# Throw a ball AT a moving train, in what direction does it reflect?

You are standing at a platform, with a ball. a train approaches and u throw the ball at it in a straight line from a small distance. in what direction des the ball reflect off? heres what i think, tell me where i am wrong.

a) the direction opposite to motion of the train: relative to the train, the ball is already moving in the opposite direction, so it must reflect off in the direction of the balls original motion ie. opposite to train.

b) same direction as the train: the momentum of the train is transfered to the ball and it moves in the same direction. (prolly wrong xD)

c) returns back straight: there is no horizontal component in the ball's motion so it just returns, like a ray of light when it is reflected off a moving mirror.

Thanks!

edits:: The ball is spherical, thrown at the side of the train.

Wouldnt the ball and train act like a system of gears and hence the rotation in the ball may be in the opposite direction?

include all friction and gravity

• which part of the train does it hit? front or side?
– user207455
May 24, 2019 at 9:44
• @SolarMike it hits the side... front wont be much of a question huh? May 24, 2019 at 17:19
• Well you could work out how much the impact of the ball slows the train... And the roof is also a possibilty?
– user207455
May 24, 2019 at 17:33
• What do you mean by a system of gears in this case? May 24, 2019 at 20:06
• @Eagle the ball acts as a pinion and the train acts like a stack dynamicscience.com.au/tester/solutions1/hydraulicus/gears1a.htm May 26, 2019 at 15:40

The ball will compress on impact to the train. While it is compressing and decompressing, the ball and train are a combined system moving together. The train imparts some of its momentum on the ball and vis a vis the ball on the train. This momentum transfer will make the ball move the train.

A large proportion of this momentum transfer will be as angular momentum. The ball will undergo shear forces on impact which will start it spinning. Due to conservation laws the train will also get some angular motion, though the track provides a righting action (even if you were to notice such a negligible change).

Now it should be noted that if the ball is thrown at the train with enough spin (such that the leading of edge of the ball is moving in the direction of the trains motion and faster than it) then you can make the ball bounce off in the direction opposite that of the trains motion.

If you want to test things out empirically I'd suggest a snooker table would be less likely to get you arrested.

• I fail to see how the likelihood of arrest is reduced if you throw a snooker table at a train.
– rob
May 24, 2019 at 12:16
• @rob tongue in cheek? I think he means checking the spin business on the sides of the table and the direction the ball takes depending on the spin? May 24, 2019 at 15:28
• wont the ball and train act somewhat like gears and the direction of movement of train will create an opposite spin in the ball? i think i am wrong, pls point out if so.. thanks! May 24, 2019 at 17:20
• @anna yes I did, but its a reply I deserved. May 24, 2019 at 22:37
• There will be an opposite effect. E.g. in the eastbound train - north thrown ball example, the trains linear motion gets partly converted to a ccw spin (from above) and that of the ball cw, i.e. the leading edge gets pulled to go with the train. May 24, 2019 at 22:43

I will assume the ball is being thrown at the side of the train. To fix co-ordinates, let's assume the ball is thrown due north and the train is travelling due east.

If the side of the train is smooth then there is no transfer of momentum in the eastward direction, so the ball is travelling due south after it bounces off the train.

If the side of the train is not smooth then there is some transfer of momentum in the eastward direction, so the ball is travelling somewhere between due south and due east after it bounces off the train.

• This is ok for a marlble. It is a ball. in the question. What about compression and elasticity and the delta(t) this introduces? May 24, 2019 at 10:46
• @annav a rugby ball? Or are you assuming one that is spherical? ie football, basketball, tennis ball, squash ball?
– user207455
May 24, 2019 at 11:42
• any ball above that has a compressibility May 24, 2019 at 15:24
• Wouldnt the ball and train act like a system of gears and hence the rotation in the ball may be in the opposite direction? May 24, 2019 at 17:28