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

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Homework Help Overview

The problem involves calculating the velocity of a ball thrown from a moving train towards a parked car, with various velocities given in different directions. The train's speed is 21 m/s North East, the man walks at 3.0 m/s [E35S] relative to the train, and the ball is thrown at 10.0 m/s [W22S] relative to the man.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss using trigonometry to find components of the velocity vectors and suggest adding these components to find the resultant velocity. There is uncertainty about how to handle multiple relative motions and the use of components versus trigonometry.

Discussion Status

Some participants have provided guidance on using components to solve the problem, and one participant reports success with this approach. There is acknowledgment of the unusual nature of the resulting velocity, but no consensus on the realism of the scenario.

Contextual Notes

Participants note the complexity introduced by the different directions and relative motions involved, as well as the potential for unrealistic outcomes in the context of the problem.

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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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