Forces as Vectors (AQA GCSE Physics)

Forces as Vectors

• Forces are vector quantities as they are described by both magnitude and direction

  • The magnitude of a force is measured in Newtons

  • The direction of a force can be described as left, right, up, down or even using angles

A free body diagram of an object with two forces acting on it

• The direction of a force can be imagined using a number line

  • Numbers to the left of zero are negative as are forces pointing left

  • Numbers to the right of zero are positive as are forces pointing right

Vectors represented as positive or negative vectors depending on their direction

• Not all forces are directed perfectly horizontally or vertically and thus need to have an angle described

  • It is useful to describe an angle with respect to the vertical or the horizontal

A force of magnitude 100 N directed 40° to the horizontal

Force Pairs

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

  • This is known as a force pair

• 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