Direction of Frictional Forces

[amusedtodeath24]

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Hello all,

I have having a hard time differentiating between when a frictional force opposes the motion of an object and when a frictional force is in the same direction of motion. Any suggestions as to getting this straight?

Thank you very much...

 

[bluemonkey]

Hello all,

I have having a hard time differentiating between when a frictional force opposes the motion of an object and when a frictional force is in the same direction of motion. Any suggestions as to getting this straight?

Thank you very much...

Kinetic friction always opposes the direction of an object's motion.

 

[amusedtodeath24]

Right, but how can you tell with static friction? I've read 2 examples involving a car - 1 where the car is being towed, and the other where the car is moving forward under its own power. In the case where the car is being towed, I am told that friction is static and opposes the forward motion of the car. When the car is moving on its own, however, I am told that the friction is static and is in the direction of movement. How would you know, for instance, that in the first case static friction is opposite to the direction of movement?

 

[Kaustikos]

Right, but how can you tell with static friction? I've read 2 examples involving a car - 1 where the car is being towed, and the other where the car is moving forward under its own power. In the case where the car is being towed, I am told that friction is static and opposes the forward motion of the car. When the car is moving on its own, however, I am told that the friction is static and is in the direction of movement. How would you know, for instance, that in the first case static friction is opposite to the direction of movement?

Are you sure? I'm almost positive it's in the directino of movement regardless of whether or not it's being towed. The frictional force is between the tire and the road. In both cases, the tires are moving in the same direction - opposite the motion of the car when it's coming in contact with the surface. Thus, the static force is opposite that - in the direction of motion.

 

[amusedtodeath24]

Well, the 2 examples are from Examkrackers 1001 Physics, and those are the answers given to the 2 problems. I'm really confused as to why it would be opposite when towed but in the same direction when traveling on its own.

They gave some kind of rational about "imagine if the friction were reduced - e.g. the car is suddenly put on ice," but I'm at work (obviously being productive) and don't have the book to know exactly what it was it said...

???

 

[amusedtodeath24]

Also,

In both cases, the tires are moving in the same direction - opposite the motion of the car when it's coming in contact with the surface.

Aren't the tires moving in the same direction as the motion of the car? Isn't that how the car is moving forward?

So confused..

 

[unsung]

Static friction on the car's wheels (assuming it's not skidding here) needs to be forward in the same direction as the car's direction of motion, if it is moving forwards.

Imagine trying to walk forward on ice on shoes without any traction. You wouldn't go anywhere, right? That's because when your foot exerts a force on the ice backwards (to propel you forwards), there's no force countering that force which would allow your foot to stay still on the ice. If there's no force countering the force your foot exerts, your foot slips and the result is you haven't take a step forwards.

I'm not sure what the towing problem is, perhaps if you post it, we can help you better. But basically in these problems, the main thing you have to focus on is HOW is this thing moving? For example, if the car's wheels are skidding as it's moving forward, then kinetic friction would be the applicable principle here. (Imagine a box "skidding" forwards across the floor.) And moreover, the direction of that friction would be backwards, opposite to the car's direction of skidding/motion. But, if the car's wheels are spinning normally, then what's operable here would be static friction, and the direction of the static friction would be in the same direction as the car's motion.

So basically, things with wheels are a bit tricky because you can imagine them moving forwards in two different ways. One way's the normal way, where the wheels are spinning and due to static friction the thing is propelled forward. But another way the thing can move forward is just being "dragged" forwards, even if the wheels are somehow locked in place. In this 2nd case, the thing behaves exactly like a thing without wheels that you're used to dealing with in physics problems (like a box). Just figure out from the problem how the thing is moving and from that think about whether you've working with kinetic friction or static friction. If a thing moves forward based on a static friction, then it has to be via something like our normal locomotion- we exert a force backwards which is countered by that frictional force, allowing us to be propelled forwards. Whereas if a thing moves forward and deals with kinetic friction, it's more likely the thing is being dragged or pushed or something (such as a box being dragged or pushed). These are just generalizations.

Does that help at all?

 

[Quantum Chem]

I think that for these two questions a force diagram would help make sense. I too would have thought that both cases were static friction towards the movement of the car, but after drawing the forces, the towing answer makes sense.

In the towing example the force acting on the car is coming from the tow truck, so on the diagram there would be force Ftow in one direction. Since there is friction, the static friction acts to "balance" out the Ftow, which means the friction must point away from the direction of force, hence movement.

In the car moving on its own, the force comes from the tires pushing the car the forward. The frictional force here acts opposite of the pushing force, thus in the direction of movement.

I'm assuming that this is the rational behind the answers.

 

[amusedtodeath24]

The force diagram explanation does make sense - so what you are saying is that in the towing example, the static friction necessarily opposes the direction of motion because otherwise the car would just be pulled into the back of the tow truck. Well, thank you all very much, that did help clear things up.