Forces as Vectors (AQA GCSE Combined Science: Trilogy: Physics): Revision Note

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 by the other

  • This is known as a force pair

• Examples of these force pairs include:

  • A laptop resting on a desk

    • The laptop exerts a downward force on the desk

    • The desk exerts an upward force on the laptop

  • A basketball player throwing a basketball

    • The basketball player exerts a push force on the ball

    • The ball exerts a push force on the basketball player

  • A person standing on the Earth

    • The Earth exerts a gravitational pull force on the person

    • The person exerts a gravitational pull force on the Earth

• 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 person pushes on the rock, and the rock pushes on the person

• The rock pushes on the ground, and the ground pushes on the rock

• The person pushes on the ground, and the ground pushes on the person