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Free Body Diagrams (Complete Guide)
Normal Force, Friction, Tension, Gravity, and Net Force
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A free body diagram is a simplified drawing that shows one object and all the external forces acting on it. It helps students turn a physical situation into a solvable physics problem. By focusing only on the chosen object, you can apply Newton's laws more clearly and avoid mixing in irrelevant details. Free body diagrams are essential in mechanics, from simple blocks on tables to cars, elevators, and objects on slopes.
To build a correct diagram, first isolate the object, then replace every interaction with a force arrow. Common forces include weight, normal force, tension, friction, air resistance, and applied force. The direction of each arrow matters because forces are vectors, and the net force determines the object's acceleration through Newton's second law. Once the diagram is complete, you can resolve forces into components and write equations such as ΣFx = max and ΣFy = may.
Key Facts
- A free body diagram shows only one object and all external forces on it.
- Weight acts downward: W = mg.
- Newton's second law connects net force and motion: ΣF = ma.
- For motion in two dimensions, write separate equations: ΣFx = max and ΣFy = may.
- On an incline at angle θ, weight components are mg sin θ parallel to the slope and mg cos θ perpendicular to the slope.
- Friction opposes relative motion or attempted motion, and kinetic friction is often modeled as fk = μkN.
Vocabulary
- Free body diagram
- A drawing that isolates one object and shows all external forces acting on it as arrows.
- Net force
- The vector sum of all forces acting on an object.
- Normal force
- The support force exerted by a surface on an object, usually perpendicular to the surface.
- Tension
- The pulling force transmitted through a rope, string, or cable.
- Friction
- A force parallel to a surface that resists sliding or attempted sliding between surfaces.
Common Mistakes to Avoid
- Drawing forces that act on other objects, because a free body diagram must include only forces acting on the isolated object. If you include action-reaction partners on the same diagram, you mix different bodies and create wrong equations.
- Assuming motion direction and force direction are always the same, because an object can move one way while accelerating another way. The net force points in the direction of acceleration, not necessarily velocity.
- Forgetting to tilt the normal force on an incline, because the normal force is perpendicular to the surface, not always straight up. Drawing it vertically gives incorrect components and friction values.
- Including internal forces within a chosen system, because internal interactions cancel when you treat multiple parts as one system. Only external forces belong on the free body diagram for that system.
Practice Questions
- 1 A 5.0 kg block rests on a horizontal table. Draw the free body diagram and find the weight and the normal force if the block is not accelerating vertically. Use g = 9.8 m/s^2.
- 2 A 10 kg crate is pulled to the right with a 50 N horizontal force while friction acts to the left with magnitude 18 N. Draw the free body diagram and calculate the crate's horizontal acceleration.
- 3 A box slides down a ramp at constant speed. Explain what the free body diagram must look like and what it tells you about the forces parallel and perpendicular to the ramp.