EK Physics 1001 Friction questions #224-225

[mtravis2190]

Hello all,

I'm just going over concepts that gave me trouble from a little while ago, now that I'm only a month away from my big day. And the following two questions pertaining to friction from EK just have me baffled. I've always learned that friction acts in the direction opposite to motion, but now I'm totally confused.

Q.224: When a car is towed at constant velocity, the frictional force between the road and its tired is:
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
Answer: B

Q.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:
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
Answer: A

I also referred to the EK forum. Most of it made sense, except at the very end when Crayton said "...regardless of whether friction is static or kinetic, its direction is always opposite the motion or intended motion of the surfaces in contact (if friction weren't there)." But based on the answer to question 224, this doesn't seem to apply. Anyway, I'd appreciation any clarification you guys can offer.

Thank you for taking the time to read this post and for replying!

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[FCMike11]

Hello all,

I'm just going over concepts that gave me trouble from a little while ago, now that I'm only a month away from my big day. And the following two questions pertaining to friction from EK just have me baffled. I've always learned that friction acts in the direction opposite to motion, but now I'm totally confused.

Q.224: When a car is towed at constant velocity, the frictional force between the road and its tired is:
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
Answer: B

Q.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:
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
Answer: A

I also referred to the EK forum. Most of it made sense, except at the very end when Crayton said "...regardless of whether friction is static or kinetic, its direction is always opposite the motion or intended motion of the surfaces in contact (if friction weren't there)." But based on the answer to question 224, this doesn't seem to apply. Anyway, I'd appreciation any clarification you guys can offer.

Thank you for taking the time to read this post and for replying!

224 was a weird one (I got it wrong). But yes friction there is opposite to the motion of the car being towed. Draw a free body diagram friction is always opposing the motion. As for the reason it is static, I concluded its because the towed tires cars weren't sliding (actually doing the spinning) it was just being pulled.

However on 225, the tires are using the static friction to propel it. Its the same thing when you take a step, you put your foot down and push off the ground, you are pushing off via friction and propelling yourself forward.

 

[mtravis2190]

224 was a weird one (I got it wrong). But yes friction there is opposite to the motion of the car being towed. Draw a free body diagram friction is always opposing the motion. As for the reason it is static, I concluded its because the towed tires cars weren't sliding (actually doing the spinning) it was just being pulled.

However on 225, the tires are using the static friction to propel it. Its the same thing when you take a step, you put your foot down and push off the ground, you are pushing off via friction and propelling yourself forward.

Hi! Thank you for such a quick reply! I think I finally get the distinction between static and kinetic friction. (Correct me if I'm wrong) Static friction is applying a force when something is rolling or rotating, while kinetic friction is in action while something is sliding or slipping.
And I think I have a better grasp of why friction is opposite to motion in question 224, but question 225 is still giving me trouble. I just don't get how friction could be in the same direction, especially after reading the EK forum where Crayton said its direction is always opposite the motion. If you or anyone else can clarify that particular part of the question I would REALLY appreciate that!

 

[FCMike11]

Static is the initial force to get something moving and kinetic is friction opposing motion.

Think about this for 225. You put a car on ice and slam on the gas, the tires spin (notice coefficient of friction would be lower than that of pavement) and dont accelerate forward. If you slam on the gas on pavement, your tires are able to grip the road and you move. When you are driving your tires are moving counterclockwise. Now look at the contact point (bottom of the wheel) friction needs to oppose the motion (counterclockwise motion friction will resist this and is forward in this case with spinning)

On 224 again I think it is static friction because the towed cars tires are not actively doing the spinning/accelerating and object as a whole id being pulled.

 

[NextStepTutor_1]

The reason that frictional forces for question 225 point in the same direction as the motion is related to how you would have to put the torque on the wheels to cause forward motion. So, to move a car to the right, the wheels would have to undergo clockwise motion. To initiate that clockwise motion though, the frictional force must point towards the right as well. Thus, in this case, friction does point in the opposite of motion--the motion of the tires. The motion of the car has nothing to do with the frictional force.

 

[mtravis2190]

Thank you so much everyone! this clarifies it!