A classical mechanics problem by A Former Brilliant Member

Classical Mechanics Level 2

You are making a video for your "Lit-Sports-Shots" channel on YouTube. You plan to drop a tennis ball from a 20 story building, and simultaneously shoot a tennis ball upwards from the ground with a tennis ball machine at 200km/hr. The upper ball is dropped from 65m above the machine opening.At what time in s after the drop/shot will the balls collide? (Neglecting air resistance)


The answer is 1.17.

A Former Brilliant Member by A Former Brilliant Member

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2 solutions

Saurav Kumar
Dec 21, 2017

using relative motion, observe the ball coming down from the ball thrown up, relative acceleration is zero because both have same acceleration, so the ball will be seen coming with uniform velocity, relative velocity equals {0-(-65)} i.e. 65km/hr or 55.555m/s, travelling distance 65m, so answer is 65/55.555 i.e. 1.17

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Write the expression for the y-position of each, and find the time when they have the same value. ydrop=65−4.9t2 yshot=55.56t−4.9t2 Then, you equate these the acceleration terms cancel and you find: t=1.17s

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You should say "neglect air resistance" or something to that effect. Great problem!

Laszlo Mihaly - 3 years, 6 months ago

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@Laszlo Mihaly , thanks 4 the feedback ;)

A Former Brilliant Member - 3 years, 6 months ago

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