Need Basic Physics Help

[Dry]

Kinetic friction is proportional to the contact surface

No.

Oops, right, I thought I removed this part. Seems I didn't...

[Dauphin]

Wheels tend not experience that much kinetic friction with the ground, unless they are skidding.

[snoopy369]

In Nascar [and other racing], that friction is rather important, however...

[Dauphin]

Driving full stop requires friction. Heck, walking requires friction - mostly not kinetic.

[snoopy369]

Ah, right. I am not 100% clear on the difference between kinetic and static friction, I think...

[KrazyHorse]

If the guy is a physicist, he just notice that the guy on the train is in an inertial frame of reference, so he can do his calculations from the observer on the train point of view.
And the same maybe applied to the space potato and the 2 gremlins. You can see it move at constant 1m/s in some inertial frame of reference, but you can also see it not moving at all in another, or see it move at 100m/s in a third one.
Makes no difference.

Gee, thanks. I didn't realize you could do that. I certainly wasn't trying to explain, from a mechanical perspective WHY that works properly.

[N35t0r]

Besides, if the question was ¿Why does the box remain travelling at 1m/s? and you answered that, you'd probably get a 0 for being a smartypants.

[Japher]

Ah, right. I am not 100% clear on the difference between kinetic and static friction, I think...

Skid marks..

[Dauphin]

Ah, right. I am not 100% clear on the difference between kinetic and static friction, I think...

In simple terms, kinetic friction occurs when two surfaces are moving with respect to each other and causes things like carpet burns, skid marks etc. Static friction is frictional force between two surfaces that are in stationary contact and must be "overcome" before an object's relative surface can be moved along by a pushing/pulling force.

When wheels roll or feet walk, the contacting surfaces don't move with respect to the ground surface, unless you slip/skid etc.

[snoopy369]

Ok, that makes sense, I wasn't thinking about it quite that way. What's the 'one level up' terms explanation of kinetic friction, then? I know that static friction derives simply from the normal force and the coefficient of friction of the contact surface; why is kinetic friction different from that [ie, from what I understand, it's similarly calculated from the normal force and a (different) coefficient of friction; why is that coefficient different?]

[KrazyHorse]

Wheels tend not experience that much kinetic friction with the ground, unless they are skidding.

True

The major lossy energy pathway between nonskidding wheels and the ground is due to deformations of the wheels themselves.

[KrazyHorse]

Ok, that makes sense, I wasn't thinking about it quite that way. What's the 'one level up' terms explanation of kinetic friction, then? I know that static friction derives simply from the normal force and the coefficient of friction of the contact surface; why is kinetic friction different from that [ie, from what I understand, it's similarly calculated from the normal force and a (different) coefficient of friction; why is that coefficient different?]

Think about it as two surfaces becoming somewhat "glued" together when they're at rest relative to each other. When they're moving against each other the molecules don't have enough time to get that stuck to each other.

That's a cartoon idea, and is probably wrong, but frictional forces are only vaguely understood from a microscopic perspective, as far as I know (this is not at all my area of expertise).

[snoopy369]

Hmm, that's how I thought it originally [ie, it's strictly less than static friction, but otherwise from the same forces]. That makes Dauphin's statement [that there is little kinetic friction btw the wheel and the ground] make less sense to me, though, unless he's trying to say that there is MORE friction between the wheel and the ground [as it's static], but that's obviously false, isn't it? Or is it, hmm. After all, anti-lock brakes exist because skidding slows you down less efficiently than not skidding and instead slowing the rotation of the wheels ... gah, don't know enough about cars to answer that. Maybe I just answered my own question...

I think a car's forward motion derives form the [whatever] friction between the wheel and the ground opposing the motion of the wheel, ie, rather than the wheel just spinning in place, because the (rubber) wheel has so much friction with the ground, the motion of the wheel drives the car forward [unless you're on ice or an oil slick!].

Hmm, yeah, that means my interpretation of Dauphin's statement is right and not nonsense - cars have static friction (ie, more) and not (much) kinetic friction (ie, less) while not skidding. Right?

[KrazyHorse]

That makes Dauphin's statement [that there is little kinetic friction btw the wheel and the ground] make less sense to me, though, unless he's trying to say that there is MORE friction between the wheel and the ground [as it's static], but that's obviously false, isn't it?

Please read more carefully. He said that there is little KINETIC friction. Not little friction in general.

Also, the amount of static friction is NOT simply equal to mu_static * weight; that is an UPPER BOUND; static friction increases as you increase the force trying to move the object up until you reach the breaking point of mu_static*weight

[KrazyHorse]

If you put an object on a flat, horizontal tabletop then it experiences no static friction. If you push on it lightly the static friction will be equal and opposite to your push. If you push on it harder the static friction will increase to again equal your push. At some point you break the maximal static friction force and the object begins to move.