In unloading a truck a man allows a 120kg crate to slide down a plank inclined at 30.0(adsbygoogle = window.adsbygoogle || []).push({}); ^{o}to the horizontal. The crate accelerates at 3.90ms^{-2}. What is the size of the frictional force?

I know I need an equation for frictional force on an incline plane to slove this but thats where my problem lies I didn't learnt it in class and I can't find the equation.

my guess is: F_{f}= umgcosa

but if it is i'm also stuck on how to find u in this problem?

u=F/N but then how do i find N? and F? I'm really stuck and going in circles.

Hope my ramble makes sense to someone lol Thank you for any help

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# Homework Help: Frictional force problem

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