Conservation of Mechanical Energy

[Lma12684]

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 problem is that I do not know how to find R? Please help!

The Attempt at a Solution

 

[alphysicist]

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?

 

[Lma12684]

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

 

[alphysicist]

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

 

[Lma12684]

Because they cancel??

 

[alphysicist]

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

 

[Lma12684]

Thank You~!