Forces and Motion

A force has the potential to cause a body to:

  • Accelerate
  • Decelerate
  • Change direction
  • Change shape

Important forces

There are some important forces that you need to know:

  • Weight: the weight of an object is the pull of gravity on its mass.  Weight always acts vertically downwards.
  • Friction: is a force which opposes motion. Friction always acts in the opposite direction of any force.
  • Air resistance: is the force of friction when an object moves through air or water.
  • Upthrust: is the upward push of a liquid or a gas on an object.

Resultant forces

The resultant force is the single force that has the same effect as 2 or more forces.

To make the picture simpler, let’s take a look at the example below:

There is a box which needs to be pushed across a room. You apply a force of 10N to push the box. As it is pushed, there is a force of 5N caused due to friction acting on the box as well. Hence here the resultant force will be 10N-5N=5N to the right.

Force, mass and acceleration

There is a relationship between force, mass and acceleration which can be demonstrated using the equation:

Force = mass x acceleration

F = ma

Here is an example:

Mass, weight and gravity

  • The mass of an object is how much matter the object is composed of. It is measured in Kilograms.
  • The weight of an object is the force of gravity multiplied by the mass of the object. It is measured in Newtons.
  • The mass of an object remains the same everywhere, whereas the weight of an object depends on the gravitational force exerted by the planet.
  • Altogether, there is an equation that summarises the relationship between mass, weight and acceleration:

W = mg


Weight = mass x acceleration due to gravity.

  • The gravitational pull exerted by the earth is 10m/s2
  • The earth’s gravitational field is equal at all points; if a hiker climbs a mountain, his weight will be he same when at ground level. This is known as a uniform gravitational field.
  • This causes all objects to fall over the earth’s surface with the same acceleration.
  • Air resistance is a force which can alter the acceleration of an object; see the example below:

Parachutists make use of air resistance:

  • When a parachutist falls from the air, the force exerted by his weight is greater than the force exerted by air resistance.
  • However, as he falls, his air resistance and the force exerted by his weight become equal; the parachutist stops accelerating and falls at a steady rate. This is known as terminal velocity.
  • At this point, the parachutist opens his parachute. This greatly increases his air resistance and he begins to slow down. He needs to reach another point of terminal velocity where he can surface safely.

Scalar and Vector quantities

Forces can be represented using arrows because they act in a particular direction and have a magnitude as well.

This means that forces are vector quantities.

Here are some scalar and vector quantities:

Scalar quantities Vector quantities
Speed Velocity
Mass Force
Energy Weight
Density Acceleration

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