07 Strong Magnet, Weak Paperclip

Aim

To show that Newton’s third law remains valid, even in the more complex cases.

Subjects

• 1H10 (Action and Reaction)

Diagram

Fig. 89 .

Equipment

• Strong horseshoe magnet, attached to a rope.

• Paperclip, attached to a string.

• Two force-sensors.

• Bars (aluminium) and clamps to build the set-up (see Diagram).

• Interface and computer with data-acquisition software.

• A camera and a large screen monitor.

• Projector to project the graphs.

Presentation

The demonstration is presented as a tug-of-war between a heavy, strong horseshoe magnet and a light paperclip. After demonstrating the strength of our magnet, the setup is as shown in the Diagram. Using a camera, the magnet and paperclip are presented in more detail on a large monitor screen. The graphs, still blank, are projected using a projector (see Figure 90).

Fig. 90 .

First, the force sensors are set to zero. Students see this in the projected graph while we use the monitor mode of the data-collection program. The effect of pulling by hand on the force sensors is shown. (We make the graphs in such a way that when pulling the magnet downward, its graph goes negative. Vise versa, when pulling the paperclip upwards, its graph goes positive.)

Then, the data is collected as a recording and is acquired by sliding the lower force sensor upward along the aluminium bars. The paperclip is brought closer to the magnet until it touches one of the poles. Afterward, the lower force sensor is lowered again (detaching the paperclip from the magnet pole) to its point of origin. The data collection is stopped.

The recorded data are discussed now; a region of interest can be selected (see Figure 90). It can be observed that the force-time relationship is a complicated one; nevertheless, Newton’s third law is valid: at every moment in time, we see \(F_{\text {paperclip }}=-F_{\text {magnet }}\).

Explanation

There is no explanation here, since Newton’s laws are just a set of hypotheses which appear to agree with our everyday experience. Our demonstration is another experiment demonstrating the validity of the third law.

Remarks

• In the demonstration, inevitable vibrations arise, especially in the data of the force sensor with the heavy magnet. An appropriate sampling rate and data-averaging should be selected to minimize these vibrations in the presented data. However, the students could also visually take the average of the graph of the magnet and see that it is in accordance with the graph of the paperclip.

Sources

• The Physics Teacher, Vol.39, October2001, pag. 392-393