Section 2.1: Newton’s Laws of Motion

Newton’s Laws describe how objects move in response to forces.

First Law (Law of Inertia):

An object remains at rest or in uniform motion unless acted upon by a net external force.

Second Law:

The acceleration of an object is proportional to the net force acting on it and inversely proportional to its mass:

\[ \vec{F}_{\text{net}} = m \vec{a} \]

Third Law:

For every action, there is an equal and opposite reaction. Forces always occur in pairs.

Example 1

A 10 kg box is pushed with a force of 50 N across a frictionless floor. Find its acceleration.

\( a = F/m = 50/10 = 5 \text{ m/s²} \)

A book of mass 2 kg rests on a table. What is the reaction force exerted by the table?

Reaction force = weight = \( mg = 2*9.8 = 19.6 \text{ N} \)

A 5 kg object is pulled by a 20 N horizontal force. Find acceleration.
An object in space remains stationary until a 15 N force acts on it. What does Newton’s first law predict?
A 50 N force acts on a 10 kg mass. Find acceleration and direction.
Two skaters push off each other: Skater A mass 60 kg, Skater B 40 kg. Compare their accelerations.
A box rests on a table. Draw all forces acting on it and identify Newton’s third law pair.