PPT ActivityBreaking Strength

Developer Notes

It would be nice to have an activity that shows that strength is not relative to volume, but I can’t think of an easy one.

Goals

Show that the breaking strength of materials is relative to their cross-sectional area, not to their diameter.

Concepts & Skills Introduced

Area / Concept
physics / the breaking strength of objects increases proportional to their cross-sectional area

Time Required

45 min

Presentation

Introduce the topic by asking how you make an object less likely to break. There should be quite a few answers ranging from using a different material to combining materials to changing shape to making it bigger. Bigger is the one we’re interested in here. If you make it twice as thick, is it twice as strong? Let’s find out.

Assessment

Warm-up Question

Writing Prompts

Relevance

Background / History

How strong is a rope? If you make the rope twice as thick, is it twice as strong? If you want to save on construction materials, so you make the diameter of columns in a building half as wide, will they be half as strong?

Problem

What is the strength of a material relative to? Thinner pieces of spaghetti will break more easily than fatter pieces. But if the noodle is twice as thick, does it take twice as much force to break it?

Materials

1spring scale (5 N, or 500 g)

10uncooked spaghetti noodles

10uncooked spaghettini noodles

10 uncooked angel hair noodles

2small blocks

1paper clip bent in an “s” shape

Procedure

  1. Work in groups of two.
  2. Find the diameter of the noodles.
  3. Make a table and record the relative breaking strengths of the noodles.
  4. Set up the blocks so they are about 30 mm apart. Be sure to keep them at exactly the same spacing for the whole activity.
  5. Place a noodle so it spans the space between the blocks. Hook the paper clip around the noodle and hook the spring scale through the other end of the paper clip. Pull slowly until the noodle breaks. Record the data.

  1. Break all 10 noodles – 10 trials for each size of noodle. (Don’t use dis-colored noodles. If they have light areas, those are fractures which will give you bad data.)
  2. Clean up.
  3. Analyze your data, comparing the diameter of the noodles and their breaking strength. Can you find a pattern?

Summary

  1. Are the thicker noodles harder to break than thin ones? Why?
  2. Is the breaking strength of noodles proportional to their diameter? How can you tell from your data?
  3. Is the breaking strength proportional to some other function of their diameter? What? Show it from your data.
  4. Do you think your conclusion is generally true for the strength of materials? Why?

Exercises

  1. If you make a rope longer, does that make it stronger? How about if you make it thicker?
  2. The diameter of a human femur (thigh bone) is about 2.0 cm, and a human weighs about 60 kg. If there was a human who weighed 600 kg, what would the diameter of the femur have to be to have the same strength to weight ratio?
  3. If a cylindrical piece of cement breaks when it is compressed at 280 kg/cm2, how much pressure would a piece of cement with twice the diameter hold?

Challenge/ extension

  1. Try breaking the same kind of noodles while changing the spacing between the blocks to find that pattern.
  2. Actually, some ropes are less likely to break if they are longer, even if they’re not thicker. Climbers use nylon ropes for safety in case they fall. Why might a longer nylon rope be less likely to break than a shorter one during a fall?

statics act breaking strength 030108 dk01.doc1 of 3Printed: 10/9/20183:35 PM

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