Blog Contest #3
Whitey's gift certificate for the first correct answer. No deadline, but you may want to get your Whitey's before you go home for the summer (if you are leaving at all or if you are even here).
When you roll a tire, it will slow to a stop. The force slowing the motion of the tire is called rolling resistance.
Please address all of the following in your response:
What is the source of the rolling resistance of tires?
What is the direction of the force?
What object exerts the force on the tire?
Would there still be a force if the tire were in a perfect vacuum?
Would there still be a force if the tire were perfectly rigid?
Would there still be a force if the tire were on a perfectly frictionless surface?
Hmmm....
I guess probably buying a car is what got me thinking about this. I was a little reluctant to have another mechanics-type question dealing with resistance, but that's what I thought of and I had Whitey's certificates burning a hole in my pocket. So to speak. Anyway, viva la resistance, or something!
6 comments:
What is the source of the rolling resistance of tires?
-the deformation of the surfaces in contact, which then touch at more than one point.
What is the direction of the force?
- it has a force opposite the CoM motion to slow down the linear velocity and a torque to slow down the angular velocity.
What exerts the force on the tire?
-the extended surface in contact with the tire.
Would there still be a force if the tire were in a perfect vacuum?
-yes
Would there still be a force if the tire were perfectly rigid?
- not if the road is also perfectly rigid
Would there still be a force if the tire were on a perfectly frictionless surface?
- yes
Gabe
I'll take a stab....
What is the source of the rolling resistance of tires?
Deformation of the tire and the ground.
What is the direction of the force?
Opposite the direction of movement.
What exerts the force on the tire?
Gravity causes distortion and affects friction, which in turn causes rolling resistance.
Would there still be a force if the tire were in a perfect vacuum?
Yes, but that sure would help with air resistance...
Would there still be a force if the tire were perfectly rigid?
Yes, but it would be much smaller. The ground would still deform.
Would there still be a force if the tire were on a perfectly frictionless surface?
I don't think the car would be able to start rolling on a frictionless surface unless it was jet powered. But that wasn't your question. I think rolling resistance would go away without friction.
I like these answers! And so quickly, too.
We are very, very close, but...
Notice that the two answers disagree on what would happen without friction. (and, yes, Shawn, I did mean if it were already rolling when it met up with the frictionless surface).
I can see reasons for both your answers:
Gabe says yes, because the deformation of the tire would not go away, and the tire would still be losing energy due to that.
Shawn says no, becasue friction is the only force that acts in the right direction to slow the tire down.
Both reasons make sense, but it can't be both yes and no. There's one more piece to the puzzle. Hmmm...
What is the source of the rolling resistance of tires?
The continual deformation of the tire as it rolls (so that it touches the ground over an area and not a mathematical line), the continual deformation of the ground in the same manner, a small (negligible) amount of friction, and air drag.
What is the direction of the force?
Opposite to the direction of movement, what else?
What exerts the force on the tire?
The ground that it is rolling over, and any surrounding air.
Would there still be a force if the tire were in a perfect vacuum?
Yes, air drag would disappear, but the other force components would be unaffected.
Would there still be a force if the tire were perfectly rigid?
"Perfectly rigid" is ill-defined when atomic scale phenoma are considered, but clasically, no, at least not if the ground weren't also perfectly rigid. It is worth nothing that the pressure on all points along the mathematical line of contact with the ground would be infinite in such a case.
Would there still be a force if the tire were on a perfectly frictionless surface?
Yes
Would there still be a force if the tire were on a perfectly frictionless surface?
- I still think it's yes, because wouldn't there still be a normal force acting on it that has a non-vertical component due to the deformation...
Gabe
OK, Gabe.
But while we're in a perfection rut (perfect vacuum, perfect rigidity, perfectly frictionless), what if the surface is perfectly flat?
(Even if it's not perfect, the component of the normal force parallel to the CM motion is probably quite small.)
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