# Cover

What is a force?
What are examples of internal forces?
What are examples of external forces?
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?
B
C
A
F
D
E
G
Answers: A internal B internal C external D external E external F external G external H internal
H
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