Deck 3 — Free-Body Diagrams & Newton's Laws

Free-Body Diagram (FBD)

A diagram showing all forces acting on one object.

Purpose of a Free-Body Diagram

To represent all forces acting on a single object.

Rule for Drawing an FBD

Draw the object as a box.

Force Arrows in an FBD

Arrows should touch the object and be labelled.

Arrow Length in an FBD

Longer arrows represent larger forces.

Weight in an FBD

The weight arrow starts from the centre of the object.

Common FBD Mistake

Do not include forces exerted by the object on another object.

Balanced Forces

Forces with a resultant force of 0 N.

Resultant Force of Balanced Forces

0 N.

Newton's First Law

An object remains at rest or moves with constant velocity unless acted upon by an unbalanced force.

Balanced Forces

They do not mean no forces; they mean the forces cancel out.

Unbalanced Forces

Forces with a resultant force that is not equal to 0 N.

Resultant Force of Unbalanced Forces

Not equal to 0 N.

Effects of Unbalanced Forces

Can start motion, stop motion, speed up, slow down or change direction.

Newton's Second Law

Unbalanced forces cause changes in motion.

Newton's Third Law

For every action force, there is an equal and opposite reaction force.

Action-Reaction Forces

Have equal magnitude, opposite direction, are the same type of force, act on different objects and always occur in pairs.

Walking Action-Reaction Pair

Foot pushes the ground backward; the ground pushes the foot forward.

Rowing Action-Reaction Pair

Oar pushes water backward; water pushes the boat forward.

Balloon Action-Reaction Pair

Balloon pushes air backward; air pushes the balloon forward.

Rocket Action-Reaction Pair

Rocket pushes exhaust backward; exhaust pushes the rocket forward.