Friction coefficient and work-energy problems with multiple forces

[satycorn]

Hey guys, I have a test tomorrow in physics and although I'm a good student, physics is getting tougher this year (high school 1st year for me) and our teacher is really bad, her teaching skills are very poor. Plus, I've been absent for a couple of days when we had physics and I can't solve the "hard" problems. The lesions these problems are related to are Power, Work and Kinetic Energy.
1) A cube is sliding on the floor for 4 meters, it changes it's speed from 1meters/second to 0,2 meters/s. Find the friction coefficient (no weight of cube given ?)
2) A cube is horizontally moving on a table, there are 4 forces on it, G, N and F1 and F2, F1 is on the right of the cube pulling it, F2 is pulling it from the left. F1=3F2. If during Δt, ΔEK of ONLY F1 is 3Ek, then ΔEk during this time of all of the forces is?
a) 2Ek; b)Ek; c)0,75Ek; d) 0,25Ek
Thanks a lot guys!

 

[tiny-tim]

welcome to pf!

hey satycorn! welcome to pf!

show us what you've tried, and where you're stuck, and then we'll know how to help!

(in 1, call the mass "m", it'll cancel)

 

[satycorn]

Ive really tried but I'm clueless with both I am sorry :(.
At the second one I did the Work=Delta(Kinetic Energy) theorem, but I think I'm wrong. Wht formula do i have to use?

 

[PhanthomJay]

For part 2, using the work energy theorem, as you have stated, is a good way to start. When only F1 is acting you can find the work done by F1 , since delta KE is given. Also, it is given that F2 is 1/3 F1, so you van find the work done by F2, which will be a negative value. The algebraic sum total of the work done by both forces is the change in KE of the block.