# Physics Help

• May 5th 2007, 04:38 AM
janvdl
Physics Help
This problem makes use of Newtons Universal Gravitation Formula (F = (G.m1.m2)/(r^2))

In an experiment, using Cavendish's Apparatus(whatever this is!!!) to measure G, the mass of each small sphere is 1,0 kg and the mass of each bigger sphere is 5,0 kg.

The distance between the midpoints of the smaller spheres and the midpoints of the bigger spheres is 80 millimetres. (From the edges of the horizontal beam)

A force of 10^-7 Newton is noted.

Calculate G, with the mass of the beam excluded. :eek:

I have no idea what this Cavendish Apparatus is, so i cant imagine what this thing looks like or how it works.
• May 5th 2007, 07:47 AM
CaptainBlack
Quote:

Originally Posted by janvdl
This problem makes use of Newtons Universal Gravitation Formula (F = (G.m1.m2)/(r^2))

In an experiment, using Cavendish's Apparatus(whatever this is!!!) to measure G, the mass of each small sphere is 1,0 kg and the mass of each bigger sphere is 5,0 kg.

The distance between the midpoints of the smaller spheres and the midpoints of the bigger spheres is 80 millimetres. (From the edges of the horizontal beam)

A force of 10^-7 Newton is noted.

Calculate G, with the mass of the beam excluded. :eek:

I have no idea what this Cavendish Apparatus is, so i cant imagine what this thing looks like or how it works.

See here for a description of the apparatus and experiment.

RonL
• May 5th 2007, 08:58 AM
janvdl
Thank you very much Captain Black.
• May 5th 2007, 09:07 AM
janvdl
Ok, so:

F = G(1 x 5)/(0,08m)^2
10^-7 = G(5)/0,0064

G = (10^-7 x 0,0064) / 5

But now my answer is = 1,28 x 10^-10

And the answer should be: 6,4 x 10^-11

If only the number under the line was 1000 instead of 5 it would have worked. Where did i go wrong?? :confused:
• May 5th 2007, 06:28 PM
topsquark
Quote:

Originally Posted by janvdl
Ok, so:

F = G(1 x 5)/(0,08m)^2
10^-7 = G(5)/0,0064

G = (10^-7 x 0,0064) / 5

But now my answer is = 1,28 x 10^-10

And the answer should be: 6,4 x 10^-11

If only the number under the line was 1000 instead of 5 it would have worked. Where did i go wrong?? :confused:

There are TWO sets of spheres in the Cavendish experiment, so
F = 2*G*m1*m2/r^2