Forces and Force Diagrams

Section: Physics | Syllabus: Cambridge Primary Stage 5 Science (0097)

A Range of Forces

A force is a push or a pull. Many different named forces act in different situations.

Force	What it does
Gravity	Pulls objects towards Earth
Applied force	A push or pull from a person or object
Normal force	A support force from a surface, acting against whatever presses on it
Upthrust	An upward force from a fluid, such as water, on an object within it
Friction	Opposes movement between two touching surfaces
Air resistance	Opposes movement of an object through air
Water resistance	Opposes movement of an object through water

Several named forces can act on an object at the same time

An object can have several forces acting on it at the same time, even when it is not moving.
A stationary book on a table has gravity pulling it downward and a normal force from the table pushing upward.
If the forces acting in opposite directions are equal in size, they are balanced, and the object stays at rest or keeps moving at a steady speed in a straight line.
If one direction has a greater force than the other, the forces are unbalanced and the object's motion changes in that direction.

Draw each force as a straight arrow starting on or beside the object.
The arrow points in the direction the force acts.
Label each arrow clearly with the name of the force.
A longer arrow can represent a larger force when the diagram is comparing the sizes of forces.

A simple force diagram for a book resting on a table

Worked Example: A Ball on a Table

Question: A ball rests on a table. Name the two vertical forces and give their directions. [2]

Earth pulls the ball downward by gravity. The table supports the ball with an upward normal force. Answer

Gravity acts downward and the normal force acts upward.

MistakeA stationary object has no forces acting on it.

FixSeveral forces can act on a stationary object; they may simply be balanced.

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