Free Body Diagrams

FBDs make it easier to solve physics problems because:

• They show all external forces clearly.
• They help identify balanced and unbalanced forces.
• They simplify calculations using Newton’s laws.

Choose the object you want to study — a block, a ball, a car, etc. Draw it as a simple box, dot, or shape.

List all the forces acting on the object. Common forces include:

• Weight (gravitational force)
• Normal reaction
• Applied force
• Friction
• Tension
• Air resistance

For each force, draw an arrow starting from the object. The arrow shows:

• Direction of the force
• Length (roughly showing magnitude)

Write the symbol or name next to each arrow, like:

• \( W \) for weight
• \( N \) for normal reaction
• \( F \) for applied force
• \( f \) for friction

For a block lying on a table, two forces act:

• Weight (downward)
• Normal reaction (upward)

These forces are equal and opposite, making them balanced.

If a block is pulled horizontally:

• Applied force (forward)
• Friction (opposes motion)
• Weight (downward)
• Normal reaction (upward)

A mass hanging from a string has:

• Weight (downward)
• Tension in the string (upward)

Once all forces are drawn, we can easily see whether forces are balanced or unbalanced, and calculate the net force:

\( F_{net} = \sum F \)

FBDs help write equations like:

\( F - f = ma \)

or

\( T - W = ma \)

depending on the forces involved.

Only draw external forces. Forces between parts inside the object are not shown.

Avoid adding forces without reason. Every force must come from an interaction.

Direction matters. Always draw arrows cleanly pointing outward from the object.