Magnetic force pairs

This demonstration shows that the forces of attraction between two magnetically interacting objects are equal and opposite. You can use it as an example of Newton’s third law.

Equipment

• Two identical small, strong neodymium magnets with holes in – the diameter of the hole should be large enough to thread string through (e.g. 2.5 mm)
• Three 30 cm lengths of string
• Two clamps and stands
• Small bulldog clip
• Two G-clamps(optional)

Preparation & safety

Rare-earth magnets are brittle and shatter easily. Don’t drill a hole into an existing magnet. Source neodymium magnets with pre-made holes or make a harness out of string or wire. When handling or moving magnets towards each other ensure that they don’t collide.

Before the activity, thread strings through each of the magnets and secure with a knot. Also tie one end of a string to the bulldog clip. Suspend the two magnets from a clamp stand so they hang around 10 cm below where the string is tied.

Setting the distance between the magnets and magnet and clip can take a bit of practice. Try it out beforehand. Mark positions on the bench and/or secure stands with G clamps to allow a quicker set-up next time.

Procedure

1. Attach one magnet by its string to a clamp and secure the string tightly so that the magnet hangs around 10 cm below where the string is tied.
2. Hang a second magnet from the second clamp. Arrange the stands so that the two magnets are close and attracting each other strongly with their strings almost horizontal.
3. Repeat step 2 but replace one of the magnets with the bulldog clip.

Discussion prompts

• What causes the magnetic force on the left magnet? What about the right magnet?
• What causes the forces on the magnet and clip?
• How do the size and direction of the forces compare?

Teaching notes

Students will be aware that magnets can attract each other and so will accept that two identical magnets pull equally on each other. The force on the left magnet is due to the right magnet; the one on the right is due to the left.

They may be surprised to see the same effect with the magnet and clip. These need to be closer to produce the same size forces but as previously the size of the forces are equal in size and opposite in direction. Like all interactions, magnetic interactions create Newton’s third law force pairs.

Learning outcome

Students identify Newton’s third law force pairs for objects that interact magnetically.

This experiment was safety-checked in March 2020.