Force diagrams

This lesson covers: 

  1. What force diagrams show
  2. Adding and subtracting forces on force diagrams

What are force diagrams?

Force diagrams are visual representations that show the forces acting on an object.

Diagram showing forces acting on a car with arrows indicating direction and magnitude.

The diagrams indicate:

  • The direction and magnitude of forces using arrows.
  • Whether the forces are balanced or unbalanced.
  • If an object is accelerating, decelerating, or moving at a steady speed.

Showing forces on force diagrams

Diagram showing forces acting on a car with 3N friction and 5N driving force.

To draw a force diagram:

1. Identify all the forces acting on the object.

2. Draw arrows to represent each force.

  • Arrow direction shows force direction.
  • Arrow length shows force magnitude.
  • Longer arrows for larger forces, shorter for smaller.

3. Label each force with its value in newtons (N).

Adding and subtracting forces

When forces act along the same line, you can add or subtract them to determine the overall force.

Calculating the overall force - example 1

Diagram showing a car with driving force 5N and friction 3N, resulting in an overall force of 2N and acceleration.
  • The forward force is larger than the backwards force so the forces are unbalanced.
  • The overall force is positive so the object is accelerating.

Calculating the overall force - example 2

Diagram showing a car with balanced forces of 5N friction and 5N driving force resulting in steady speed.
  • The forward force is the same size as the backward force so the forces are balanced.
  • The overall force is 0 so the object is moving at a steady speed.

Calculating the overall force - example 3

Diagram showing a car with friction force of 5N and driving force of 3N resulting in an overall force of -2N indicating deceleration.
  • The backwards force is larger than the forwards for so the forces are unbalanced.
  • The overall force is negative so the object is decelerating.