Friction (EK Physics)

[phillips101]

# 222 page 22 in EK's 1001 Physics:

Which of the following statements is true concerning friction?

A. The frictional force on an object is always in the opposite direction to the motion of that object.
B. A frictional force opposes the motion of the object to which it is applied relative to the motion of the object that applies the force.

Choices C-D are obviously wrong, so I won't include them.

Between choices A and B, I picked choice A, and the book says it's B. Now I understand why B is correct (even though the wording is very confusing). However, how is A wrong?

An answer offered on the EK website is that a car moves FORWARD because the friction between the car and the road is pushing it FORWARD. Thus, the force of friction and motion may not always be in opposite directions (as this example shoes). I don't exactly think this is correct, because (and please tell me if I'm wrong) the tires are pushing BACK on the road as it's rotating, and the road pushes forward on the tires ... thus, the car moves forward. The interaction is between the tires and the road, not the whole entire car and the road. Also, let's say that a car is stuck in muddy water, and the driver tries to get out by stepping on the accelerator. The tires rotate back, pushing the mud to the back. I've never seen (or see in my head anyways) a car's tires rotating to push mud or dirt towards the front of the car. Thus, the direction of friction opposes the direction of motion, right?

Is my logic correct here? If not, please correct me. This is a very fundamental question, and I don't want to misunderstand such a simple concept.

Also, I'd like to add that it's complete BS that you have to "sign up" in order to see the mistakes they have in the book. I think BUYING the book should entitle me to know the author's mistakes if there are any. But that's just me ranting as I am now on a high from doing so many physics problems.

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[kobe200LATE]

Friction opposes the motion of the two surfaces that are sliding across one another/producing the friction.

A very good example that will you understand this concept is when you are running or walking.

When you are running or walking, you slide your feet backwards against the ground. Because the direction of motion of your feet is moving backwards, the force of friction is actually moving forward.

However, the whole motion of your body (ie the motion of the object) is actually moving forwards when you walk/run, in the same direction as the force of friction.

Just remember that an object's total motion can be in the same direction of the force of friction, but the force of velocity is always opposite to the direction of motion of the surfaces that cause the friction.

 

[matth87]

No offense but that is a bull **** question.

Even if considering the friction on a tire friction is opposing the motion on the tire even though the car is moving forward.

Friction always acts in the opposite direction of motion or in the case of static friction, intended motion.
remember this and you will be fine for the mcat.

The only way I could see A being wrong is if they were taking into the account that when velocity = 0 there is no motion and no frictional force.

 

[phillips101]

Thanks for the replies! Kobe, I get what you're saying, and I understand the "car" example on EK too. I guess it depends on what you count as your "system"- the system can be the car and the road, or the tires and the road.
Thanks, matth for confirming what I had in mind, just wanted to make sure I didn't misunderstand a whole entire chapter.

P.S. I realized I had a spelling error in my original post, but yall get the idea 🙂

 

[Fort]

Frictional force always opposes direction of the motion of an object. I do not think this question is very fair to ask. Their explanation is shady to me.