phy 121
Your 'cq_1_21.1' report has been received. Scroll down through the document to see any comments I might have inserted, and my final comment at the end.
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Seed 21 1
A ball is tossed vertically upward and caught at the position from which it was released.
Ignoring air resistance will the ball at the instant it reaches its original position be traveling faster, slower, or at the same speed as it was when released?
answer/question/discussion:
It will be traveling at the same speed as it was when it was released. As it goes up it loses velocity at a constant rate due to gravity and then as it falls back down it gains velocity at the same constant rate. Since it is traveling the same distance it will be going at the same speed when it returns at is was when it left.
What, if anything, is different in your answer if air resistance is present? Give your best explanation.
answer/question/discussion:
I think the air resistance will be the same going up and coming down, so it will still act in the same way, the difference will be that the air resistance will increase the effects of gravity going up causing it to slow down quicker, but the air resistance will decrease the effects of gravity coming down so it will speed up slower. The end result would be that it travels less distance, but will still return at the same speed it was going when it was released.
Air resistance will enhance the slowing effect of gravity on the rising ball, which will as a result not rise as far. As a result of the decreased maximum height, the falling ball won't drop as far, resulting in a lesser final speed.
Air resistance will then oppose the speeding up of the falling ball, so that the final velocity will be even less.
For a dense ball tossed gently upward, the effect of air resistance will be small, perhaps negligible with respect to the accuracy of our instruments. If the ball is less dense, and/or speeds are greater, air resistance will have a greater effect.
In terms of energy conservation, assuming still air (i.e., no wind, no rising and falling of air), air resistance acts always in the direction opposite motion and therefore does negative work on the object. PE doesnt change between the beginning and the end of the interval, so the result is a lesser KE at the end than at the beginning.
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5 mins
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Good thinking, but air resistance has different effects on the rising and the falling ball. See my note.