# Homework Help: A Couple of Practice Questions

1. Dec 12, 2008

### MartyVice

I need help with some physics questions. (Grade 11 Physics) Any help appreciated...

1. Assuming that the effective force of friction is 225N, what is the minimum work needed to push a 1.00 x 10^3kg car 45.0m up a 12.5 degree incline?

2. A 2.20kg pendelum has a period of 4.0 sec

a) What is it's frequency? (Already figured out to be 0.25)
b) What is the length of the pendelum? (Already figured out to be 3.97m)
c) What must be true about the angle (Theta) for calculated value to be correct?
d) What is the value of the restoring force when the angle (Theta) is 11.0 degrees?
e) Assuming the angle in d), what is the velocity of the pendelum bob when it passes through the equilibrium point?
f) What is the tension in the pendelum string when it passes through the equilibrium point in e)?

3. A 6200kg satellite is in a circular earth orbit that has a radius of 3.3 x 10^7m. A net external force must act on the satellite to make it change to a circular orbit that has a radius of 7.0 x 10^6. How much work must the net external force do?

Can anyone show their work and explain how they got to the answer, I realize this is very simple compared to some of the other stuff on here, but I am having trouble with physics due to a very poor professor.

Thanks

2. Dec 12, 2008

### cepheid

Staff Emeritus
For question 1, the *minimum* work required occurs when you're pushing just hard enough to overcome friction (so that the force applied is equal to the friction force, but in the opposite direction i.e. up the ramp vs. down the ramp). Does that help?

3. Dec 12, 2008

### Elbobo

In 3, I assume you're supposed to use Newton's Law of Universal Gravitation, in which case you need the mass of the earth and the gravitational constant G. Consider using a free body diagram to determine the forces acting upon the satellite and their positivity or negativity.

4. Dec 12, 2008

### cepheid

Staff Emeritus
For question 2, you're assuming simple harmonic motion. For a simple pendulum, that is only true under certain conditions. What are they? The answer to this question is the answer to 2c.