A force is a push or a pull. Forces causes objects to accelerate, change direction, or change shape. The strength of a force is measured in Newtons ( N).

Forces can be grouped into two types, contact forces and non contact forces. Contact forces act on objects they are in contact with and non contact forces act at a distance.

Friction is an example of a contact force and it is a force that opposes motion.

Gravity is a non contact force and it is the force that pulls of objects of large masses together. Gravity is the force that pulls you back to earth when you jump.

Newton’s first law of motion

What this means is,

  • A stationary object will only move if it is pushed or pulled.
  • A moving object will only stop or change direction or change speed if it is pushed or pulled. However, a ball rolling along a flat surface will slow down and eventually stop. This is because of friction , which is a force that slows the ball down.
  • Every object has a reluctance to start moving or stop moving on its own. This reluctance is called inertia.

The greater the mass of an object, the greater the inertia. Therefore, mass is a measurement of inertia.

Newton’s first law of motion is also called the law of inertia.

Newton’s second law of motion

What this means is,

  • If you increase the resultant force acting on any object, the object accelerates by the same factor i.e if you double the force, the acceleration also doubles.

Mathematically this is represented as F∝a whereby F is the force and a is the acceleration.

Introducing the constant “mass” gives us the equation F = ma.

Gravitational force causes any object with a mass to accelerate downwards with an acceleration of approximately 9.81 m/s2. The acceleration due to gravity i denoted by the letter g.

Using F=ma we can derive the fomular for weight, W=mg whereby W is the weight of an object, m is its mass and g is the acceleration due to gravity.

Weight and mass are two different properties as we shall see later.

Newton’s third law of motion

What this means is,

  • If you push an object, that object pushes back at you.

Imagine a book resting on a table. The weight of the book is pushing downwards at the table and the table is providing an upwards reaction on the book.

Imagine a rocket lifting off. What causes it to move upwards? The action of forcing the exhaust gases downwards produces an opposite upward reation that lifts the rocket. The momentum of the gases is equal to resultant momentum of the rocket.

Momentum = mass x velocity

Mass vs Weight

The terms “mass” and “weight” are often used interchangeably, but however they don’t mean the same thing.

Mass vs Weight
Mass is a property of matter and so it is the same everywhereWeight depends on the effect of gravity on that location.
Mass can never be zero.Weight can be zero if no gravity acts upon an object, eg in space.
Mass is a scalar quantity. It has magnitude only.Weight is a vector quantity. It has both magnitude and is directed downwards toward the center of the Earth.
Mass usually is measured in grams and kilograms.Weight often is measured in newtons, since it is a force.