Conservation of Mechanical Energy

AI Thread Summary
The discussion focuses on a physics problem involving the conservation of mechanical energy, where a 2kg block falls 82 cm in 2 seconds. The participants clarify the equations needed to solve for the mass, including potential energy (PE) and kinetic energy (KE) formulas. A key point raised is the confusion over the radius (R) and its relevance, which is resolved by noting that it cancels out in the calculations. The importance of correctly calculating acceleration (a) and final velocity (vf) is emphasized for solving the energy equation. The conversation concludes with a confirmation that the problem can be solved without needing the radius.
Lma12684
Messages
24
Reaction score
0

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

 
Physics news on Phys.org
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~!
 
Back
Top