# Homework Help: Force of gravity involving astrology

1. Jul 6, 2012

### problemswithp

1. The problem statement, all variables and given/known data
People who believe in astrology think the positions of the planets at the time of birth have a large effect on the personality and destiny of the baby. The only known interaction between a planet and the baby is the force of gravity.

Calculate the force of gravity (in Newtons) between a newborn baby (mass = 4.5 kg) and the planet Mars (mass = 6.4 X 1023 kg), when Mars is at its closest to the Earth (distance = 5.6 X 1010 m).

Calculate the force of gravity (in Newtons) between a newborn baby of mass 4.5 kg and the obstetrician of mass 100 kg, who is 0.5 m from the baby.

2. Relevant equations

3. The attempt at a solution
Fg= g ( 4.50 kg)(100.0 kg)/0.5m^2 =Fg= (6.67 E -11 N*m2/kg2)( 4.50 kg)(100.00 kg)/0.5m^2

I arrive at -1.32E4 but it gives me incorrect answer. I am completely lost with this problem.

2. Jul 6, 2012

### ehild

How did you get a negative result by multiplying positive numbers?????

ehild

3. Jul 6, 2012

### cepheid

Staff Emeritus
Welcome to PF,

Your method is fine. The physics of this problem is simple. Newton's law of gravitation tells you the gravitational force between any two masses given their separation. So, use that to compute the force. You're doing it right, in other words.

Since your physics is fine, the trouble must be with your math. If you got the wrong answer using the right equation, then you must have made an arithmetic error. I'm assuming that you just used a calculator, so short of punching in the wrong thing, I don't know what could have gone wrong. Maybe you could describe what you did to compute the answer...

4. Jul 6, 2012

### problemswithp

I get a negative number by multiplying 6.67E-11 by the masses of the objects, I am not confortable with my calculator and I have probably gone wrong in the calculation, however I try repeatedly and any positive answers I get are incorrect...

5. Jul 6, 2012

### ehild

Different calculators use different keys for the magnitudes (power of 10). In the calculator supplied for my Windows XP, it is the key "EXP". It can be also "EE". E-11 means 10-11. Calculate the value without 10-11 and then multiply the result with it.

ehild

6. Jul 6, 2012

### Redbelly98

Staff Emeritus
As a check, you can do the calculation without the "e-11" or any other exponents. Then apply the exponents to the value you calculate, to get a final answer.