Lesson D8: Internal Forces

Key Ideas   Unit Checklist

  Video Lesson

In addition to the outside forces that act on a structure, forces come from within the structure, too. Watch this video to learn more about the various internal forces that act on structures.

  Lesson D8: Internal Forces


Figure D.2.8.1 – A one-storey house experiences compression force from the roof.

Figure D.2.8.2 – A Mongolian yurt has a sturdy roof to support the compression force from heavy snow in winter.


Figure D.2.8.3 – Multiple floors of apartment buildings push down on each other with compression force.
Reading and Materials for This Lesson

Science in Action 7
Reading: Pages 296-299

Materials:
spaghetti, marshmallows, wooden skewers, tape, plastic spoon, rubber band

Internal Forces in Homes - Compression

When humans build homes like the ones you see above, they need to consider the internal forces acting on the structure so that the home is safe. Houses look different around the world, but most need to account for compression force so that the structure can hold up the roof and multiple levels of the house.

Home builders must account for shear force, which can be caused by strong winds or earthquakes. In many countries, building codes ensure dwellings are constructed safely.

Internal Forces in Homes - Tension

Traditional First Nations tipis are very unique. They rely on tension rather than compression to remain standing.

Click here to try an online tipi-building activity. This activity shows you how tension force is used construct a tipi. This activity uses Flash, so if your computer can't run it, don't worry – this activity is optional, move on to the Amazing Stronginsky game below.

 Watch More

Five Forces

Watch this animated video to see some more examples of the types of internal forces that occur in everyday life.


  Try It!

Spaghetti Forces

At home, try an experiment involving internal forces.

Materials:
  • handful of dry spaghetti

Instructions:

  1. Hold a small handful of spaghetti with your hands on opposite ends of the spaghetti bundle.
  2. Bend slowly the handful of spaghetti until some of the spaghetti starts to break. What do you observe?

Figure D.2.8.4  -  Spaghetti.
Questions:

Think about the following questions very carefully; then, type or write your answers. After you have your answers, click the questions for feedback.

Tension and compression work together to cause bending.
When you bend the spaghetti bundle, you are applying compression to the inside of the bundle and tension to the outside of the bundle. When the tension becomes too great on the outside of the bundle, the spaghetti snaps.

 Watch More

Medieval Catapults

Watch this video to see a medieval catapult in action. Look carefully to see how the twisted ropes make the catapult work. The twisted ropes (near the base) show torsion force in the structure.


  Try It!

Build a Catapult

Catapults have been used as weapons since ancient times. A catapult uses tension (tightness) and torsion (twisting) forces to fling a heavy object a long distance.

Try building your own catapult at home!

Materials:

  • 5 marshmallows
  • 7 skewers
  • tape
  • plastic spoon
  • rubber band

Figure D.2.8.5 – Catapult.
Instructions:

Watch this video for instructions on how to build your catapult:


Questions:

Think about the following questions very carefully; then, type or write your answers. After you have your answers, click the questions for feedback.

A triangle is a strong shape. Building the catapult base as a triangular pyramid makes a strong structure.
When you pull back the rubber band and spoon in your catapult, you apply tension force. When you let go of the spoon, the tension is released, causing the marshmallow to fly into the air.



  Make sure you have understood everything in this lesson. Use the Self-Check below, and the Self-Check & Lesson Review Tips to guide your learning.

Unit D Lesson 8 Self-Check

Instructions


Complete the following 6 steps. Don't skip steps – if you do them in order, you will confirm your understanding of this lesson and create a study bank for the future.

  1. DOWNLOAD the self-check quiz by clicking here.

  2. ANSWER all the questions on the downloaded quiz in the spaces provided. Think carefully before typing your answers. Review this lesson if you need to. Save your quiz when you are done.

  3. COMPARE your answers with the suggested "Self-Check Quiz Answers" below. WAIT! You didn't skip step 2, did you? It's very important to carefully write out your own answers before checking the suggested answers.

  4. REVISE your quiz answers if you need to. If you answered all the questions correctly, you can skip this step. Revise means to change, fix, and add extra notes if you need to. This quiz is NOT FOR MARKS, so it is perfectly OK to correct any mistakes you made. This will make your self-check quiz an excellent study tool you can use later.

  5. SAVE your quiz to a folder on your computer, or to your Private Files. That way you will know where it is for later studying.

  6. CHECK with your teacher if you need to. If after completing all these steps you are still not sure about the questions or your answers, you should ask for more feedback from your teacher. To do this, post in the Course Questions Forum, or send your teacher an email. In either case, attach your completed quiz and ask; "Can you look at this quiz and give me some feedback please?" They will be happy to help you!


Self-Check Time!
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Self-Check Quiz Answers

Click each of the suggested answers below, and carefully compare your answers to the suggested answers.

If you have not done the quiz yet – STOP – and go back to step 1 above. Do not look at the answers without first trying the questions.

The roof of the Parthenon pushes down on the columns due to gravity. This is an example of compression in a structure.
The Parthenon is made of stone – marble, to be specific. Marble is a heavy, strong material. The marble columns of the structure are strong enough to withstand the compression pushing down from the roof.
When people jump on a trampoline, they push down on the trampoline, which is compression. This pulls down on the springs on the edges of the trampoline further than usual. When a group of people jump at once, a large amount of tension is applied to the springs. When this tension force is released, people are bounced higher into the air than if a single person bounces. This might result in injuries when they land.
To build a tire swing, you use a rope to hang a tire from a tree branch. When people sit on the tire, their weight pulls down on the rope. This pulling on the rope is tension force, and you should be concerned about the strength of both the rope and the branch to withstand that tension.
If you were building a plank bridge, you should think about making the bridge strong enough not to bend. The two complementary forces involved in bending are tension and compression.