# Conservation of Mechanical Energy

## Homework Statement

A massless string is wrapped around a solid cylinder as shown in the diagram. A block of mass 2kg hangs from a string. When released, the block falls a distance of 82 cm in 2.0 seconds. Calculate the mass using the conservation of mechanical energy.

## Homework Equations

d=speed(i) + 1/2at^2
a=.205 m/s

V(f)=v(i) + at
v(f)=.41 m/s^2

PE=KE(block) + KE(cylinder)
mgy=1/2mv^2 + 1/2(1/2MR^2)(v/r)^2

## The Attempt at a Solution

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alphysicist
Homework Helper
Hi Lma12684,

I think you neglected the 1/2 in your equation when you calculated a.

For the value of R, what do R and r represent in your final equation?

My apologies, they represent the radius. I didn't mean to use different symbols.

alphysicist
Homework Helper
So you can see now why you do not need to know the radius?

Because they cancel??

alphysicist
Homework Helper
That looks right; once you correct a and vf I think you can solve the energy equation.

Thank You~!!!