Rolling and sliding friction

[chiddler]

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1. Imagine you put your hand on top of a book and exert forward force to slide it forward. What would the free body diagram look like?

2. I'm trying to understand these questions.

I illustrated my musing of the matter:

 

[MedPR]

Nevermind, rethought that and it didn't work. Now I'm confused too.. Makes me want to skip ahead and be where you are in studying

 

[chiddler]

Nevermind, rethought that and it didn't work. Now I'm confused too.. Makes me want to skip ahead and be where you are in studying

i'm actually behind and i've forgotten to do a lot of things that my schedule says I should which is slowing me down even more!

I'm really, really glad I gave myself a lot of extra buffer time so i'm still in an ok position.

Are you following SN2's schedule and how far are you?

 

[MedPR]

i'm actually behind and i've forgotten to do a lot of things that my schedule says I should which is slowing me down even more!

I'm really, really glad I gave myself a lot of extra buffer time so i'm still in an ok position.

Are you following SN2's schedule and how far are you?

Yea, I'm following the schedule. Today (December 29) was day 13 for me. I'm caught up but trying to get a few days ahead over christmas vacation.

 

[chiddler]

But this is physics chapter 2 stuff!

 

[MedPR]

But this is physics chapter 2 stuff!

That's true, haha.

Is it static friction? If yes, wouldn't static friction be opposite the direction the car is being pulled thereby preventing the tire from slipping?

 

[chiddler]

That's true, haha.

Is it static friction? If yes, wouldn't static friction be opposite the direction the car is being pulled thereby preventing the tire from slipping?

The question:

When a car is towed at constant velocity, the frictional force between the road and its tires is:

Answer: static and in the opposite direction of the motion of the car.

Explanation: The force opposes the motion of the car; if the friction is reduced (say the car is put on ice), the car does not move forward.

What I don't understand is that if friction is reduced and the same force is applied to the car, it would slide across instead of roll across.

I'm imagining tires skidding on a frictionless surface and friction is slowly added to the surface. The tires start rolling which means friction is causing this rotation. The rotation is opposite the direction of movement. This means friction is in the same direction as the rotation because the friction causes the rotation.

...right?

Q224 and 225 in EK book.

 

[MedPR]

The question:

When a car is towed at constant velocity, the frictional force between the road and its tires is:

Answer: static and in the opposite direction of the motion of the car.

Explanation: The force opposes the motion of the car; if the friction is reduced (say the car is put on ice), the car does not move forward.

What I don't understand is that if friction is reduced and the same force is applied to the car, it would slide across instead of roll across.

I'm imagining tires skidding on a frictionless surface and friction is slowly added to the surface. The tires start rolling which means friction is causing this rotation. The rotation is opposite the direction of movement. This means friction is in the same direction as the rotation because the friction causes the rotation.

...right?

Q224 and 225 in EK book.

I don't know if this will work for you, but for this example I think of static friction as allowing the tires to rotate rather than causing them to rotate. The static friction is opposite the direction of the car because, if there was no static friction, the tires would just spin and stay in place. The static friction allows the car to "pull" on the ground (think of each of the tire treads pulling on the road in the same way that you would climb up a rope).

 

[chiddler]

I don't know if this will work for you, but for this example I think of static friction as allowing the tires to rotate rather than causing them to rotate. The static friction is opposite the direction of the car because, if there was no static friction, the tires would just spin and stay in place. The static friction allows the car to "pull" on the ground (think of each of the tire treads pulling on the road in the same way that you would climb up a rope).

Yeah that works...

I don't like it! But it works.

I don't think i'll get a question correct if you give me something similar though. :<

 

[MedPR]

Yeah that works...

I don't like it! But it works.

I don't think i'll get a question correct if you give me something similar though. :<

I just think of static friction as the force that prevents sliding/slipping.

If you push on a wall and don't fall forward on your face, that's static friction between your feet and the ground.

If you are ice skating and are able to push off with your foot, that's static friction w/ the ice.

If you are driving in your car and slam to a stop, that's kinetic friction between your tires and the road. But when you start from 0m/s, static friction is what allows your tires to grip the road and pull the car forward.

If you put a book flat on a table, place your hand flat on the top of the book, and then push your book forward + release your hand from the book, kinetic friction is what eventually makes the book stop moving, but static friction is what allowed your hand to transfer the force to make the book move. If there wasn't static friction, your hand would've slipped off the book when you tried to push it.

 

[Pisiform]

hmm I don't know, I have asked several people about this question when I was doing EK and no one was able to give me a definitive answer. They told me there are two forces involved, frictional force and rolling resistance which depends on torque and rolling direction respectively.

 

[MedPR]

hmm I don't know, I have asked several people about this question when I was doing EK and no one was able to give me a definitive answer. They told me there are two forces involved, frictional force and rolling resistance which depends on torque and rolling direction respectively.

But none of those factors will change the direction of the frictional force right?