From inertia to the concept of net force

Reminder: Newton’s first law and inertia

Before moving on to the second law, let us recall the principle of inertia (Newton’s first law): an object remains at rest or in uniform linear motion as long as no net force is acting upon it. In other words, if the forces acting on it cancel each other out (their vector sum is zero), the velocity does not change, either in direction or magnitude.

What is the resultant force?

An object is often subject to several forces at the same time: weight, normal reaction from the support, tension in a string, friction, etc. The resultant force (denoted as the sum of the F’s) is the vector sum of all these forces. It is this, and this alone, that determines whether the motion changes.

Example: a box resting on a table is subject to its weight P (downwards) and the normal reaction N from the surface (upwards). If P = N, the resultant is zero: the box remains at rest.

Why go further?

The first law tells us what happens when the resultant force is zero. But what happens when it is not? This is precisely the question answered by Newton’s second law, which we will explore in the next lesson. It links the resultant force to the change in velocity, that is, to acceleration.

Key points

Concept	Symbol	Unit
Force	F	Newton (N)
Mass	m	kilogram (kg)
Acceleration	a	m/s²

Common pitfall: do not confuse ‘constant velocity’ (zero net force) with ‘stationary object’: an object moving in a straight line at a constant speed also has a net force of zero, even though it is moving.