Need Help With A Physics Problem

[shahalam]

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Hey everyone. I really need your help with a physics problem my professor posted. I'd appreciate a nice thorough answer.
IF ACAR THAT IS COASTING DOWN A HILL LOCKS ITS FRONT BRAKES, IT WILL STILL TRAVEL IN A REASONABLE STRAIGHT LINE. IF IT LOCKS ITS REAR BRAKES, IT WILL BEHAVE ERRATICALLY. WHY?

Once again, thnx for all your help. Don’t study too hard for those midterms!
also, a websitte t
hat helps with physics would be nice.

 

[JustR]

Hey everyone. I really need your help with a physics problem my professor posted. I'd appreciate a nice thorough answer.
IF ACAR THAT IS COASTING DOWN A HILL LOCKS ITS FRONT BRAKES, IT WILL STILL TRAVEL IN A REASONABLE STRAIGHT LINE. IF IT LOCKS ITS REAR BRAKES, IT WILL BEHAVE ERRATICALLY. WHY?

Once again, thnx for all your help. Don’t study too hard for those midterms!
also, a websitte t
hat helps with physics would be nice.

I don’t want to make fun of your physics question really, but I don’t know somehow it is making me laugh. Dohhhh!!!It is so obvious that if you lock the front and the rear breaks on a car going down the hill, then the natural to behavior of a car would be to go erratically and not in a straight line. But but but, tell ya truth that a car does not behave erratically unless the driver makes the stupid mistake, which I am sure did not happen. If the erratic driver was me, then it might have. This is just because your question makes me laugh out loud, and not relavent to how the problem might be solved.

And, I don’t really know the exact answer to your physic problem indeed, on a serious note.

I wish I could help, but I am not a physics expert.

 

[amandil]

This question deals with the difference between rolling friction and sliding friction. For a given surface, the coefficient of rolling friction is greater than the coefficient of sliding friction.

Locking the brakes causes the wheels to slide rather than roll over the surface. Because the coefficient of rolling friction is greater, the rolling wheels will have greater friction with the surface and will lag behind the sliding wheels.

So, if you lock the front brakes: the front wheels slide while the back wheels lag behind. The whole car moves generally in a straight line.

If you lock the back brakes: the back wheels slide while the front wheels lag, resisting the movement, so the car moves erratically down the incline.

 

[shahalam]

can you explain why for a given surface, the coefficient of rolling friction is greater than the coefficient of sliding friction. that would be very much appreciated!

 

[glp]

can you explain why for a given surface, the coefficient of rolling friction is greater than the coefficient of sliding friction. that would be very much appreciated!

rolling friction is the same thing as static friction. sliding friction is the same thing as kinetic friction. by definition, static friction is always greater than kinetic friction for a given surface. this is because static friction is the force that completely impedes motion while kinetic friction is the force that merely resists motion.

 

[MattD]

can you explain why for a given surface, the coefficient of rolling friction is greater than the coefficient of sliding friction. that would be very much appreciated!

Rolling friction is analogous to static friction, because even though the wheel is turning, if you look at the contact area between the wheel and the ground, there is no slip. So really the question is why is static friction greater than kinetic friction?

Static friction is caused by a combination of surface roughness and molecular attraction. Because no surface is perfectly smooth, there are many bumps and valleys, which can lock together when two surfaces are in contact much like teeth in a gear. This brings a large amount of the surface into close proximity, which allows molecular interactions to pull the materials together (if they are electrically prone to doing so...). This means that not only do you have to push one surface hard enough to separate the 'teeth' from each other, you may also have to overcome the molecular attraction.

In kinetic friction, the surfaces are already moving across one another. This means that instead of the 'teeth' being locked together, they are hopping and skipping across each other, providing less resistive force. Also, because the surfaces are not coming into such intimate contact as in the static situation, the molecular attraction is also severely reduced.

All this is relatively unimportant for a freshman physics class though. Just remember that static is bigger than kinetic, and you'll do fine!