Have you ever been close to something that smells bad? Have you noticed that the farther you get from that something, the less it smells, and the closer you get, the more it smells? Well forces sort of work in the same way.
Forces behave according to a fancy law called the inverse-square law. To be technical, an inverse-square law is any physical law stating that some physical quantity or strength is inversely proportional to the square of the distance from the source of that physical quantity.
The inverse-square law applies to quite a few phenomena in physics. When it comes to forces, it basically means that the closer an object comes to the source of a force, the stronger that force will be on that object. The farther that same object gets from the force’s source, the weaker the effect of the force.
Mathematically we can say that doubling the distance between the object and the source of the force makes the force 1/4th as strong. Tripling the distance makes the force 1/9th as strong. Let’s play with this idea a bit.
Here’s what you need:
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- magnet
- paper clip
- string
- tape
Using a magnet (the stronger the better), paper clip, string (or yarn or even dental floss!), and tape, you can make a flying paper clip.
OPTIONAL: If you happen to have a spring scale and ruler, get those out, too…otherwise, just skip these items – they are not essential to understanding the concept here.
Download Student Worksheet & Exercises
1. Tie about 4 inches of string to a paper clip.
2. Tape the magnet to the table.
3. Hold the end of the string that is not tied to the paper clip and let the paper clip dangle.
4. Slowly bring the paper clip closer and closer to the magnet.
5. Notice that the closer you get to the magnet, the stronger the force of the magnetic field is on the paper clip.
If you have a spring scale:
6. Attach the paper clip to the spring scale.
7. Move the paper clip closer to the magnet until the magnetic field affects the paper clip.
8. Measure how far the paper clip is from the magnet with a ruler.
9. Measure how much pull there is on the paper clip. Use newtons if your spring scale shows that measurement, but grams are OK if it doesn’t.
10. Bring the paper clip a half inch closer and measure the force of the pull again either in grams or Newtons.
11. Continue to get closer to the magnet half an inch at a time, measuring the force until you can’t get any closer.
What you may have noticed here was that the closer you got the paper clip to the magnet (the object causing the force field) the stronger the force was on the paper clip. You have just seen the inverse-square law in action!
Exercises:
- Circle one: The closer you get to the magnet, the (stronger | weaker) the force of the magnetic field is on the paper clip.
- Why does it matter which way you orient the magnet in this experiment?
- Which magnet has the strongest magnetic field?
- Is the north or south pole stronger on a magnet?
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