# Newton's Gravity Theory Question

#### GeorgeGR

I need help with this question from "Physics for scientists and engineers by Randall Knight, Chapter 13".
In the following figure, what is the graditude and direction of gravitational force over a mass of 20 kg?
The author's answer is 3*10^(-7) N but I found 6.28*10^(-7) N.

#### ChipB

PHF Helper
I don't know what "gratitude" is, but I calculate the magnitude of the force acting on the 20 Kg mass to be 6.09 x 10 ^(-7) N. It seems the book answer is incorrect.

topsquark

#### Cervesa

what is the graditude and direction of gravitational force over a mass of 20 kg?
I assume you mean magnitude vice "graditude", also I'm not sure what the exact meaning of "gravitational force over a mass of 20 kg". If the statement means the net gravitational force exerted on the 20 kg mass by the two 10 kg masses, then your solution is close to what I calculated,

$F_{net} = 2 \cdot \dfrac{G \cdot 200}{(0.05^2+0.20^2)} \cdot \dfrac{0.20}{\sqrt{0.05^2+0.20^2}} = 6.09 \times 10^{-7} \text{ N}$

topsquark

#### GeorgeGR

I assume you mean magnitude vice "graditude", also I'm not sure what the exact meaning of "gravitational force over a mass of 20 kg". If the statement means the net gravitational force exerted on the 20 kg mass by the two 10 kg masses, then your solution is close to what I calculated,

$F_{net} = 2 \cdot \dfrac{G \cdot 200}{(0.05^2+0.20^2)} \cdot \dfrac{0.20}{\sqrt{0.05^2+0.20^2}} = 6.09 \times 10^{-7} \text{ N}$
Thank you! It was Google's translation.

#### GeorgeGR

I don't know what "gratitude" is, but I calculate the magnitude of the force acting on the 20 Kg mass to be 6.09 x 10 ^(-7) N. It seems the book answer is incorrect.
Thank you! I translated from Google translator. I thought books like this doesn't have mistakes.

#### Cervesa

Thank you! I translated from Google translator. I thought books like this doesn't have mistakes.
You'd be surprised at the number of mistakes in the answer section of a text. The author (usually a professor) has his/her grad student assistants work out the problems.