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# Gravity watch

We were discussing Newton's theory of gravitation. If and apple is placed somewhere above the Earth, you can measure the force using

F=G(M1M2)/r^2
Where F=Force of gravity,
M1 = Mass of Earth
M2 = Mass of apple
r= distance between he centre of the two objects

He then asked me how can we find G, we don't know M1 or r, but we can find M2 using a weighing scale, and F using a weighing scale. With this information only, how can we find G? Not necessarily an exact value, but how can we calculate it or what experiment can be done to find it?

Thanks

Kyx
2. (Original post by kennethdcharles)

We were discussing Newton's theory of gravitation. If and apple is placed somewhere above the Earth, you can measure the force using

F=G(M1M2)/r^2
Where F=Force of gravity,
M1 = Mass of Earth
M2 = Mass of apple
r= distance between he centre of the two objects

He then asked me how can we find G, we don't know M1 or r, but we can find M2 using a weighing scale, and F using a weighing scale. With this information only, how can we find G? Not necessarily an exact value, but how can we calculate it or what experiment can be done to find it?

Thanks

Kyx
I don't know what assumptions your interviewer had in mind for calculating G given the information above, but I don't think that's possible with an apple and the earth when, in fact, G was initially calculated to work out the mean density (and hence mass) of the earth. You can browse Cavendish experiment online and find out more about how G is usually calculated.
3. Mass of earth = 6.00 x 10^24 kg.

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