## Forces

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).

**Discuss!**

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.