2.3 Newton's Laws

Newton's First Law states: An object will stay at rest or continue to move at a constant velocity unless a resultant force acts on it.

This property of objects resisting a change in their motion is called inertia. More mass means more inertia.

This means:

• If the forces on an object are balanced (resultant force is 0 N), a stationary object will stay stationary.
• If the forces are balanced, a moving object will keep moving at the same speed and in the same direction.

For example, a car travelling at a constant 70 mph has balanced forces. The driving force from the engine is perfectly balanced by the backward forces of air resistance and friction.

Newton's Second Law states: The acceleration of an object is directly proportional to the resultant force acting on it and inversely proportional to its mass.

This gives us the most important equation in mechanics:

Resultant Force = Mass × Acceleration

F = m × a

Where:

• F is the resultant force in newtons (N)
• m is the mass in kilograms (kg)
• a is the acceleration in metres per second squared (m/s²)

This law means that a large force will produce a large acceleration, but a large mass will resist acceleration (this is inertia).

Newton's Third Law states: For every action, there is an equal and opposite reaction.

This means that forces always act in pairs. If object A exerts a force on object B, then object B will exert an equal and opposite force on object A.

Example: When you push against a wall with a force of 5 N, the wall does not move. This is because the wall is pushing back on you with an equal and opposite force of 5 N.

When you sit in a chair, your weight (an action force) pushes down on the chair. The chair (reaction force) pushes up on you with an equal force, called the normal contact force.