Newton's laws of motionIB MYP Sciences: Revision notes

An object stays at rest, or keeps moving at a constant velocity, unless a resultant force acts on it.

This means that when the forces on an object are balanced (the resultant force is zero), its velocity does not change.

Objects resist changes to their motion. This property is called inertia. The bigger the mass, the bigger the inertia, and the harder it is to change the object's motion.

A car moving at a steady 20 m/s has balanced forces: the driving force equals friction plus air resistance.

The acceleration of an object is directly proportional to the resultant force on it and inversely proportional to its mass.

F = m × a

force in newtons (N), mass in kilograms (kg), acceleration in m/s².

Rearranged: a = F ÷ m and m = F ÷ a.

Worked example: a 1500 kg car has a resultant force of 4500 N. a = 4500 ÷ 1500 = 3.0 m/s².

Double the force and the acceleration doubles; double the mass and the acceleration halves.

When object A exerts a force on object B, object B exerts an equal and opposite force on object A.

These two forces:

• are the same size and in opposite directions
• are the same type of force
• act on different objects

Examples: when you push on a wall, the wall pushes back on you with the same force. A swimmer pushes the water backwards and the water pushes the swimmer forwards.

Because the two forces act on different objects, they do not cancel each other out.

When an object falls through air, two forces act: its weight downwards and air resistance upwards.

1. At the start, speed is low, air resistance is small, so there is a downward resultant force and the object accelerates.
2. As speed increases, air resistance increases, so the resultant force and the acceleration become smaller.
3. Eventually air resistance equals the weight. The forces are balanced, so the object falls at a constant speed called the terminal velocity.

Opening a parachute greatly increases air resistance, so the skydiver slows down until a new, smaller terminal velocity is reached.

Newton's laws explain safety features in vehicles.

• Seat belts: in a crash, a passenger tends to keep moving forward (first law). The belt applies a force that slows the passenger with the car.
• Seat belts stretch slightly, which increases the stopping time and reduces the force on the body (F = ma).
• Other features such as airbags and crumple zones also increase the time taken to stop, so the force is smaller.

Worked example: a 70 kg passenger slows from 20 m/s to rest in 0.10 s. a = 20 ÷ 0.10 = 200 m/s², so F = 70 × 200 = 14 000 N. The passenger pushes on the belt with an equal force of 14 000 N (third law).