- #1

Perseverence

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- 7

## Homework Statement

Princeton p 411 #6

A 7 kg ball is dropped from 20m. if the speed just before it hits the ground in 18 meters per second, what is the work done by air resistance?[/B]

## Homework Equations

KE= (mv^2)*1/2

PE=mgh[/B]

## The Attempt at a Solution

Mgh equals 1400 joules. PE equals KE. using the kinetic energy formula I was able to derive that the velocity of the ball without air resistance would be 20 meters per second. Which would be 2 meters per second less than 18 meters per second. Then I calculated the amount of kinetic energy there would be for 2 meters per second he found that it would be 14 joules. Why isn't this the answer for the amount of air resistance? I know how to find the right answer, but why doesn't this work?[/B]