Newton's First Law of Motion

Newton’s First Law explains what happens when forces are balanced. It describes how objects behave when there is no overall (resultant) force acting on them.

What the law says

Statement: An object remains at rest or moves in a straight line at constant speed unless acted on by a resultant force.

In symbols:

$$\sum \vec{F} = \vec{0} \;\Rightarrow\; \text{velocity is constant (at rest or straight-line, constant speed)}$$

Key ideas

• Resultant force is the single force that has the same effect as all forces combined. If forces are balanced, the resultant is zero.
• Zero resultant force means no change in velocity. The object stays still or keeps the same speed and direction.
• A change in speed or direction needs a non-zero resultant force.
• Inertia is an object’s tendency to resist changes in motion. Greater mass means greater inertia (harder to start, stop, or turn).
• In real life, friction and air resistance act on moving objects. To keep a steady speed, a driving force often balances these resistive forces.

Real‑world connections

• A puck on an air table glides in a straight line at constant speed because friction is tiny, so the resultant force is close to zero.
• When a car suddenly stops, passengers move forward. Their bodies tend to keep the same velocity (inertia), so seatbelts provide the force to change it.
• Cycling at steady speed: thrust from pedalling balances friction and air resistance; the resultant force is zero.

How to analyse a situation

1. List the forces (weight, normal reaction, thrust/engine force, friction, air resistance).
2. Compare sizes and directions to find the resultant in each direction.
3. If the resultant is zero, the object is at rest or moving at constant velocity. If not zero, its velocity changes in the direction of the resultant.

Common misconceptions

• “A force is needed to keep something moving.” Incorrect. A force is needed to change motion. With no resultant force, motion stays the same.
• “At rest” and “moving steadily” are different in this law. Both mean zero resultant force.
• Turning a corner at constant speed is not First Law behavior; direction changes, so a resultant force acts.