# Homework Help: Related rates of man walking

1. Feb 23, 2007

### Rasine

A man starts walking north at 6 ft/s from a point P. Five minutes later, a woman starts walking south at 2 ft/s from a point 500 ft due east of P. At what rate are the people moving apart 30 min after the woman starts walking?

so i was ableto set up a right triangle using the x-y plane (y being north and x being east)

so i was trying to relate the components with distance^2=x^2+y^2
then where i differientate i get dd/dt=dx/dt+dy/dt

i know dy/dt=8 and i want to know dd/dt

there dx/dt=0 becuase there is no change in x.....x is always 500 ft

so would dd/dt=dy/dt=8 ????????

2. Feb 23, 2007

### HallsofIvy

Then you differentiated wrong! If $dd^2= x^2+ y^2$ then, differentiating, 2dd d(dd)/dt= 2x (dx/dt)+ 2y (dy/dt). Since dx/dt= 0,
2dd d(dd)/dt= 2y(dy/dt) so dd d(dd)/dt= 2y dy/dt. Now, 30 minutes after the woman starts walking (35 minutes after the man starts walking) what are dd and y?

Very close but not exactly!

3. Feb 23, 2007

### silvagirl19

Pyramid shaped tank...?

Does anyone know how to do this problem on related rates?:

The base of a pyramid-shaped tank is a square with sides of length 8 meters, and the vertex of the pyramid is 8 meters above the base. The tank is filled to a depth of 2 meters, and water is flowing into the tank at the rate of 4 cubic meters per minute. Find the rate of change of the depth of water in the tank. (Hint: the volume of a pyramid is given by V=1/3bh, where b is the base area and h is the height of the pyramid)

4. Feb 23, 2007