Forces as Vectors (OCR GCSE Physics A (Gateway))

Force Pairs

• When there is an interaction between two objects, a force is exerted on each object

  • This is known as a force pair

• Force pairs are described by Newton's Third law

• Examples of these force pairs include:

  • The downwards force due to the weight of a laptop resting on a desk, the desk exerts a normal force back up on the laptop

  • The force exerted by a basketball player throwing a basketball is opposed by the reaction force of the basketball on the player

  • The tension force exerted along a cable to a suspended object is opposed by the force of the weight of the object

• Force pairs can be represented by arrows in vector diagrams

The force pairs present with respect to a rock being pushed by a person

• The applied force that the person exerts on the rock is opposed by a reaction force from the rock

• The weight of the rock on the ground is opposed by a normal force

• The weight of the person is also opposed by a normal force

• The force applied by the person driving their feet into the ground is opposed by friction

Forces as Vectors

Net Force

• Vector diagrams include arrows in a particular direction which represent the different forces on an object

• The size of the arrow corresponds to the size of the force

• Net, or resultant, forces can be calculated by adding or subtracting all of the forces acting on the object

  • Forces working in opposite directions are subtracted from each other

  • Forces working in the same direction are added together

• If the forces acting in opposite directions are equal in size, then there will be no resultant force – the forces are said to be balanced

Resultant forces on an object