Confusion regarding static & kinetic friction

[dmission]

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I'm wondering if someone can please help me clarify static & kinetic friction. My understanding is that static friction is when the two objects are not moving, and kinetic is when they are moving relative to eachother.

However, one of the questions I just missed was about a car driving forward. First of all, how does one gather than the direction of the friction from the ground is the same as the direction of motion of the car? Doesn't friction oppose motion?

Secondly, I thought it would be kinetic friction, since the car is moving. It's static though, and I'm clueless as to why.

Would appreciate any help, thanks.

 

[kalilza]

The fact that the force of friction is in the same direction as the motion of the car may be easier to grasp if you change your point of reference. Yes, the car is moving right (for example), but how is it doing that? Because the tires are moving clockwise. If you were to somehow observe at eye-level the point where the tire touches the ground, you would see that the tire is moving left with respect to the road where the two meet. Since the direction of the motion of the tire with respect to the road is to the left, then friction would be in the opposite direction, or to the right (also the same direction as the car).


The fact that it is static friction seemed weird to me at first, but I think I get why, I just don't know if I can explain it well. Kinetic friction is when two objects are "sliding" relative to one another. In order for the tire to "slide" along the road surface, it would have to be stationary/not rotating while the car moved relative to the road (in other words, have the car break on and have it towed; of course, the fact that the tires aren't moving would mean that you would burn rubber really quickly🙂)

 

[dmission]

The fact that the force of friction is in the same direction as the motion of the car may be easier to grasp if you change your point of reference. Yes, the car is moving right (for example), but how is it doing that? Because the tires are moving clockwise. If you were to somehow observe at eye-level the point where the tire touches the ground, you would see that the tire is moving left with respect to the road where the two meet. Since the direction of the motion of the tire with respect to the road is to the left, then friction would be in the opposite direction, or to the right (also the same direction as the car).


The fact that it is static friction seemed weird to me at first, but I think I get why, I just don't know if I can explain it well. Kinetic friction is when two objects are "sliding" relative to one another. In order for the tire to "slide" along the road surface, it would have to be stationary/not rotating while the car moved relative to the road (in other words, have the car break on and have it towed; of course, the fact that the tires aren't moving would mean that you would burn rubber really quickly🙂)

Very helpful 👍 Thanks. Is that what they mean when they say friction does not oppose motion, but rather, relative motion?

 

[karafox]

It is reasonable to assume that if a car is moving, then there must be kinetic friction at play. However, you must think of the TIRE, not the car. A tire is only momentarily in contact with the ground.

Kinetic friction occurs when surfaces slide against one another (take an object and slide it against a surface without picking it up - e.g. skidding tire, "sliding" against the floor with socks on, a box sliding on a floor).

You can think of static friction as one object being static relative to the surface (e.g. a tire rotating, feet walking -- they are not in constant contact with the surface).

The reason the static friction vector is pointing in the same direction of the car's motion is because it is opposing the force on the road by the rotating tire.

 

[dmission]

Thanks. So would the direction of friction always oppose a contact force like the tires pushing [left for example] on the road?

 

[karafox]

I'm not comfortable with the use of "always", but in the case of the scenario described here, yes. 👍

 

[kalilza]

Very helpful 👍 Thanks. Is that what they mean when they say friction does not oppose motion, but rather, relative motion?

Yes, because in this example there is not just one thing moving relative to the ground, but two (the car and the tires). The only reason the car moves is because of the friction between the tires and the ground (not because of any interaction between the metal body of the car and the ground), so that is the system you need to be looking at in order to answer this question. Probably the hardest part in these physics questions is figuring out exactly what system you need to look at in order to answer the given question 🙁

 

[dmission]

Thanks for all the help, appreciate it