# Free body diagram help

## Homework Statement

http://postimg.org/image/vnn9m9au9/ [Broken]

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?

## The Attempt at a Solution

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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.

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
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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.
james26 said:
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
Homework Helper
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.

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
Homework Helper
@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