Free-body diagram — Science definition

A simplified sketch of a single object with an arrow drawn for every force acting on it, pointed in the correct direction and sized to show relative strength.

Where it is taught

In use

That system is the free-body diagram, the single most-used tool in all of mechanics, and it is the bridge between a messy real scene and the clean equation ΣF = ma.

Free-Body Diagrams & Net Force

Questions using free-body diagram

Problem 1

In a free-body diagram of a book resting on a flat table, which forces should be drawn?
  1. Only the normal force, upward
  2. An applied force and friction
  3. Weight downward and the normal force upward
  4. Only the weight, downward
Answer and explanation

Weight downward and the normal force upward

A resting book feels gravity pulling it down and the table's normal force pushing it up. The two are equal, so it stays in equilibrium.

Problem 2

An object moves in a straight line at constant velocity. Its free-body diagram must show forces that…
  1. include no forces at all
  2. grow larger over time
  3. point in the direction of motion
  4. add up to zero
Answer and explanation

add up to zero

Constant velocity means zero acceleration, so the net force is zero — the forces present must cancel out. (Forces can exist; they simply balance.)

Problem 3

List the forces you would draw on a free-body diagram of a sled being pulled across snow by a horizontal rope, and give the direction of each.
Answer and explanation

Weight pulls straight down; the normal force from the snow pushes straight up; the rope's tension pulls forward (horizontal); and kinetic friction from the snow pushes backward, opposing the sled's motion. If the sled moves at steady speed, the up/down pair cancels and tension equals friction.

Problem 4

A lamp hangs motionless from a single cord. Its free-body diagram shows…
  1. only the tension, upward
  2. weight up and tension down
  3. tension up and weight down, equal in size
  4. only its weight, downward
Answer and explanation

tension up and weight down, equal in size

A hanging lamp at rest is in equilibrium: the cord's tension (up) exactly balances the weight (down).

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