PHYS 2211 Module 5.1


Recommended Reading

5.1 Forces

Learning Objectives

By the end of this section, you will be able to:

  • Distinguish between kinematics and dynamics
  • Understand the definition of force
  • Identify simple free-body diagrams
  • Define the SI unit of force, the newton
  • Describe force as a vector

Forces and Net Force

Free Body Diagrams

A free-body diagram is a sketch showing all external forces acting on an object or system.

The forces are represented by vectors extending outward from the free body.

Tactics for Identifying Forces

Identify the object of interest. This is the object whose motion you wish to study.

Draw a picture of the situation. Show the object of interest and all other objects – such as ropes, springs, surfaces – that touch it.

Draw a closed curve around the object. Only the object of interest is inside the curve; everything else is outside.

Locate every point on the boundary of this curve where other objects touch the object of interest. These are the points where contact forces are exerted on the object.

Name and label each contact force acting on the object. There is at least one force at each point of contact; there may be more than one. When necessary, use subscripts to distinguish forces of the same type.

Name and label each long range force acting on the object. For now, the only long range force is the force of gravity (or weight).


Consider how you would answer these questions. Then bring this to class for a group discussion.

The figure shows three objects with forces (red vectors) acting on them. Assume each square is equivalent to 1 N of force. 

For each object, determine the net force. Express the net force as a vector with magnitude and direction.