Forces have Agents Each force has a specific, identifiable, immediate cause called the agent. How to solve for agents: 1. Create a pictorial model of situation (the free body diagram). 2. Circle the system and identify every place where the system touches the environment. 3. It is at these places that contact forces are exerted. 4. Then identify any long-range forces on the system, ie. Force of gravity (Fgrav).1

Example of Forces having Agents

jA physics book resting on a desk.

2

Equilibrium of forces Equilibrium - if the net force on an object is zero, then it is said to be in equilibrium. An object is in equilibrium when: at rest or moving at constant velocity.

3

4

5

Some Examples from Real LifeA soccer ball is sitting at rest. It takes an unbalanced force of a kick to change its motion.

Two teams are playing tug of war. They are both exerting equal force on the rope in opposite directions. This balanced force results in no change of motion.6

Net Force After you have added and subtracted all the forces you are left with the net force acting on the object. There are several common forces acting on objects that you need to memorize:

ForceFriction Normal Applied Net Weight

SymbolFfric or Ff FN FAppl or Fa FNet Fgrav or Fg7

Free Body Diagrams To keep track of how all these forces are affecting a single object, it is a good idea to draw a free body diagram. A free body diagram is just a simple sketch of the object showing all the forces that are acting on it. Draw a quick sketch of the object. Draw an arrow showing every force acting on the object. To calculate the net force, add any vectors acting on the same axis (x and y), making sure to pay attention to the directions.8

Examples of Free Body Diagrams

9

Examples of FNet

10

Examples of FNet

11

12