Thread: [SOLVED] Hard Vector problem... ??

I don't know where to start with this vector problem ?

a. Find the magnitude of force required to keep a 3500-pound car from sliding down a hill inclined at 5.5° from the horizontal.

b. Find the magnitude of the force of the car against the hill.

If you are being assigned this type of problem, you should have covered how to work with forces at angles by using trigonometry. The car's weight is the result of the mass of it times to acceleration to gravity directed straight down. The force of the car along with hill isn't this number though. Think of the hill as the hypotenuse of a right triangle. You know the angle and the vertical leg of the triangle, so finding the hypotenuse should be simple.

Yea, but this is not law of sines/cosines ...
I am asked about the force rather then the hypotenuse side.

The way i thought about it is, 3500sin(5.5°) = 3484lb ?
And this to be the magnitude of the force, I don't have the book that explains this by the way.

I didn't say use law of sines and yes I get that you are trying to find a force. Your thinking is correct and is what I was trying to get you to do. So if that is the force the car directs down the hill the amount of force needed to keep it from doing so should be obvious.

Draw a free body diagram.

If the hill is inclined at 5.5 degrees, you'll find the force normal to the hill (the force required to keep the car from sinking into the ground) is equal to 3500*cos(5.5 degrees) = 3484 lb.

The force required to keep it from sliding downhill (such as if you are standing behind it pushing it uphill just enough so that it doesnt slip) is 3500*sin(5.5 degrees) = 335 lb

Thanks, Mentia
That's what I needed.