Free Body Diagrams

A Free Body Diagram (FBD) is a visual representation of all the forces acting

on a single object

FBDs are extremely powerful problem solving tools that bridge the gap between

qualitative analysis and a quantitative mathematical representation

Drawing an FBD…

• A simple dot can be used to represent the object in question

• Identify the forces acting ON that object only• Draw one arrow to represent each force acting

ON the object– All arrows should be drawn from the dot or the center

of the object– The arrows point in the direction of the force.– The length of the arrow represents the relative

magnitude of the force…ie longer arrows = larger force

Example 1

Normal ForceFN

FW

Weight/Force due to Gravity

A block of wood resting on a desktop

Example 2A block of wood moving with a constant velocity across a surface

FN

FW

Applied ForceFA

Frictionf

Example 3A block of wood accelerating to the right across the surface

FN

FW

FAf

Example 4A block of wood accelerating to the right when pulled by a rope at an angle

FN

FW

TensionT

f

Example 5A tetherball while swinging at a constant speed around a pole

FW

TensionT

Notice there is nothing in the original description that says the tetherball is being hit. A diagram for the ball as it is being hit would show an additional force acting on the ball.

Example 4

Try it…Draw a FBD and write the 2nd law equations for a

lawn mower being pushed at a constant speed