How Do You Calculate the Velocity of a Ball Thrown from a Moving Train?

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SUMMARY

The velocity of a ball thrown from a moving train can be calculated using the equation VBG = VBM + VMT + VTG, where VBG is the ball's velocity relative to the ground, VBM is the ball's velocity relative to the man, VMT is the man's velocity relative to the train, and VTG is the train's velocity relative to the ground. In this scenario, the train moves at 21 m/s North East, the man walks at 3.0 m/s [E35S], and the ball is thrown at 10.0 m/s [W22S]. By resolving each velocity into its x and y components and summing them, the resultant velocity of the ball when it hits the car is calculated to be 12 m/s [E49N].

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Homework Statement


A man on a moving train throws a ball at a parked car. Find velocity of the ball when it hits the car if:
train is moving 21 m/s North East;
the man is walking 3.0 m/s [E35S] (relative to train)
ball is thrown 10.0 m/s [W22S] (relative to man)

Homework Equations


VBG = VBM + VMT + VTG
(B=ball, M=man, T=train, G=ground)

The Attempt at a Solution


I know how to do it if they were all either North or south, but the different directions with the angles confuses me. I've done a bunch of practice questions with only 2 other relative motions (Vxy=Vxz+Vzy), where I use trigonometry to solve for the resultant, but I don't know how to solve it with three relative motions solving for the fourth. I attempted to use trig, but got nowhere with it, and I'm assuming that's not what I should use. My guess is that I should use components? But so far all I wrote was this:
VBG = VBM + VMT + VTG
 
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You use trig to find the components, don't you?

I take it that E35S means 35° South of due East, etc.

Just find the x-component & y-component for each velocity vector & add them. Then find the magnitude & direction of the resultant. (Usually the positive x-axis points East, & the positive y=axis points North.)

The result looks to me like it will be pretty strange. Also the person throwing the ball must be a contortionist and left handed.
 
Last edited:
Components completely slipped my mind. Thank you, it worked. I got 12m/s [E49N] so that does sound a bit odd but it's just a random question so I'm sure it doesn't matter if it's realistic
 
Yes, that's a reasonable answer.

The train is moving at more than twice the speed he throws the ball, and he throws the ball in nearly the opposite direction of the train's motion.
 

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