- #61
TonyCross
- 66
- 12
Thanks very much, I really must phrase my questions better in future...(:Dale said:
Friction/heat kinetic energy - reactive force
- Context: Undergrad
- Thread starter TonyCross
- Start date
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- Tags
- Energy Force Kinetic Kinetic energy
Click For Summary
Discussion Overview
The discussion revolves around the relationship between friction, heat, and kinetic energy in the context of disc brakes on vehicles. Participants explore how the conversion of kinetic energy into heat affects the forces experienced by brake pads and the implications of these interactions on energy conservation and mechanical behavior.
Discussion Character
- Debate/contested
- Technical explanation
- Conceptual clarification
Main Points Raised
- One participant questions whether the heat generated by disc brakes reduces the reactive force experienced by the disks and if the absence of heat would increase the lateral force on the brake pads.
- Another participant suggests that the kinetic energy of a squash ball, when it hits a wall and warms up, may affect the force with which it rebounds.
- Some participants argue that energy and force are distinct concepts and cannot be directly decomposed into one another, challenging the idea of dividing kinetic energy into thermal and reactive components.
- There is mention of "brake fade" as a real-world phenomenon that relates to the discussion of energy conversion in braking systems.
- Participants discuss the implications of Newton's Third Law in the context of forces exerted during collisions, with some emphasizing the importance of the term "reactive force" while others argue it lacks physical significance.
- One participant proposes that energy loss can be experimentally deduced by measuring the height to which a ball rebounds after being dropped, linking mechanical energy loss to kinetic energy changes.
Areas of Agreement / Disagreement
Participants express multiple competing views regarding the relationship between kinetic energy, heat, and force. There is no consensus on the significance of the term "reactive force" or how energy should be conceptualized in this context.
Contextual Notes
Participants highlight the complexity of linking energy and force, noting that while they are related, they are fundamentally different quantities. The discussion includes unresolved questions about definitions and the implications of energy conservation laws.
Who May Find This Useful
This discussion may be useful for individuals interested in the mechanics of braking systems, energy conversion processes, and the conceptual challenges in understanding the relationship between force and energy in physical systems.
- #62
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Again, you need to define hitting harder.TonyCross said:
The peak force could be exactly the same, indeed, the whole force profile during compression could be the same, but the change in momentum will be greater since the forces during expansion will be greater.
Consider instead a spring mechanism. In one scenario the springs rebounds perfectly elastically; in another, a latch operates at maximum compression to inhibit any rebound.
- #63
jbriggs444
Science Advisor
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Those numbers suggest a rebound that is four times as fast as the impact. Is the ball coated with explosives?TonyCross said:
- #64
A.T.
Science Advisor
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No. You can have a very soft but elastic ball, that will have a lot of rebound but low peak force, because the impulse is spread out over a long time.TonyCross said:
- #65
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I modeled it as a spring with two different constants, a higher one during compression and a lower during relaxation. As you can see, the rebound takes longer.
Read the next chart as clockwise around the loop. The enclosed area represents the net KE lost.
And for completeness:
Read the next chart as clockwise around the loop. The enclosed area represents the net KE lost.
And for completeness:
Attachments
- #66
Physics guy
- 23
- 10
First of all before giving my opinion you seem to be a heck lot confused with the concept of force and energy. Maybe because you think a little too much about it. Force and energy are concepts which help us understand and study the motion around us. They are compatible with each other but each helps us understand motion in a different way.TonyCross said:
Energy can be of any type but one thing that we know for sure is that the total energy is conserved for an isolated system no matter in what situation. This has been strongly established. Kinetic energy is the energy due to the motion of the body and potential energy is the energy due to the position of a body with respect to another body. In ideal situations when there are no dissipative forces we can conclude that the total mechanical energy (sum of kinetic and potential energy) is conserved. The question of hammering a nail if you want to study it by forces then we apply an external force on the nail which causes the nail to move and it hits the wall bit stops because the wall exerts force causing the nail to stop (note that this has nothing to do with the reaction force due to the force of the nail on the wall). If we were to study it in terms of energy then we use the energy stored in the body to be converted to kinetic energy of the hammer which inturn transfers the energy to the nail causing it to enter the wall. But the dissipative forces exerted by the wall cause the kinetic energy to be converted to any other form of energy about which we are not concerned. I don't understand what potential energy has to do in this situation.
- #67
TonyCross
- 66
- 12
I have attempted to use your graph with some imaginary velocities, does this seem correct?haruspex said:
- #68
A.T.
Science Advisor
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To much imaginary stuff there, especially that "make it a triangle for the sake of ease" part.TonyCross said:
Why would the rebound take longer, if the rebound force is the same as the compression force? You have now more impulse transmitted on the rebound, than on the compression. Also, if rebound force is the same as the compression force, then you have no energy dissipation, so why is the final speed smaller than the initial speed.
- #69
TonyCross
- 66
- 12
All good points how would you approach the problem?A.T. said:
- #70
A.T.
Science Advisor
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What is the question?TonyCross said:
- #71
- #72
TonyCross
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So do I...Dale said:
- #73
Dale
Mentor
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OK, so just use that and don’t change it.TonyCross said:
- #74
TonyCross
- 66
- 12
Thanks Dale, you actually answered my initial question earlier in the thread, haruspex made a really good examination of the physics. I think we can now call this question well and truly answered. Thanks to everyone who gave their input to the discussion.Dale said: