- #1
Taulant Sholla
- 96
- 5
Homework Statement
Homework Equations
work-kinetic energy theorem
Work done by torque: wheel turning about a curb
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Homework Help Overview
The discussion revolves around the work done by torque when a wheel turns about a curb, exploring concepts related to the work-kinetic energy theorem and static equilibrium. Participants are examining the conditions under which the net work can be zero and the implications of forces acting on the wheel as it rises.
Discussion Character
- Exploratory, Conceptual clarification, Assumption checking
Approaches and Questions Raised
- Participants question the assumption that net work can equal zero with a constant force and explore the necessary conditions for the wheel to rise. There are discussions about using the work-kinetic energy theorem versus virtual work methods, as well as considerations of rotational kinetic energy and the relationship between linear and angular displacements.
Discussion Status
There is an active exploration of different interpretations regarding the application of the work-kinetic energy theorem and the concept of average force. Some participants have provided hints and suggestions for reconsidering the approach, particularly regarding the calculations of forces and energy changes involved in the problem.
Contextual Notes
Participants are navigating assumptions about the geometry of the problem, including angles and the relationship between linear and angular motion. There is also mention of the need to clarify the definitions of average force in the context of the problem.
work-kinetic energy theorem
- #2
TSny
Science Advisor
Homework Helper
Gold Member
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You've assumed that it is possible for Wnet to equal zero when F is constant. Try to see why this can't be true.
Can you figure out how much force is required to get the wheel to start to rise?
Can you figure out how much force is required to get the wheel to start to rise?
- #3
- 42,888
- 10,521
Further to TSny's hints...
You write that you have to solve it by the Work-KE theorem. I suspect you have misunderstood the requirement. As TSny writes, you have no guarantee that ##\Delta KE=0##.
You can solve statics problems using virtual work. Maybe that is what you are supposed to be using? But that method considers infinitesimal changes in position, not integrating over a substantial change.
You write that you have to solve it by the Work-KE theorem. I suspect you have misunderstood the requirement. As TSny writes, you have no guarantee that ##\Delta KE=0##.
You can solve statics problems using virtual work. Maybe that is what you are supposed to be using? But that method considers infinitesimal changes in position, not integrating over a substantial change.
- #4
Taulant Sholla
- 96
- 5
Ah, yes. Thank you. It would take a variable force to result in a final kinetic energy of 0. I know torque equilibrium yields the constant force required to offset the torque produced by gravity. Thank you again.TSny said:
- #5
Taulant Sholla
- 96
- 5
Right. Thank you. I need to find the rotational kinetic energy of the wheel once it rises to the top of the curb.haruspex said:
- #6
Hamal_Arietis
- 156
- 15
You assumed that ##ds=Rd\theta## but ##ds>0## and ##d\theta <0##. So it is wrong
If you assumed that ##ds=-Rd\theta## I think you will get answer
If you assumed that ##ds=-Rd\theta## I think you will get answer
- #7
Monsterboy
- 305
- 96
Isn't that angle = 30o ?
- #8
- 42,888
- 10,521
Taulant's error was to calculate a funny kind of average force (averaged over horizontal distance, which is not what is meant by "average force") necessary to provide the PE gain. Instead, the force to be found is the minimum constant force that will get it over the step. The next stage is to find the residual KE that results. That could be done by integration, but it is not necessary.Hamal_Arietis said:
Taulant set θ as the angle to the vertical, the 60o. That reduces as the wheel rises.Monsterboy said: