Unbalanced Forces (Edexcel IGCSE Science (Double Award))

Unbalanced forces

• When multiple forces act on a single object, the forces can be added together to produce a resultant force

• When the forces acting on an object completely cancel out

  • the forces are balanced

  • the resultant force is zero

• When the forces acting on an object do not completely cancel out

  • the forces are unbalanced

  • there is a resultant force

Balanced forces

• Balanced forces mean that the forces have combined in such a way that they cancel each other out and no resultant force acts on the body

  • For example, the weight of a book on a desk is balanced by the normal force of the desk

  • As a result, no resultant force is experienced by the book, the book and the table are equal and balanced

A book resting on a table is an example of balanced forces

Unbalanced forces

• Unbalanced forces mean that the forces have combined in such a way that they do not cancel out completely and there is a resultant force on the object

  • For example, imagine two people playing a game of tug-of-war, working against each other on opposite sides of the rope

  • If person A pulls with 80 N to the left and person B pulls with 100 N to the right, these forces do not cancel each other out completely

  • Since person B pulled with more force than person A the forces will be unbalanced and the rope will experience a resultant force of 20 N to the right

A tug-of-war is an example of when forces can become unbalanced

Unbalanced Forces, Mass & Acceleration

• When forces combine on an object in such a way that they do not cancel out, there is a resultant force on the object

• This resultant force causes the object to accelerate (i.e. change its velocity)

  • The object might speed up

  • The object might slow down

  • The object might change direction

• The relationship between resultant force, mass and acceleration is given by the equation:

F = m × a

• Where:

  • F = resultant force, measured in newtons (N)

  • m = mass, measured in kilograms (kg)

  • a = acceleration, measured in metres per second squared (m/s²)

• This equation is also known as Newton's second law of motion