# Coefficient of static friction?

• kselanders

#### kselanders

I have this question that I am working on, I think I know the formula to figure it out, but I am not too sure. I just need some help explaining how to do it, what to figure out first. The rest I can obviously do myself. Thank you so much!

A 1450 kg car is towing a trailer of a mass of 454 kg. The force of air resistance on both vehicles is 7471 N (backwards). If the acceleration of both vehicles is 0.225 m/s^2, what is the coefficient of static friction for the wheels on the ground?

I think the formula is Ff=uFn
Is the Ff 7471N?
How do I figure out Fn?

Thank you in advance for anyone who helps me out

## Answers and Replies

This question will be moved to the Homework Help section. This problem is not worded correctly, as it should be asking for the force of static friction, not the coefficient of static friction. That is because the formula for static friction is not quite as you have noted...the formula should be Ff (the force of static friction) is less than or equal to uFn. Ff equals uFn only when the tires are about to slip with respect to the ground, and this cannot be determined without more information. Nevertheless, assuming that Ff does equal uFn, you have set it equal to the air resistance. If this was so, the car could not accelerate, because the net force in the horizontal direction would be zero. Use Newton's 2nd law in the horizontal direction to find Ff. Note that the friction force Ff must point in the direction opposite to the air resistance, that is, in the forward direction of the car-trailer's movement. The normal force can be determined by applying Newton's first law in the vertical direction.