Finding Velocity Using Work-Energy Principle

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Homework Help Overview

The problem involves a system of two blocks connected by a rope over a pulley, where one block is on a flat surface and the other is hanging. The task is to find the velocity of the blocks after they have moved a certain distance using the work-energy principle. The scenario includes parameters such as mass, friction, and gravitational forces.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss calculating the work done by friction and gravity, and how these relate to the change in kinetic energy. There are attempts to set up equations involving forces and energy changes, but some participants express uncertainty about their calculations and the inclusion of all relevant factors, such as the pulley’s kinetic energy.

Discussion Status

The discussion is ongoing, with participants providing guidance on calculating work and energy changes. Some participants have expressed uncertainty about specific aspects of their calculations, while others have indicated they may have found a solution.

Contextual Notes

There is a mention of confusion regarding the height of the hanging mass and the overall setup of the problem, indicating that assumptions about the system may need clarification.

Bones
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Homework Statement




The system of two blocks is released from rest. The pulley is a solid cylinder of mass 5.00kg and radius of 10.0cm. MA is 10.0kg and MB is 20.0kg. The coefficient of kinetic friction between MA and the table is 0.2. Find the velocity of the blocks after moving 3.00m using work-energy principle.

Block A is on a flat surface and block B is hanging. They are connected by a rope over a pulley.

Homework Equations





The Attempt at a Solution



I don't remember how to do this, please help!
 
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Welcome to PF!

Bones said:
The system of two blocks is released from rest. The pulley is a solid cylinder of mass 5.00kg and radius of 10.0cm. MA is 10.0kg and MB is 20.0kg. The coefficient of kinetic friction between MA and the table is 0.2. Find the velocity of the blocks after moving 3.00m using work-energy principle.

Block A is on a flat surface and block B is hanging. They are connected by a rope over a pulley.

I don't remember how to do this, please help!

Hi Bones! Welcome to PF! :smile:

Calculate the work done by the friction force on MA and by gravity on MB.

(and remember that the two blocks, and the rope on the pulley, will have the same speed!)

That will equal the change in KE. :smile:
 
Work Force of friction: 0.2*98N*(d)(cos180)=-19.6(d)
Change in KE=1/2(20kg)(V)^2-1/2(20kg)(0)

-19.6(d)=1/2(20kg)(V)^2

I don't think I am doing this right...
 
Bones said:
Work Force of friction: 0.2*98N*(d)(cos180)=-19.6(d)
Change in KE=1/2(20kg)(V)^2-1/2(20kg)(0)

-19.6(d)=1/2(20kg)(V)^2

What about gravity? and KE of the pulley?
 
mAgh+0.5mAvA^2=mBgh+0.5mBvB^2

(0.2)(98N)(3.00m)(cos180)=(10kg)(9.8m/s^2)(0)+(0.5)(10kg)(vA)^2=(20kg)(9.8m/s^2)(h)+(0.5)(20kg)(vB)^2

I am not sure about the height of mass B. I don't think this is right either...
 
Last edited:
I think I figured it out. Thanks for your help!
 

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