# Kinematics bouncing ball problem

1. Aug 6, 2013

### mridulag

given below is a more than one choice type MCQ,

A ball is bouncing elastically with a speed 1 m/s between walls of
a railway compartment of size 10 m in a direction perpendicular
to walls. The train is moving at a constant velocity of 10 m/s parallel
to the direction of motion of the ball. As seen from the ground,
(a) the direction of motion of the ball changes every 10 seconds.
(b) speed of ball changes every 10 seconds.
(c) average speed of ball over any 20 second interval is fixed.
(d) the acceleration of ball is the same as from the train.

i could not understand what observer at ground will see,is it ball velocity i.e 1m/s or ball velocity relative to train.
plz give answer with an explanation
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Aug 6, 2013

### ehild

Hi mridulag, welcome to PF.

1 m/s is the speed of the ball with respect to the train.

ehild

3. Aug 7, 2013

### mridulag

is it then 1st option be correct .Will the ball will retain its velocity 1m/s unjustyfyng option b?.
IN answer sheet b,c,d options are given correct

4. Aug 7, 2013

### ehild

Inside the compartment, the direction of motion of the ball changes every time it bounces off from the compartment wall. How do you express the velocity v of an object observed from the ground, if it has some velocity v' with respect to an object, travelling with velocity V with respect to the ground ?
What is the velocity of the ball observed from the ground when it moves in the same direction as the train, and what is it when it moves backwards inside the compartment?

ehild

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