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I'm doing a problem in my book ("Check your understanding" 4.3 in Cutnell/Johnson, 7th ed.). The book says the answer is x=43 degrees, but I don't know how to find it exactly. Given that it's an in-chapter problem, I'm guessing there's something wrong with my brain, but I'd appreciate your help.

Here's the problem:

A box has weight 150 N and is being pulled across a horizontal floor by a force of 110 N. The pulling force can be applied to the box either horizontally or can point above the horizontal at an angle, x. When the pulling force is applied horizontally, the kinetic frictional force acting on the box is 2x as large as when applied at angle x. What's x?

I know that fk = uk(Fn).

I started by writing

F_{x} #1 = 110 - fk = ma = 0

and

F_{x} #2 = 110cos(x) - fk = ma = 0

and making those equal to each other, but I end up with multiple variables, and I'm stuck.

Thanks! :D

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# Homework Help: Kinetic frictional force question

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