Force diagrams

Objective. Represent weight, normal reaction, tension and thrust.

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Learn: the key idea

Force diagrams show all forces acting on an object as labelled arrows, including weight (acting vertically downward, magnitude mg), normal reaction (perpendicular to a surface, pushing away from it), tension (pulling along a string towards the string), and thrust (pushing along a rigid rod away from it). For example, a book resting on a table has weight mg downward balanced by normal reaction R upward, with R=mg if stationary. Drawing a correct force diagram is essential before applying Newton's laws to find unknown forces or accelerations.

Worked example

A 5 kg box rests on a smooth horizontal table and is pulled by a horizontal force of 20 N. Find its acceleration.

1 $Draw the force diagram: weight 5g down, normal reaction R up, 20 N horizontally.$
2 $Vertically there is no acceleration, so R = 5g; the table is smooth, so there is no friction.$
3 $Horizontally, Newton's second law: F = ma gives 20 = 5a.$
4 $a = 20 \div 5 = 4 m/s².$

Answer: 4 m/s²

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Common mistake. Students draw tension in a string pushing an object, when string tension can only pull, never push. Always draw tension arrows pointing along the string towards the point of attachment.

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