# Nice gravity experiment

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1. Sep 12, 2015

### shpotes

I want to explain the law of universal gravitation but i need an experiment to ilustrate it, any can help me?

What do you think are the best gravity experiment?

Last edited by a moderator: Sep 12, 2015
2. Sep 12, 2015

### UncertaintyAjay

Jumping off a cliff
Nah, I'm kidding. What exactly do you mean by gravity experiment?

3. Sep 12, 2015

### Andrew Mason

In order to show gravity you would have to do something like the Cavendish experiment using a torsion balance with large masses. It is probably best to show a video of the experiment rather than doing it because it is very difficult to set up.

AM

4. Sep 12, 2015

### Staff: Mentor

What do you mean by a "good" experiment? Simple yet profound? An intensely demanding technical effort?

5. Sep 12, 2015

### Staff: Mentor

You can let two things of different mass and shape fall down and compare the time they need. If you have access to a vacuum, you can use a feather and some compact dense object (like a steel ball) to make the experiment better.
The same experiment has been done on the moon - you don't have access to the moon in the classroom, but you can show the video.

6. Sep 12, 2015

### UncertaintyAjay

That is a beautiful experiment. You should definitely check it out

7. Sep 20, 2015

### Creator

I kinda like this one:

8. Oct 28, 2015

### tony873004

The gravitational constant G is too small to measure in most school labs. To recreate Cavendish would be too difficult for most high schools or universities.

But you can experimentally derive a maximum value for G.

Two buckets of water, each about 7 kg, 1 meter apart, would take about 9 hours to "fall" to each other if placed on a perfectly friction-free tabletop.

Intuitively the students know this won't happen.

Find the coefficient of static friction between the bucket and the table by inclining the table until the bucket slides. Then measure the slope. (either rise over run or tan of the angle) .

Let's assume it's 0.3.

The force of static friction must be at least as strong as the gravitational force between the buckets, or the buckets will begin to slide towards each other.

Now the competition between the students is to see who can experimentally measure the smallest value. They'll never get close to 6.67e-11. The competition is really who can find the densest objects (so they minimize the distance while maximizing mass) and the smoothest surface (but don't tell them that. Let them figure it out from the formulas). Don't use round objects. They must slide!
$$\begin{array}{l} f_s \ge \frac{{GMm}}{{r^2 }} \\ \mu _{s,\max } mg \ge \frac{{Gmm}}{{r^2 }} \\ \mu _{s,\max } g \ge \frac{{Gm}}{{r^2 }} \\ \frac{{\mu _{s,\max } gr^2 }}{m} \ge \frac{G}{{}} \\ G \le \frac{{\mu _{s,\max } gr^2 }}{m} \\ G \le \frac{{0.3\left( {9.8\,{\rm{m/s}}^{\rm{2}} } \right)\left( {1\,{\rm{m}}} \right)^2 }}{{7\,{\rm{kg}}}} \\ G \le 0.42\,\frac{{{\rm{m}}^{\rm{3}} }}{{{\rm{kg - s}}^{\rm{2}} }} \\ \end{array}$$

Last edited: Oct 29, 2015
9. Oct 29, 2015

### Staff: Mentor

You can improve that limit by at least one order of magnitude with a mass attached to a string. Probably two orders of magnitude. It will also make the experiment easier because you don't have to measure the sliding angle.
You get several orders of magnitude more if you do a Cavendish-like experiment to derive an upper limit.

We had that experiment in university, and the result was reasonable (~10% uncertainty).

10. Oct 29, 2015

Staff Emeritus
I believe Iowa State has their undergrads measure G. It's much easier than in Cavendish's day, with cameras and lasers available and cheap. It's important to understand how this experiment scales, or rather, how it doesn't. A bigger experiment is no better.

https://www.fourmilab.ch/gravitation/foobar/ has some interesting stuff on home gravity experiments.

11. Oct 30, 2015

### Jakecp

12. Oct 30, 2015

### Jakecp

13. Nov 19, 2015

### whitsona

I've been looking at this one lately as well. Has anyone done it?

Cavendish had to spend so much time avoiding air currents, etc. that I've been wondering about how feasible it is.

14. Nov 19, 2015

### whitsona

Oh! Just went to the link that Vanadium 50 posted. The water break seems to be the piece of it that I was missing! Can't wait to try this.