AQA GCSE Free Body Diagrams Part 2

Free Body Diagrams

You can think of a free body diagram as a simplified force diagram for an object. 

A free body diagram will show only the forces acting on an object. 

We are going to look at a box sliding downwards on a ramp which is a more complex example than the previous page.

There are several forces acting on this square box. 

1. Force of the Earth’s gravity acting on the weight of the box. This arrow will go from the box, directly downwards. Force arrow 1

2.As the box is sliding downwards, there will be a force of friction which is in the opposite direction. Force arrow 2

3. Reaction force from the ramp pushing upwards on the box. Force arrow 3

There are other forces, but these are not acting on the box. So, these have not been included. Lets put those forces onto the diagram.

In most cases the reaction force from the ramp (Force 3) is normally shorter than Weight force (Force 1). There is a reason, but it goes beyond GCSE.

In a true free body diagram we should not include the ramp. So, the free body diagram can be found below

The box and the ramp have been obscured in the background, so you can just see them to allow you to see where the force arrows are located.

However, only the box should be present, which has been represented in the free body diagram as an orange circle.