Potential Energy and Kinetic Energy

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Homework Help Overview

The discussion revolves around a physics problem involving two blocks of different masses connected by a string over a pulley, focusing on the concepts of gravitational potential energy and kinetic energy. Participants are examining the changes in energy as one block descends.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants discuss the calculation of gravitational potential energy changes and question the sign of the energy values. There is an exploration of the relationship between potential energy and kinetic energy as one block descends. The movement of both blocks is considered in the context of energy conservation.

Discussion Status

Some participants have provided calculations and expressed uncertainty about the correctness of their values. There is an ongoing examination of the relationship between the kinetic energies of both blocks and how they relate to the potential energy changes. Multiple interpretations of the energy values and signs are being explored.

Contextual Notes

Participants are navigating the implications of energy conservation in a system with two moving masses, and there is a focus on ensuring that all aspects of the system's energy are accounted for. The discussion reflects the complexity of the problem setup and the assumptions being made about the system.

Manh
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Homework Statement


Two blocks are hung by a string draped over a pulley, a 1.5-kg block on the left and a 3.0-kg block on the right. The two blocks start out at rest and at the same height.

A. What is the change in the gravitational potential energy of the system of blocks and Earth when the 3.0-kg block has dropped 0.60 m ?
B. What is the change in the kinetic energy of the system between release and this instant?
C. What is the velocity of the 1.5-kg block at this instant?

Homework Equations


U = m*g*h
K = 1/2 m*v^2

The Attempt at a Solution


A. U = (3.0 kg)(9.8 m/s^2)(0.60)
= 17.64 J
I wonder if the answer should have a negative sign!

B. K = 17.64 J
Since U is converted to K, therefore K has the same value of U. Am I correct?

C. 17.64 = 1/2*(1.5)*v^2
v = 4.85 m/s
These are my works and answers. I hope somebody can help me check them. Thanks!
 
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Manh said:
A. U = (3.0 kg)(9.8 m/s^2)(0.60)
= 17.64 J
I wonder if the answer should have a negative sign!
Don't ignore the movement of the 1.5 kg mass. (As far as sign, when something lowers the change in gravitational PE is negative.)

Manh said:
B. K = 17.64 J
Since U is converted to K, therefore K has the same value of U. Am I correct?
Yes, but you have the wrong value. (See A.)

Manh said:
C. 17.64 = 1/2*(1.5)*v^2
v = 4.85 m/s
These are my works and answers. I hope somebody can help me check them. Thanks!
Realize that the total kinetic energy is for both masses.
 
Doc Al said:
Don't ignore the movement of the 1.5 kg mass. (As far as sign, when something lowers the change in gravitational PE is negative.)
A. U1 = (1.5 kg)(9.8 m/s^2)(0.6 m) = 8.82 J
U2 = (3 kg)(9.8 m/s^2)(-0.6 m) = -17.64 J
delta U = -17.64 J + 8.82 J = -8.82 J
Doc Al said:
Yes, but you have the wrong value. (See A.)
B. delta K = 8.82 J
Doc Al said:
Realize that the total kinetic energy is for both masses.
C. K = 1/2 m1*v1^2 + 1/2 m2*v2^2. Where K = 8.82 J. Is this correct?
 
Manh said:
C. K = 1/2 m1*v1^2 + 1/2 m2*v2^2. Where K = 8.82 J. Is this correct?
Yes, but what is the relationship between the two velocities?
 
wouldn't you end up with 2 variables v1 and v2?
 

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