# Gravity Meter Problem

## Homework Statement

A superconducting gravity meter can measure changes in gravity of the order = 1×10-11 (delta g)/g. You are hiding behind a tree holding the meter, and your 78 kg friend approaches the tree from the other side. How close to you can your friend get before the meter detects a change in g due to his presence?

## Homework Equations

I know that Fgrav = G (m1m2)/r^2 and G = 6.672e-11 Nm^2/kg^2

## The Attempt at a Solution

My first thought was to assume the m2 was 1kg and set the Fgrav = 1e-11 N, but this was wrong.

Last edited:

Kurdt
Staff Emeritus
Gold Member
You are using the equation that describes the force between two objects due to gravity. What a gravity meter detects is variance in the gravitational field. What physics do you know of the gravitational field?

Very Little

I dont think we have discussed yet in class, let me check the book for any references. This is a first year Mechanics course for natural science majors.

Hmm, there is a very brief explaination in the chapter we are in showing that the field is equal to G(M)/r^2, which is simular to what i did before but this would give me N/kg, do i factor his mass in again to get some cancelations?

Kurdt
Staff Emeritus
$$g =G\frac{M}{r^2}$$