# Circular Motion Question

1. Apr 22, 2014

### Shauta

1. The problem statement, all variables and given/known data
What must be the period of rotation of the Earth on its axis so that a person at the equator will have a reading on the scale that is approximately one fifth as much as he would at the North Pole?

(Given on the formula sheet)
Radius of Earth = 6.37 x 10^6 m

2. Relevant equations
Fc = m*ac
ac = (4*(pi^2)*r)/(T^2)
Fg = m*g

3. The attempt at a solution
I had tried making Force of gravity (Fg) equal to Centripetal Force (Fc), then solving for T. Though this gives me an answer that is close to the one that my teacher gave me, the process doesn't feel right. I then tried repeating the process, but first multiplying Fg by 1/5 (as stated in the question), however, this also doesn't give me the right answer.

2. Apr 22, 2014

### Simon Bridge

Modelling the Earth as a uniform sphere and neglecting air pressure ;)

If the force of gravity is equal to the centripetal force, then you are orbiting the Earth ... how hard would you be pressing on the ground?

Start by drawing a free body diagram for the object being weighed.

3. Apr 22, 2014

### Shauta

I think I understand what you are trying to say. If I'm orbiting the Earth, then I'm not exactly pressing on the ground right? But, now I'm confused, because wouldn't that make Fc equal zero then?

4. Apr 22, 2014

### Simon Bridge

The centripetal force needed to keep you on a circular path is not the same thing as your weight, which, in turn, is not the same thing as how hard you press into the ground.

Do not confuse the weight on a set of scales with the force of gravity.
The second is the technical meaning of the word weight in physics and the first is the casual everyday meaning of the word weight.

In your free body diagram - you have a person standing on the scales on the surface of the Earth at the equator.
What are the physical forces acting on the person? Which direction do they act?