It is done in undergraduate intro physics classes. You, on the other hand, have changed it into a relativistic problem in which a whole set of different rules apply. So in essence, you have attempted to hijack the thread, which we do not allow on here.

The question of velocity addition at relativistic speeds has been addressed ad nauseum on here in the SR/GR forum. Please look it up there.

Zz.

 

[DaveC426913]

You guys might want to brush up on your SR 101. This is basic stuff.

1] You cannot travel at the speed of light in your car. Nothing with mass can. You can however, travel at a speed arbitrarily close to c.

2] From your vantage point in the car you will see the beams of light leave your car at the speed of light. Period. This will be the same whether you are stationary or whether you are moving at .999c relative to an object of your choosing (maybe Earth or maybe the Milky Way Galaxy or maybe your grandmother in her scooter). Notice how the object you choose to measure your speed relative to has no effect on the speed at which light leaves your headlights. In a nutshell, the same beam of light is observed to be moving at c from ANY vantage point of your choosing - yes the same beam of light. That's the beauty of relativistic time dilation.

 

[Redbelly98]

The headwind should not be a big factor as the ball will experience the same air resistance as any baseball traveling at 100 mph. Pitchers do throw baseballs at close to that velocity and air resistance does not seem to slow them appreciably.

I've thought about this some more. It takes http://www.google.com/search?hl=en&q=60.5+ft+/(100+mph)&btnG=Search" for a 100 mph fastball to reach home plate. So the ball doesn't seem to slow appreciably in 1/2 second. But a 100 mph wind exerts quite a force on a hand-sized object. If you give the ball some arc so that it's in the air for many seconds, the slow-down should be quite noticeable.

Or ... we could just do the experiment with a whiffle ball instead!

 

[kate.k]

Last night my friends and I had a discussion about a science riddle that one of them had brought up. The question triggered some debate and the four of us were split in half with our answers. The question/riddle is:

If you're standing on top of a train traveling at 50mph and you throw a baseball in the direction that you're moving which leaves your hand at 50mph, what will happen to the baseball?

This experiment would be done under controlled conditions i.e. no wind or anything other than the atmosphere acting as an opposite force.

With respect to this question.
The baseball is already in your hands when the train is traveling at 50mph. It already possesses kinetic energy with respect to a stationary object on the ground.

When you throw it, you are giving it additional KE for it to travel at 50mph.

Baseball would continue moving until all energy is used to work against fluid resistance due to atmosphere?

 

[Grogerian]

You guys might want to brush up on your SR 101. This is basic stuff.

1] You cannot travel at the speed of light in your car. Nothing with mass can. You can however, travel at a speed arbitrarily close to c.

2] From your vantage point in the car you will see the beams of light leave your car at the speed of light. Period. This will be the same whether you are stationary or whether you are moving at .999c relative to an object of your choosing (maybe Earth or maybe the Milky Way Galaxy or maybe your grandmother in her scooter). Notice how the object you choose to measure your speed relative to has no effect on the speed at which light leaves your headlights. In a nutshell, the same beam of light is observed to be moving at c from ANY vantage point of your choosing - yes the same beam of light. That's the beauty of relativistic time dilation.

Question, i understand why thing's can't travel at the speed of light, It's clearly obvious but the "Nothing with mass can" why doesn't light have a mass, i mean it does succumb to extremely high gravitational energy like a black hole right or is this caused from something else?

 

[spikenigma]

Can I modify the OP just a little bit?

what does an outside observer see through the windows of the train if the ball is thrown in the opposide direction of the trains movement

i.e.

<<---
[]_[]_[]_[]_[]
1000 miles per hour (train)

--->> 0
1000 miles per hour (ball thrown on the train)


* will he/she see the ball float in the air for a while before it loses momentum and falls to the ground?

* if another indentical ball was dropped by the outside observer at the exact moment the ball on the train is thrown, would there be any difference in the atomic makeup of both balls to account for the difference in behaviour?

 

[Doc Al]

Can I modify the OP just a little bit?

what does an outside observer see through the windows of the train if the ball is thrown in the opposide direction of the trains movement

i.e.

<<---
[]_[]_[]_[]_[]
1000 miles per hour (train)

--->> 0
1000 miles per hour (ball thrown on the train)

I presume the ball is thrown inside the air-tight train? In fact, why don't we remove the air from within the train to eliminate that complication.

The train moves 1000mph West with respect to the ground; the ball moves 1000mph East with respect to the train. Everything viewed by a person on the ground.

* will he/she see the ball float in the air for a while before it loses momentum and falls to the ground?

What momentum? The ball just falls.

* if another indentical ball was dropped by the outside observer at the exact moment the ball on the train is thrown, would there be any difference in the atomic makeup of both balls to account for the difference in behaviour?

What difference in behavior?

 

[ZapperZ]

Question, i understand why thing's can't travel at the speed of light, It's clearly obvious but the "Nothing with mass can" why doesn't light have a mass, i mean it does succumb to extremely high gravitational energy like a black hole right or is this caused from something else?

Please read our FAQ in the General Physics forum.

Again, do NOT derail the thread. All questions regarding the relativistic aspect of this should be in the SR/GR forum. In fact, many of them have already been addressed sufficiently there.

Zz.

 

[spikenigma]

I presume the ball is thrown inside the air-tight train? In fact, why don't we remove the air from within the train to eliminate that complication.

The train moves 1000mph West with respect to the ground; the ball moves 1000mph East with respect to the train. Everything viewed by a person on the ground.


What momentum? The ball just falls.


What difference in behavior?

oops!, yes they both hit the ground at the same time

 

[Redbelly98]

With respect to this question.
The baseball is already in your hands when the train is traveling at 50mph. It already possesses kinetic energy with respect to a stationary object on the ground.

When you throw it, you are giving it additional KE for it to travel at 50mph.

True initially: it travels at 50 mph relative to you, the thrower. It is traveling 100 mph relative to an observer on the ground. That much has been determined earlier in the thread.

After that it's a question of, does the ball slow down enough that the thrower sees it slow down noticeably, or even reverse direction before hitting the ground?

Baseball would continue moving until all energy is used to work against fluid resistance due to atmosphere?

Either that, or it hits the ground, bounces several times, and rolls to a stop. But whether it is stopped by air resistance or ground friction, in the end the thrower on the train sees the ball reverse direction, from 50 mph forward to 50 mph backwards.

I alluded in a previous thread to using something with more air resistance than a baseball. An inflatable beach ball for example. The effect would be much more dramatic then.