Question

A 4 kg book and a 7 kg lamp are both in the living room. If the force of gravity between them is 2.99 10-10 N, how far apart are they?

Answer 1

The answer is 2.5 m. 

Hope this helps. 

Answer 2

When applying newton’s law of universal gravitation: Fg = G(m1•m2) divided by r^2

we have to rearrange to find the distance, or r^2

Plug in your known values:

G = 6.67x10^-11
m1 = 4 kg
m2 = 7kg
Fg = 2.99x10^10

Unknown:
r^2

Rearranging, we get:

r^2 • Fg = Gm1m2
divide by Fg to get r^2 alone

and boom! take the square root of 6.67x10^-11 • 4 • 7 (divided by) 2.99x10^10 and you will get 2.49 or 2.5 m as your final answer! Hope this was detailed enough to prevent future issues!