Video: Instructor’s video
Description: A counter intuitive demonstration that exposes students’ common misunderstandings about tension and scales.
- One table
- Two 1kg masses
- One hanging Newton scale
- Two table clamps
- Two vertical posts
- Two s-clamps
- Two pulleys
- Two thin strings, each approximately 1m long
- Set up the posts in the table clamps.
- Attach the pulleys to the tops of the posts with the s-clamps. Make sure the pulleys are at roughly the SAME LEVEL.
- Tie loops in the ends of the strings.
- First, hang a 1kg mass vertically from the spring scale, to show that the scale reads about 10N, as one would expect.
- Next, ask the class what the scale will read when the weight is hung on a string, which passes through one pulley and then attaches horizontally to the scale, which is affixed to a stiff post (see photo).
- Show class that the scale still reads about 10N* in this configuration, proving that the only thing that changes is the direction of the string tension, NOT the magnitude.
- Ask the class what the scale will read if instead of affixing the scale to a stiff post, you hang a SECOND 1kg mass from a string, around a pulley, and then attached to the other end of the scale. Nearly all of them will guess 20N!
- Show them that even in this configuration, the scale reads about 10N*. A free-body diagram of the scale can help show when affixed to the stiff post, the scale felt two (roughly) equal and opposite forces: the tension of in string and the force from the post. In the last configuration, the free-body diagram is almost the same; the two forces are both tensions in the strings. In both cases, the scale reads the magnitude of one force, not both.
Note: In the second configuration-and even more so in the third-the scale will read slightly LESS than 10N. This is because when the scale is horizontal, part of the 10N tension goes to a vertical component that counteracts the weight of the scale, but doesn’t contribute to the scale’s reading. In the second configuration, the stiff post contributes some of the upward force, and thus less has to come from tension, so the effect is less noticeable there.