Conservation of Mechanical Energy

  • Thread starter Lma12684
  • Start date
  • #1
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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

 

Answers and Replies

  • #2
alphysicist
Homework Helper
2,238
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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?
 
  • #3
25
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My apologies, they represent the radius. I didn't mean to use different symbols.
 
  • #4
alphysicist
Homework Helper
2,238
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So you can see now why you do not need to know the radius?
 
  • #5
25
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Because they cancel??
 
  • #6
alphysicist
Homework Helper
2,238
1
That looks right; once you correct a and vf I think you can solve the energy equation.
 
  • #7
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Thank You~!!!
 

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