Homework Help: Pulling a crate uphill with friction (need help)

1. Oct 8, 2009

killat943

1. The problem statement, all variables and given/known data
A crate is pulled by a force (parallel to the incline) up a rough incline. The crate has an initial speed of 1.39 m/s. The crate is pulled a distance of 8.04 m on the incline by a 150 N force. The acceleration of gravity is 9.8 m/s^2 (The incline is 20 degrees, the coefficient of friction is 0.358 and the weight of the crate is 11 kg.

a) What is the change in the kinetic energy of the crate in units of J?

b) What is the speed of the crate after it is pulled the 8.04 m in units of m/s?

2. Relevant equations
KE = 1/2mv^2

3. The attempt at a solution

This is the only hint our professor gave us and i feel lost. I just need help setting it up.

we take the system to be the block, then energy is being transferred out of the system (by friction) and into the system (by the applied force F). Setting this up like we set up other conservation of energy problems may be confusing, as you figure out how to deal with the force F. Here's another suggestion: You can calculate how much energy was pulled away from the block by friction and how much energy was added to the block by the force F. When you do that, you will have a number in joules which represents a net gain of energy by the block. That energy had to go somewhere. It went into two places: adding KE to the block and adding PE to the block. You can calculate a number for how much PE was added from the information given. That means that the rest of the energy added went to increasing the KE.

Thanks!

2. Oct 8, 2009

Delphi51

Have a go at it - we're eagerly awaiting your attempt so we can make suggestions! At least figure out the friction force, the force of gravity down the ramp - might as well find the acceleration, too. Then you can get the Vf directly as a check on the energy approach.

3. Oct 8, 2009

willem2

Why don't you just try to calculate all those energies like your prof said.

The energy added by a force, also called the work done by the force is force * distance (for a constant force that acts in the direction of movement)