Conservation of Mechanical Energy

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SUMMARY

The discussion centers on applying the conservation of mechanical energy to solve a physics problem involving a block and a solid cylinder. The block, with a mass of 2 kg, falls 82 cm in 2 seconds, leading to the calculation of its velocity and acceleration. Key equations include the potential energy (PE) and kinetic energy (KE) formulas, specifically PE = KE(block) + KE(cylinder). The radius (R) of the cylinder is determined to be unnecessary for the calculations as it cancels out in the energy equation.

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  • Understanding of conservation of mechanical energy principles
  • Familiarity with kinematic equations
  • Knowledge of potential and kinetic energy formulas
  • Basic algebra for solving equations
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Lma12684
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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

 
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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?
 
My apologies, they represent the radius. I didn't mean to use different symbols.
 
So you can see now why you do not need to know the radius?
 
Because they cancel??
 
That looks right; once you correct a and vf I think you can solve the energy equation.
 
Thank You~!
 

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