Week 24 - Measuring Forces and Designing Structures to Resist Forces

Exercise 2.3

Lesson 2.3: Internal Forces within Structures

ACTIVITY A: Internal Forces - Compression, Tension, Shear, and Torsion

Textbook Readings

Science in Action 7
pages 296-298

Science Focus 7

pages 307 to 310

Internal forces are the forces that act within the materials from which structures are built. Some forces are pulling forces and some are pushing

Compression is the force that acts to push things together. Think of the word compress - this means to squeeze things together.

Tension is the force that acts to pull things apart. Materials will stretch to carry a load. At the point where the material can not take any more force the material can snap or break due to the force of tension.

Watch the "What is Tension/Compression?" video then read the section in the textbook before answering the questions that follow.


These forces do not have to act alone. Often these forces act together at the same time. When two internal forces act at the same time they are said to be complementary forces. The bending of a beam as you apply your weight to it is an example of complementary forces. The top of the beam is being compressed or squished and the bottom of the beam stretches or tries to pull apart due to the tension in the beam.

Shear is another effect that is simply the combination of tension and compression. It is the force that pushes parts that are in contact with one another apart in opposite directions. 

The last internal force to point out is torsion. Torsion forces twist a material by turning the ends in opposite directions.

Next, watch the video, then read the section in the textbook before answering the questions below.


Exercise 2.3: Internal Forces Within Structures