Physics riddle (relocate if necessary)

1. Oct 4, 2008

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.

2. Oct 4, 2008

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.

3. Oct 4, 2008

Staff: Mentor

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

4. Oct 4, 2008

Staff: Mentor

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

5. Oct 4, 2008

DaveC426913

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.

6. Oct 4, 2008

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.

7. Oct 4, 2008

Staff: Mentor

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

8. Oct 4, 2008

Staff: Mentor

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

9. Oct 4, 2008

Staff: Mentor

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.
True.
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.

10. Oct 4, 2008

DaveC426913

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?

11. Oct 4, 2008

Redbelly98

Staff Emeritus
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.

12. Oct 4, 2008

schroder

The headwind should not be a big factor as the ball will experience the same air resistance as any baseball travelling 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, travelling 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 travelling 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!

Last edited: Oct 4, 2008
13. Oct 6, 2008

Harrybarlow

another riddle is if you were in a car travelling 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 cant go at 2c because it cant travel faster than itself, so would you see the light rays beside you or what?

14. Oct 6, 2008

Grogerian

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 :)

15. Oct 6, 2008

Harrybarlow

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

16. Oct 6, 2008

Grogerian

I assume that if you are travelling 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

17. Oct 6, 2008

Harrybarlow

ouch i feel the brain ache aswell, i still dont really know what waves are, since ive 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

18. Oct 6, 2008

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.

19. Oct 6, 2008

Staff: Mentor

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.

Right.

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).

20. Oct 6, 2008

Grogerian

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?