Newton’s first law and resultant force

What you will learn

Rest, constant velocity and the effect of a non-zero resultant force.

Specification mapping: B2.1

Prerequisites: B1.3

Physical principles

Newton’s first law states that an object remains at rest or moves at constant velocity unless acted on by a non-zero resultant force. The resultant is the vector sum of all external forces on the object. Motion does not require a continuing net force; a change of velocity does.

A vehicle at steady speed in a straight line has driving force balanced by resistive forces. If drive exceeds resistance, it accelerates forward. If resistance exceeds drive, forward velocity decreases while resultant force points backwards. Turning also changes velocity and requires a resultant force even if speed is constant.

The first law is framed in an inertial reference system. In everyday road problems the ground is a suitable approximation. Real moving objects often stop when propulsion is removed because friction and drag provide an unbalanced force, not because motion is naturally “used up”. A force diagram should include only forces acting on the chosen object.

Scientific diagram and interpretation

Weight acts downward, normal reaction upward, drive right and resistance left. Horizontal arrows need not balance; N = mg only under the stated vertical conditions. Schematic lengths are not force values.

Worked examples and applications

Balanced driving

A car has 800 N forward drive and 800 N backward resistance. Resultant = 800−800 = 0 N. It can continue at constant forward velocity.

Non-zero resultant

Drive rises to 1100 N while resistance initially remains 800 N. Resultant = +300 N, so velocity changes in the forward direction. Constant speed is no longer consistent with those forces.

Common misconceptions

A moving object can have zero resultant force. Zero resultant does not mean that no individual forces act.

Examination technique

Use “constant velocity” rather than just “constant speed” when stating the law.

For an explanation, connect the physical cause to its effect. For a calculation, state the model, rearrange before substitution, retain intermediate digits, and finish with units. For a data question, refer to the actual readings and distinguish a trend from proof of a cause.

Check your understanding

Five questions. Choose an option, then Check Answer. Read the feedback and use Try Again to reset. Use Tab between controls, arrow keys within radio groups, and Enter or Space on buttons. These local practice answers are not saved.