Understanding Friction in Car and Trailer: Free Body Diagram Help

[eterna]

Homework Statement

http://postimg.org/image/vnn9m9au9/

For the car why is the friction acting towards the left. Shouldn't it be towards the right since the car is moving to the left

And why is there no friction for the trailer?

Homework Equations

The Attempt at a Solution

 

[james26]

The friction is what is making the car accelerate to the left - it's the grip that the driving wheels have on the ground. There is no friction on the wheels of the caravan because the wheels are free to turn, though there will be a small amount of friction in the wheel bearings.

 

[eterna]

The friction is what is making the car accelerate to the left - it's the grip that the driving wheels have on the ground. There is no friction on the wheels of the caravan because the wheels are free to turn, though there will be a small amount of friction in the wheel bearings.

thanks

and by "find the friction required so that the wheels of the car do not slip", what exactly do they want once I've found the friction from doing the problem?

 

[haruspex]

and by "find the friction required so that the wheels of the car do not slip", what exactly do they want once I've found the friction from doing the problem?

I think they want the (minimum) coefficient of static friction, not the frictional force.

There is no friction on the wheels of the caravan because the wheels are free to turn, though there will be a small amount of friction in the wheel bearings.

Pedantic note: to the extent that there is friction in the bearings, there will also be friction from the road acting to the right. The net torque of the two will be zero at constant speed. When the caravan accelerates, there will be additional friction from the road to provide the torque to accelerate the rotation of the wheels.

 

[lightgrav]

they are simply asking for the friction Force.
(you could calculate the required coefficient, after finding the drive wheel's Normal Force and the friction Force)
They ought to say, "find the friction required to cause this acceleration", because it is the same friction needed whether the tires slip or don't slip.

 

[vanceEE]

Use the equation $$F = \mu R$$ where $$\mu$$ is the coefficient of friction and $$R$$ is the normal force acting on the car. You can find F by resolving the forces horizontally and setting them equal to $$ma$$ where $$a = 0$$

 

[lightgrav]

@VancEE: They are calling "the friction" F ... they would ask for "the minimum coefficient" if that's what they wanted. I'd bet that eterna has calculated friction coefficients before.

most important, the horizontal acceleration is NOT zero
(so friction is not zero, and the trailer hitch Force has horizontal and vertical components).

 

[haruspex]

@VancEE: They are calling "the friction" F ... they would ask for "the minimum coefficient" if that's what they wanted.

I disagree. The text asks for the minimum friction "so that the tires do not slip". The frictional force required for the given acceleration does not depend on whether the tyres slip.
In the absence of such a clue, I find the unqualified term "friction" entirely ambiguous - it could equally well be asking for frictional force or coefficient. Given the clue, I see no ambiguity.

 

[vanceEE]

... they would ask for "the minimum coefficient"

I've never heard of a "minimum" coefficient when dealing with friction. The coefficient is constant unless conditions change e.g. rough to smooth surface. We must assume that the car is on point of moving (limiting equillibrium), therefore letting a = 0. Letting a equal zero does not imply that there is no resistance. This simply means that the car is not accelerating, which it is not if the car is simply on point of moving.