The discussion revolves around the question of whether static friction does any work when a car accelerates. Participants explore the role of friction in the context of driving and walking, examining the conditions under which work is defined and the implications of different reference frames.
Participants express differing views on whether static friction does work and how to define work in this context. There is no consensus, as multiple competing interpretations and models are presented throughout the discussion.
Limitations include the dependence on definitions of work, the assumptions made regarding reference frames, and the varying interpretations of friction's role in energy transfer.
Well said. Our assumptions about immovable anchors in experiments is well justified, Momentum is still conserved and no Energy is 'lost'.Dale said:The dilemma lies in conflating the change in momentum with a change in energy. Momentum and energy are closely related, but they are not the same thing. You can have a force which transfers momentum and does not transfer energy, which is the case with the car.
Yes, understood. Which is why I was careful not to include "energy" in the list of knowable things, given the more limited information.Dale said:But without that extra information you cannot determine the change in the energy of the system.
The extra information could be determined. Force exerted onto the vehicle = mass of vehicle · acceleration of vehicle. Assuming a flat road, you have a force exerted onto a vehicle and the current velocity of the vehicle. My premise is that the power = force exerted on vehicle times velocity of the vehicle. The acceleration of the vehicle is linear (still assuming a flat road). The angular acceleration of the entire drive train is due to the torque of the engine being slightly greater than the opposing torque related to the force exerted at the contact patch times the effective radius, so the "power" related to the force exerted onto the vehicle is only responsible for the energy change related to the linear acceleration.Dale said:But without that extra information you cannot determine the change in the energy of the system.
This premise is adequately demonstrated to be false above many many times.rcgldr said:My premise is that the power = force exerted on vehicle times velocity of the vehicle.
Ah, good point. I didn’t pick up on that the first time.jbriggs444 said:Yes, understood. Which is why I was careful not to include "energy" in the list of knowable things, given the more limited information.
The force is not the missing information. The velocity of the material at the contact point is the missing information.rcgldr said:The extra information could be determined. Force exerted onto the vehicle = mass of vehicle · acceleration of vehicle.
The velocity of the material at the contact point is zero, but's rolling motion and the tire moves at the same speed as the vehicle. If power was force times velocity of the materials, power would be zero, and over time work would be zero, but the work is known to be non-zero since the vehicle accelerates coexistent with an increase in kinetic energy.jbriggs444 said:The force is not the missing information. The velocity of the material at the contact point is the missing information.
You can’t know that if you are treating the car as a black box.rcgldr said:The velocity of the material at the contact point is zero, but's rolling motion and the tire moves at the same speed as the vehicle.
What in the world does "accounted for" mean?rcgldr said:So it's still my premise that the change in kinetic energy related to the linear acceleration is accounted for by the force exerted onto the vehicle times the velocity of the vehicle.
The response is incorrect. If the road is doing work on the car then why doesn’t the energy increase?rcgldr said:the response I received is similar to what I've seen elsewhere:
"The road is, in fact, doing work on the car to propel it forward."
The kinetic energy increases due to acceleration, the potential energy decreases due to consumption of the energy source (fuel or battery). If the energy comes from an external source, such as the Sun, then the vehicle experiences a net gain in energy, the solar energy received on the solar panel is eventually converted into kinetic energy of the vehicle, using the road to supply the external force that accelerates the vehicle.Dale said:The response is incorrect. If the road is doing work on the car then why doesn’t the energy increase?
Therefore the rate of change of energy for the car is zero: no power.rcgldr said:The kinetic energy increases due to acceleration, the potential energy decreases due to consumption of the energy source (fuel or battery).
That is not in question. The road does supply the force that changes the momentum. The energy does not change so the road cannot supply any power.rcgldr said:the road to supply the external force that accelerates the vehicle.