# Homework Help: Help on linear momentum and work/energy problems

1. Oct 13, 2004

### SJC25

Okay ya'll. I am back with mor eproblems I need help with. I had 18 problems, and I need some guidence on 6 of them.

1) A skier, of mass 60 kg, pushes off the top of a hill with an initial speed of 4.0 m/s. How fast will she be moving after dropping 10 m in elevation?

I am guessing this is a linear momentum problem, but the only equation that fits the info given is p=mv. What do I do with the drop in elevation? I feel like I am going left when I should be going right on this problem.

2) A driver, traveling at 22 m/s, slows down her 1500 kg car to stop for a red light. What work is done by the friction force against the wheels?

Is this a Work-Energy Theorem problem? W (net)= delta K? I know I can find 2 values of K (initial velocity and final)... Am I doing this right?

3) A simple pendulum with a 4.4 kg mass and a length of 75 cm, is released from rest at an angle of 50. (a) To what height does the mass swing on the other side? (b) What is the speed of the mass at the bottom of the swing?

This is the first time I have seen a problem about a pendulum. I cant even find an example in the book. Honestly, I am clueless on this one.

4) A 60 kg woman pushes a 20 kg suitcase on wheels a distance of 10 m by exerting a force of 2.0 N in the direction of motion, starting from rest. (a) How much work does she do? (b) If there were no friction acting on the suitcase, what would its final velocity be at the end of the 10 m distance?

I got part (a)... W=Fd Not sure about part (b)... is it as simple as F=ma, or am I totally wrong again?

5) A workman pushes a 200 kg crate up a frictionless metal ramp pf length 4.00 m into the back of a truck(see figure below) *note= there was no figure below. (a) If the angle made by the ramp with the ground is 25.0 and the workman pushes the block at a very slow, constant speed, how much work does he do? (b) What is the increased gravitational potential energy of the crate?

How do I find work if I dont have a F... which I dont know how to get without an acceleration... I thought about using mgy to find what i need... Am I on the right track?

6) Approximately what running speed would an 80 kg pole-vaulter have to acquire to clear a 5.0 m off the ground? Assume that the pole-vaulter's center of gravity is intially 1.0 m off the ground and that the vaulter clears the bar if his center of gravity just reaches the height of the bar.

Okay, lost on this one too. My book has about 2 paragraphs on center of gravity, and I am just not getting it.

2. Oct 14, 2004

### Galileo

1) Use an energy approach. Gravity does work on the skier which converts to kinetic energy.
2)Again, use conservation of energy. How much work is needed to put the car to a halt?
3) Once more. Conservation of energy. Gravitational + Kinetic.
4a) Use the definition of work.
b) Use the work energy theorem.
5) Basically the same as 4)
6) Basically the same as 3).

All these questions use the same principle. The total amount of energy is conserved. What is lost in kinetic energy is gained in some other form (usually potential energy).

3. Oct 19, 2004

### SJC25

Thanks for the help. I am still having problems with 5 though. How can I find K to do the work energy theorem when I dont have a velocity? Shouldn't I use U=mgy if I am looking for increased gravitational potential energy?
On 4b I have the same problem. How do I use the work energy theorem if I cant find K?