# Homework Help: Calc involving Physics

1. Nov 25, 2003

### gigi9

Someone plz help me how to do these problems below. Thanks a lot for your help.
1) On the surface of the moon the acceleration due to gravity is approximately 1/6 the sun at the surface of the earth, and on the surface of the sun it is approximately 29 times as great as at the surface of the earth. If a person on earth can jump with enough initial velocity to rise 5ft, how high wil the same initail velocity carry that person (a) on the moon? (b) on the sun?
***I had s= -16t^2+ int. v*t+ int. s
s=5, int.s=0...what should I find, how do I do this problem?

2) Find the natural length of a spring if the work done in stretching it from a length of 2ft to the lenght of 3ft is one-fourth the work done in stretching it from 3ft to 5ft.
**Force=k*x,k is a constants, what I did was the force of stretching from 2ft-3ft is F1=k*1ft, F2=4*F1...not sure if I started out right...plz show me how to do it.

3) A great conical mound of height h is built by the slaves of an oriental monarch, to commemorate a victory over the barbarians. If the slaves simply heap up uniform material found at ground level, and if the total weight of the finished mound is M, show that the work they do is 1/2h*M
**I'm totally stuck w/ this problem..what integral should i use???

2. Nov 25, 2003

### Ambitwistor

That equation tells you how high the person will rise in a given amount of time, but you need a formula for the maximum height. If you know conservation of energy, that's the easiest way; if not, then use the fact that the maximum height occurs at a time when the velocity equals zero.

It's not true that F2 = 4 F1. It's true that W2 = 4 W1. Work is the integral of force with respect to displacement. So if F(x) = k(x-L) where x is the length of the stretched spring and L is its natural length, then the work done in stretching it from length x1 to length x2 is,

$$W_{12} = \int_{x_1}^{x_2} F\,dx$$

Slice the cone up into a stack of horizontal discs, and write down a formula for the work dW needed to lift a disc of mass dm from the ground to a given height h.

Then integrate dW (you'll turn it into an integral with respect to dm, which can in turn be written with respect to dh, since the mass of a disc depends on its sizewhich for a cone depends on its height) to get the total work W.