# Homework Help: Weight decreased by the combined pull of the sun and moon?

1. Feb 21, 2010

### bd24

1. The problem statement, all variables and given/known data
at a moment of total eclipse the moon lies in a straight line from the earth to the sun. If your normal weight is 600N how much is your weight decreased by the combined pull of the sun and moon?

2. Relevant equations
Mass of sun - 2.0x10^30 kg distance from earth - 1.5x10^8 km
mass of moon - 7.4x10^22 kg distance from earth - 3.8x10^5 km

so i have calculated the gravitational pull from the sun and the moon on the person of weight 600N (w =mg , W/g = m = 61.22kg)
F = (G x 61.22kg x mass of moon) / distance from earth to moon^2
F = (G x 61.22kg x mass of sun) / distance from earth to sun^2
= 3.63 x 10^-3 N

do i now just subtract this from the 600N to find my answer?

any help is appreciated

2. Feb 21, 2010

### andytuki

Re: gravity

Total eclipse is the moon's shadow onto Earth.

Sun -------> moon (((((( Earth

The Gravitational pull from the sun and moon will be in the opposite direction than Earth.

Force total = F(earth) - F(moon) - F(sun)

3. Feb 22, 2010

### bd24

Re: gravity

so net force on person standing on the earths surface during a solar eclipse is equal to (the force of the moons gravity upon the person + the force of the suns gravity upon the person) subtract the force of the earths gravity upon the person, which is given as 600N. Is this correct?

4. Feb 22, 2010

### andytuki

Re: gravity

Yeah, but I think your calculations may be wrong if you got 3.63E-3 N for the total pull from the moon and sun. Did you use meters for distance and kg for the mass?

5. Feb 22, 2010

### bd24

Re: gravity

ok, i tried again and now have a value of 0.365N for the combined pull of the sun and the moon. hope this is the right answer..