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## Homework Statement

While doing a chin-up, a man lifts his body 0.40 m. The muscles involved in doing a chin-up can generate about 70 J of work per kilogram of muscle mass. If the man can just barely do a 0.40-m chin-up, what percentage of his body's mass do these muscles constitute? (For comparison, the total percentage of muscle in a typical 70-kg man with 14% body fat is about 43%.)

## Homework Equations

W = F * s

## The Attempt at a Solution

m = w/(g*s)

m = 70J/(9.8m/s^2 * .40m

m = 17.86 kg

Then I take this and divided by total mass 70kg to get the percentage but I got the wrong answer. I don't know why....what am I doing wrong here?

## Homework Statement

Rotating Bar. A thin, uniform 12.0-kg bar that is 2.00 m long rotates uniformly about a pivot at one end, making 5.00 complete revolutions every 3.00 seconds. What is the kinetic energy of this bar?

## Homework Equations

KE = 1/2m (vf^2 - vo^2)

## The Attempt at a Solution

I didn't know how to get this started but I gave it a shot. First, I took the 5 revolution per 3 seconds and converted them to velocity.

5rev/3s (2*pie*r/rev) = 20.9 m/s

Then I take that answer and plug it into the kinetic equation but as expected, I was wrong. This kinetic stuff is really confusing or is it just me? Pls help?

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