For every action there is an equal and opposite reaction
Introduction
Feel the force one film can of magnets exerts on another. Even though one film can contains three magnets and the other contains only one the forces they exert on each other are equal and opposite. This provides a model for gravitational attraction. The force the earth exerts on me is the same as the force I exert on the earth.
Material
Four film cans.
8, 1.25 inch diameter donut magnets (Available from Radio Shack)
Clay
paper, or Styrofoam peanuts
Assembly
Make a stack of the 8 donut magnets. In the stack all of the north
poles will point in the same direction.
Place three magnets into each of two film cans and one magnet into
each of the remaining two film cans.
Fill the empty space in the three magnet film cans with paper or
Styrofoam peanuts and close the can lids.(This will keep the magnets
pressed against the bottom of the film can.)
Check to see that all of the film cans repel each other when the base
of one is brought up to the base of another. If they attract, flip
over one set of magnets.
Remove the paper from the cans with one magnet.
Put clay into each of the film cans with one magnet until each has
approximately the same mass as the threemagnet cans.
Fill the remaining space with packing material and close the lid.
To Do and Notice
Bring the bottoms of two, onemagnet film cans together.
Feel the force of repulsion between the cans when the bottoms
touch.
Bring the bottoms of two threemagnet film cans together.
Feel the force of repulsion between the cans when the bottoms
touch.
Compare the two forces.
The force of three magnets on three magnets should be noticeably
greater than the force of one on one.
Notice that the force increases in strength as the distance between the film cans is made smaller.
Now, bring the bottom of a onemagnet film can up to the bottom of
a three magnet film can.
Notice that the force is greater than the force that one magnet
exerts on one, and less than the force that three exert on three.
Notice that the force that each film can exerts on the other is
the same.
The force that three magnets exert on one magnet is the same as the
force that one magnet exerts on the three. You cannot tell which can
has one magnet in it and which can has three magnets in it.
What’s Going On?
More magnets exert more magnetic force.
The magnetic forces between two magnets are equal and opposite so
that the force that one magnet exerts on three is the same as the
force that three magnets exert on one.
it is the same with gravitational forces. The force that the massive
earth exerts on me is the same as the force that I exert on the
earth.
So What?
The force of gravity between two masses like you and the earth acts the same way. The force that the earth exerts on you is the same as the force that you exert on the earth.
The electric force between two charges acts the same way the force of two electron charges on a single electron charge is equal to the force of one electron on two.
Going Further
In the modern theory of quantum electrodynamics the magnets exert a force on each other by exchanging particles known as virtual photons.
Etc
You might expect that three magnets will exert three times the force that one magnet will exert. However there are two effects which change the actual force from this expectation. The second and third magnets are further away than the closest magnet and so exert less force than the nearest one. In addition, some magnets particularly those made of soft iron intensify the magnetic field of a more distant magnet. The intensification proceeds to a point called saturation and then stops. Ferrite magnets like those used here do not intensify the field of other magnets.
Etc.
If you hook two spring scales together, such as those used to weigh fish, and then pull them apart, both spring scales will always have the same reading.
Scientific Explorations with Paul Doherty 

20 October 2000 