# Friction between bodies in motion

vijay_singh
Hi

I was ready a physics textbook and came across a quiz related to friction

"What is the direction of the friction force on a crate which is on a truck moving towards east" .

I think it is west, but I don't know if I am correct. As the crate is also moving with truck, the truck surface will have friction force on crate which is towards west.

Homework Helper
Welcome to PF!

Hi vijay! Welcome to PF!

Hint: What would happen if there was no friction, and you were holding the crate in place with your hand, and suddenly let go of it?

And what difference would accelerating or braking make?

vijay_singh
Hi vijay! Welcome to PF!

Hint: What would happen if there was no friction, and you were holding the crate in place with your hand, and suddenly let go of it?

And what difference would accelerating or braking make?

Thanks for replying, but i didn't really get your hint. In the scenario you described, i am holding the crate and i let it go, it will obiviously go down due to gravitational pull of earth.

Homework Helper
Thanks for replying, but i didn't really get your hint. In the scenario you described, i am holding the crate and i let it go, it will obiviously go down due to gravitational pull of earth.

oh … no … I meant the crate is on the floor of the truck, which is very slippery, and you are holding it still with your hand.

So it can't fall … but what will happen to it?

prasannab13
Hi Vijay definitely the friction force also act towards east.

Monocles
Draw a free body diagram for the box. If the net force is towards the west, then the box would be accelerating in the western direction, which obviously is not the case if the box is staying in the bed of the east moving truck.

vijay_singh
I understood it now. The relative motion of the crate will be west because of the non-inertial frame of reference it is in. And so the friction will towards east. Thanks guys.

Homework Helper
I understood it now. The relative motion of the crate will be west because of the non-inertial frame of reference it is in. And so the friction will towards east. Thanks guys.

Stop!

The truck is an inertial frame of reference. The truck-driver is an inertial observer.

Only accelerating trucks are non-inertial!

So start again … the truck is an inertial frame of reference, so the friction is … ?

vijay_singh
Stop!

The truck is an inertial frame of reference. The truck-driver is an inertial observer.

Only accelerating trucks are non-inertial!

So start again … the truck is an inertial frame of reference, so the friction is … ?

But this truck is accelerating, so the truck and the truck driver both are in non-inertial frame of reference. Why do u say truck and driver are in inertial frame of reference?

tmc
But this truck is accelerating, so the truck and the truck driver both are in non-inertial frame of reference. Why do u say truck and driver are in inertial frame of reference?

The initial question never implied that the truck was accelerating; merely that it was moving.

vijay_singh
The initial question never implied that the truck was accelerating; merely that it was moving.

I can't personally explaining the direction of friction if the truck was moving with uniform velocity. How would you explain it?

Vijay

vijay_singh
Come to think more about this, I think I am beginning to understand it, but I guess you all are better judge.

If the truck is moving at constant velocity towards east, the crate is at rest and hence because of its inertia tries to remain at rest. But because the truck moves towards east, the relative motion(w.r.t truck) of the crate is towards west and hence the friction on crate is in east direction?

Is this correct explanation?

Homework Helper
It depends if there is an aerodynamic drag force acting on the crate, which may be the case since the statement was a crate "on" the truck as opposed to "in" the truck.

Monocles
You had it correct before. There is friction acting towards the east if the truck is accelerating towards the east, and there is no friction if it is moving at a constant velocity (ignoring air drag). Remember that inertia means that objects in motion tend to stay in motion as well, not just tend to stay at rest. If there was still friction towards the east when the car was moving a constant velocity, then the free body diagram would be telling you that the box was accelerating towards the east (once again assuming that there is no air drag), which obviously is not true.

Note that which direction the truck is moving in actually has no bearing on which direction the friction is. The only thing that matters is which direction the truck is accelerating in. If the truck were moving west but was slowing down, then friction would act towards the east, because the truck is accelerating towards the east.

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Homework Helper
Hi vijay_singh!

Monocles is completely right.

If there's no acceleration, then the crate will stay where it is, even if the truck floor is completely smooth. There is no friction force!

vijay_singh
Now I am totally confused.

But I guess this confusion may be because of the way I am looking at the motion. My last comment was considering that the truck is at rest initially and then it moves in constant velocity. And as the crate is at rest, it tries to remain at rest. But as the truck is moving, its relative motion (w.r.t to truck) is towards west and hence the friction is towards east.

tmc
Where did you get the idea that the truck was at rest initially? You can consider the truck to have been in motion forever. Heck, you could even imagine the truck being bolted to the ground, and simply moving East with the rotation of the Earth (course, you have to assume the Earth is basically flat and the likes, but it's a good approximation). If you bolt a table to the ground and put a frictionless crate on top, what happens?

Direction of motion has no effects on friction, only acceleration does. Since the question never specifies that any acceleration is taking place, one can infer that there is no acceleration and thus no friction force.

vijay_singh
Where did you get the idea that the truck was at rest initially? You can consider the truck to have been in motion forever. Heck, you could even imagine the truck being bolted to the ground, and simply moving East with the rotation of the Earth (course, you have to assume the Earth is basically flat and the likes, but it's a good approximation). If you bolt a table to the ground and put a frictionless crate on top, what happens?

Direction of motion has no effects on friction, only acceleration does. Since the question never specifies that any acceleration is taking place, one can infer that there is no acceleration and thus no friction force.

If I understand it right, the crate is moving towards east at same velocity as truck, not because it is on the truck, but because of its inertia.

Homework Helper
Now I am totally confused.

But I guess this confusion may be because of the way I am looking at the motion. My last comment was considering that the truck is at rest initially and then it moves in constant velocity. And as the crate is at rest, it tries to remain at rest. But as the truck is moving, its relative motion (w.r.t to truck) is towards west and hence the friction is towards east.

Hi vijay!

The simple answer is … pretend you're a lawyer , and don't read anything into the question that isn't there!

tmc's comment:
You can consider the truck to have been in motion forever
may appear to be flippant … but he's absolutely right!

It's none of your business to enquire what the truck-driver did before the question began!

Allow the poor guy some privacy!

All you are entitled to assume is what you are told, and all you are told about in this case is the present, not the past!

That's the way physics exam questions work!

Philosophy exams? … well, that would be different!

shamrock5585
if the truck is moving at constant velocity then there is no acceleration, hence no force, hence no friction force to counter the box. There would only be friction force if the truck was accelerating or decelerating. The friction force would always be in the same direction of the trucks acceleration (if it were moving east and decelerating then the acceleration would be to the west)

Monocles
I have to apologize about my first post, because I actually read your initial question wrong as well, and I just assumed you meant accelerating towards the east. So that may have been a source of confusion for you.

vijay_singh
Thank you all for giving so many angles to this question, I have really gained a lot.