Structures & Mechanisms

Grade 5 Science Unit

Answer the following based on your prior knowledge

What is a force?

What are examples of internal forces?

What are examples of external forces?

Answer the following based on your prior knowledge

What natural phenomena may affect structures?

What are some examples of natural structures?

What are examples of man-made structures?

A force is any push or pull. It can cause an object to change its movement or shape. Structures need to be built to withstand all the forces it may be subject to. If a structure is not built properly it might fall down. There are two main types of forces that act upon structures- external and internal.

Force and Motion

External forces act upon an object from outside of the object. Gravity is a great example of an external force as it pulls structures to the Earth's centre. It acts on the structure from the outside. Wind is another example because it pushes on the outside of objects. When you sit on a chair, your weight is an external force acting on the chair.

Internal forces also act upon an object from the inside not the outside. An internal force is caused by one part of a structure acting on another part. The legs on a table must withstand the force of the tabletop.

External and Internal Forces Explained

Is an External or Internal Force acting on object?

Answers: A internal B internal C external D external E external F external G external H internal

An example of an external force that threatens a structure is weather. Bridges are built to withstand the weight of cars/trucks, wind, rain and other natural forces. Basically, it is weight! All of these outside forces are dynamic (moving) loads. A bridge or any other structure also has to deal with static (non-moving) loads such as the materials used to build it and the load of gravity. Internal forces are constantly acting on structures. There are four internal forces that act on structures- compression, tension, shear and torsion. (watch short videos on next page if unsure)

Use the pen to match the word to the definition

Torsion

A force that twists

Shear

A force that squeezes or presses something together

Tension

A force that stretches apart to expand or lengthen

Compression

A force that pushes in opposite directions

Five Forces Animated

Compression and Tension

Static vs. Dynamic Load