Solving Problems with Work, Energy, and Power

[Caraniah]

I have question about this chapter (and yes, this is my tutorial question) that i want to ask. I use the formula but i did not get the right answer. Please help me and tell me what's wrong...


Question

A block of 30 kg slides down a rough inclined plane with an acceleration 1.5 meter per second. The plane makes an angle of 28 degree with the horizontal and is 8.0 m long. Find

1) The kinetic energy of the block when it reaches the bottom of the incline. (Ans: 360.15J)

I use K=1/2 mv^2

2) The work done to overcome friction. (Ans:745.2 J)

I use K + U + W other = K + U

3)the friction force that acts on the block. (Ans: 93.2 N)

I use F - friction = ma

4) The coefficient of the kinetic between the block and the plane. (Answer: 0.36)

I use friction =kN

 

[Defennder]

1. Use the basic kinematic equations to determine the final speed of the block, then just use the KE formula.

2. Note that since energy is conserved, the work done to overcome friction + final KE of the block is a constant throughout block's downward slide. You need a formula to find the total energy. Think in terms of potential energy here.

3. Once you have done 2., you can easily determine the force from the work done to overcome friction.

4. The formula is correct but N refers to the normal force exerted on the plane by the block. Use some vector algebra and trigo to figure that out.

 

[Caraniah]

Thanks. I will try to digest your answer slowly...