Newton's first law

An object will remain at rest or in uniform motion in a straight line unless acted upon by an external force.

Key points:

• A stationary object will stay still until a resultant force acts upon it.
• An object in motion continues to move at a constant speed and direction unless a resultant force changes its speed or direction.

Newton's second law

Newton’s second law states that the rate of change of momentum is directly proportional to the net force experienced.

$\text{F = }\frac{\mathrm{\Delta }\text{P}}{\text{t}}$

Where:

• F = net force in newtons (N)
• $\mathrm{\Delta }\text{P}$ = change in momentum (kg m s${}^{-1}$)
• t = time in seconds

Newton’s second law can also be expressed as:

$F=ma$

Where:

• F = resultant force in newtons (N)
• m = mass of the object in kilograms (kg)
• a = acceleration in meters per second squared (m s^-2)

Worked example - Calculating the force acting on an accelerating object

A car of mass 1,000 kg accelerates at 2 m s^-2. Find the net force.

Step 1: Formula

F = m x a

Step 2: substitution and correct evaluation

F = 1,000 x 2 = 2,000 N

Newton's third law

Newton’s third law states that for every action, there is an equal and opposite reaction.

Interaction of Forces:

When one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

• The forces are always of the same type (e.g., both gravitational )
• The paired forces act on different objects.