EK #224 - Rolling Friction (Concept Question)

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ilovemcat

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Here's the question(s)

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

#225: When a car moves under its own power at constant velocity, the frictional force between the road and the tires that propel the car is:

Here's the choices for both questions:

A. static and in the direction of the motion of the car.
B. static and in the opposite direction to the motion of the car
C. kinetic and in the direction of the motion of the car.
D. kinetic and in the opposite direction to the motion of the car







Okay, I understand that in both scenarios the friction is static because the point of position relative to the wheel and ground doesn't change (due to the nature of rolling motion)

The answer for #224 was B. The answer for #225 was A. I'm having trouble trying to understand which direction friction points. Also, why does the direction of friction change?

Thanks.

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For 224, the friction has to be opposite direction to the tires because otherwise the car would just slide along.
For 225, the friction has to be in the same direction (Helps if you draw a free body diagram) because the wheel is rotating and pushing the road in the opposite direction and hence the friction counter acts that by pushing the tires in the same direction as the car's movement.
 
For 224, the friction has to be opposite direction to the tires because otherwise the car would just slide along.
For 225, the friction has to be in the same direction (Helps if you draw a free body diagram) because the wheel is rotating and pushing the road in the opposite direction and hence the friction counter acts that by pushing the tires in the same direction as the car's movement.

I understand your second explanation, but I'm still having understanding how the first scenario and why that would differ from the second.
 
imagine you are on ice. in the second one the the friction is to oppose sliding and the tires are trying to move and it does. the friction is trying to stop the car but the force is overcoming it so it is in the same direction.

for the second on, friction is to oppose sliding. the car is moving forward on its own so friction wants to stop it, it is in the direction opposite the car.

so if the car is stopped and wants to move forward it is in the same direction. if the car is moving forward and wants to stop it is in the opposite direction. does that make any sense?
 
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imagine you are on ice. in the second one the the friction is to oppose sliding and the tires are trying to move and it does. the friction is trying to stop the car but the force is overcoming it so it is in the same direction.

for the second on, friction is to oppose sliding. the car is moving forward on its own so friction wants to stop it, it is in the direction opposite the car.

so if the car is stopped and wants to move forward it is in the same direction. if the car is moving forward and wants to stop it is in the opposite direction. does that make any sense?

I think I'm beginning to understand. I had to read through your explanation a few times (I'm slow, sorry lol).

From what I understand:

In the second example: The wheel axle is providing the torque to spin the wheel in a clockwise direction. The friction wants to oppose this motion and acts forward (in the same direction as the car is moving).

In the first example: There is no force turning the wheel. Instead, the wheels moved based on the pull and direction of the towe truck which would be counter-clockwise? (I'm assuming it's being pulled from behind). So then, because friction acts against the direction the wheels are spinning, it works in the opposite direction the car is moving.

Does this sound right?
 
For the first example, friction IS the force that is turning the wheel. Without friction, the wheels wouldn't turn by themselves. Like Frky mentioned, if the car was on ice in the first example, the wheels wouldn't move but instead would just slide along as the car is being towed. But since there is friction on the opposite direction, the wheels turn forward or counterclockwise.
 
For the first example, friction IS the force that is turning the wheel. Without friction, the wheels wouldn't turn by themselves. Like Frky mentioned, if the car was on ice in the first example, the wheels wouldn't move but instead would just slide along as the car is being towed. But since there is friction on the opposite direction, the wheels turn forward or counterclockwise.

What you said makes sense, BUT why does friction act in opposite direction? Is it because of the way the wheels are turning, ie. counter-clockwise and not clockwise like the second example (without towe truck).

Does it matter how the car is being pulled (like, for example - the photos below): - In other words, the attachment of car (Rear/Front)

CarandHome_Car_TowTruck.jpg


Shanghai+Lady+Drives+Off+With+Tow+Truck.jpg
 
Ok, you can think of this problem in two ways. First, Let's just remove friction for example. How else would the wheels on the tires turn? You have to have a force to turn the tires and in this case friction is the force turning that tire.

Seconf friction acts in the opposite direction because the car wants to slide in the direction it is being pulled so friction resists and is the opposite force (Newton's 3rd law) and hence acts in the opposite direction.

It doesn't matter how the car is being pulled.
 
Ok, you can think of this problem in two ways. First, Let's just remove friction for example. How else would the wheels on the tires turn? You have to have a force to turn the tires and in this case friction is the force turning that tire.

Seconf friction acts in the opposite direction because the car wants to slide in the direction it is being pulled so friction resists and is the opposite force (Newton's 3rd law) and hence acts in the opposite direction.

It doesn't matter how the car is being pulled.

I think I understand now. Sorry for the trouble and thanks for your patience. :) :thumbup:
 
Can someone further explain this concept to me?

For the first example, the way I understand it is that the reason the the friction is in the opposite direction is because the car is not applying a force to move the tires. Thus, the the tires are stationary and it is the that friction acts as a force in the opposite direction of motion causing the tires to move in the clock-wise direction (it has to be this direction or the car would not move forward?).

For the second example, the car is applying a force on the wheels in order to turn them. Thus, if the tires are moving in the forward direction (their tires and moving clockwise). This force that is being applied on the tires is pushing on the road in the opposite direction. Thus, in order for the tires to move forward, the road pushes back with an equal and opposite force towards the direction that the car is moving, thus allowing the tires to spin clockwise.

The question I have is, why the hell is it static friction? It was to my knowledge that static friction was the friction we had to overcome in order to move. Once we moved, the friction would turn kinetic. If we are already moving at a constant velocity, haven't we already converted over to kinetic friction?
 
Can someone further explain this concept to me?

For the first example, the way I understand it is that the reason the the friction is in the opposite direction is because the car is not applying a force to move the tires. Thus, the the tires are stationary and it is the that friction acts as a force in the opposite direction of motion causing the tires to move in the clock-wise direction (it has to be this direction or the car would not move forward?).

The car would still move forward--after all, it is being towed. But, without friction, you would simply have the tires sliding across the concrete. Actually, it helps to visualize that exact scenario (tires sliding across a surface) and then figure out what would make the tires rotate normally. That something is friction, pointing in the opposite direction of movement.

For the second example, the car is applying a force on the wheels in order to turn them. Thus, if the tires are moving in the forward direction (their tires and moving clockwise). This force that is being applied on the tires is pushing on the road in the opposite direction. Thus, in order for the tires to move forward, the road pushes back with an equal and opposite force towards the direction that the car is moving, thus allowing the tires to spin clockwise.

Yes

The question I have is, why the hell is it static friction? It was to my knowledge that static friction was the friction we had to overcome in order to move. Once we moved, the friction would turn kinetic. If we are already moving at a constant velocity, haven't we already converted over to kinetic friction?

It's confusing, I know. Kinetic friction is only for something that's sliding. The tires aren't sliding, they are rotating. You can imagine the tire being divided into a million different pie-slices...each one one is being worked on by static friction, and none of them is being worked on by kinetic friction (because none of them is sliding).

Comments in bold. You're almost there.
 
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