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Physi1csStud3nt
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http://www.physicsclassroom.com/mmedia/energy/cs.cfmCan you guys explain why a car that is skidding to a stop has the same kinetic energy as its potential after it has stopped?
Thank you for the explanation. So would there be work done either negative or positive, or is work = 0?Merlin3189 said:No, because it hasn't. I think you misread it.
"The work done by friction on the car is related to the initial kinetic energy of the car. The work-energy relationship is often related by the equation
KEi + PEi + Wext = KEf + PEf "
Which is different from what you said.
Then they point out that the PE is the same throughout, assuming the car is going along level ground.
"Since the potential energy of the car is the same in the initial state (before braking) as the final state (after stopping),"
And then they cancel all the irrelevant and null terms
" each term can be canceled from the above equation. And since the car is finally stopped, the KEf term in the equation is zero."
So that KEi + Wext = 0 though they don't bother to explain, just substitute straight in
"Thus, the equation becomes 0.5*m*v2 + F*d*cos(180) = 0."
Basically KE = Work done by skiddingIn the absence of external gain or loss of energy, such as a ball rolling (or sliding) along a frictionless surface in the absence of air resistance, then KE + PE = KE + PE , so if it stops, it must have gained PE.
But here the lost KE went into the friction.
Physi1csStud3nt said:Then I do not understand why the answer to question 6. a) is 'none' on this worksheet: http://www.hartlandhigh school.us/subsites/Andrew-Kartsounes/documents/AP%20Physics%20C/5%20Energy/Masters/Energy%20Packet%202%20Key.pdf
That is not correct. The work done by a force is computed as the product of the force times the distance moved by the object on which the force acts. The road is not moving, so no work is done. [There is another layer of complexity if one deals with rotating or non-rigid objects, but the road is rigid and non-rotating, so we need not deal with that].Merlin3189 said:I must admit, I don't understand any of question 6. All the systems seem to be closed and any work done is exchanged between parts of the system.
I am saying that in 6a the car is doing work on the pavement and the earth.
Irrelevant. It is the motion of the target of the force that matters, not the motion of its originator.It is applying a force to them and is moving in the direction of the force.
Yes. Correctly reasoned.I'm not familiar with the idea of negative work, but I assume the Earth and pavement are doing the negative work, since the movement is in the opposite sense to the force they are applying.
I can't really dispute this as I'm not an authority on this subject, but that is not my understanding.jbriggs444 said:... The work done by a force is computed as the product of the force times the distance moved by the object on which the force acts. The road is not moving, so no work is done. ...
Your hand will heat up as well, eventually more than the road. Your force is not doing work on the road in this case.Merlin3189 said:If the road doesn't move, the work is (largely) transferred to heat in the road.
No, because the road is doing negative work on your hand. That's where the heat energy comes from.Merlin3189 said:If you hold to the notion that energy (such as the heat of friction) can appear where no work is being done, are you not breaking the conservation of energy?
You may have a point there. I am doing work, some on the road and some on my hand?A.T. said:Your hand will heat up as well, eventually more than the road. Your force is not doing work on the road in this case.
Can the road be doing negative work on me, yet I not be doing positive work on the road? In what form does the negative work come from me? Does it come as work or as heat?No, because the road is doing negative work on your hand. That's where the heat energy comes from.
No work is done on a static object. To model heating up using the work principle you would have to model the individual road atoms which are moving when your hand atoms apply a force to them.Merlin3189 said:I am doing work, some on the road ...
No, the hand cannot do work on the hand.Merlin3189 said:...and some on my hand?
Yes, and the difference in energy is dissipated at the interface.Merlin3189 said:Can the road be doing negative work on me, yet I not be doing positive work on the road?
Work is F dot s, not heat.Merlin3189 said:In what form does the negative work come from me? Does it come as work or as heat?
You have to consider a physical point of application (a bunch of atoms) not just a geometrical. point.Merlin3189 said:My position is simply that work is the dot product of the force I apply and the distance the point of application moves, irrespective of how or why that point of application moves.
Yes, if you model all that detail you will have work being done on road atoms.Merlin3189 said:However, in the specific case pushing against friction, even though the nominal pushed object does not move, the atoms/molecules of which it is made do move and then spring back, converting tiny amounts of linear KE into random vibrational KE (apart from those pieces, which I mentioned in my first post, which are torn asunder doing work against their inter atomic/molecular bonds.)
When a car is skidding, the brakes are no longer able to effectively slow down or stop the car. This is because the tires have lost traction with the road surface, causing them to slide instead of grip. As a result, the brakes cannot create the necessary friction to slow down the car.
A car can skid to a stop due to a variety of factors, such as wet or icy road conditions, worn tires, or excessive speed. In some cases, sudden braking or turning can also cause a car to skid.
Yes, driving a skidding car can be dangerous as the driver has less control over the vehicle. It can also increase the likelihood of accidents, especially if the car skids into other objects or vehicles.
Skidding can be prevented by practicing safe driving habits, such as maintaining a safe speed and keeping a safe distance from other vehicles, especially in wet or icy conditions. It is also important to regularly check and maintain tires for proper tread and inflation.
If your car starts to skid, it is important to stay calm and avoid overcorrecting or slamming on the brakes. Instead, gently ease off the accelerator and steer in the direction of the skid. Once the car has regained traction, you can slowly apply the brakes to come to a stop.