A couple of problems for you, help please (About mechanical energy)

[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.

 

[asteriatic]

Hi there,

First of all, don't worry too much about your teacher's teaching skills or your absence. Physics can be a challenging subject, but with some practice and understanding, you can definitely improve.

For the first problem, you can use the equation for kinetic energy (KE = 1/2mv^2) to solve for the friction coefficient. Since the mass and weight of the cube are not given, you can assume that they cancel out in the equation. So, you are left with the equation KE = (1/2)(v^2). Plug in the given values (KE = 1/2(0.2^2)) and solve for the friction coefficient.

For the second problem, you can use the equation for work (W = Fd) to solve for the change in kinetic energy (ΔEK) for each force. Since F1 is pulling the cube from the right and F2 is pulling it from the left, their work will be in opposite directions and will cancel out. So, the total work done by the two forces will be zero. This means that the ΔEK for all forces will be equal to the ΔEK for F1, which is given as 3Ek. Therefore, the correct answer is a) 2Ek.

I hope this helps and good luck on your test! Remember to practice and understand the concepts, rather than just memorizing formulas. That will make solving problems much easier.