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Overview of Force & Free-Body Diagrams

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  • 0:03 What Are Free-Body…
  • 1:12 Key Forces and Rules
  • 4:04 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After completing this lesson, you will be able to explain what a free-body force diagram is, some of the rules associated with them, and what needs to be considered when drawing these diagrams.

What Are Free-Body Force Diagrams?

A free-body diagram is a picture or sketch used by physicists and engineers to show the forces acting on an object. In these diagrams, arrows are used to represent forces. The longer the arrow, the stronger the force.

But hold on a minute - what exactly is a force? A force is actually pretty simple: it's just a push or a pull. When you push a shopping cart across the supermarket, you're applying a force. When you drop a ball, gravity is applying a force. And when two north poles of magnets are brought together, a force tries to push them apart. Any push or pull is a force.

It's useful to have a way to represent the forces on an object, because humans can register information with pictures much more quickly than words or numbers. The better you understand a situation, the easier it is to solve problems for that situation. An engineer needs to know what forces will be acting on a bridge they plan to build, otherwise that bridge might collapse.

So, let's go through some of the rules for drawing free-body (force) diagrams (FBD) and how to draw some of the key forces you need to know about.

Key Forces and Rules

Rule number 1: The arrow lines must be straight, and a force has to be pointed in a particular direction.

A curve doesn't really make sense. If you push two ways at once that would be a diagonal, but still straight line. Maybe a curve means the force is changing over time. But if things change over time, you need to draw more than one force diagram. All diagrams should have straight lines.

Rule number 2: Draw the forces acting on a dot, with arrows pointing away from the object.

People like to draw arrows moving away from the edge of an object, but in physics we know that forces act on a specific location. If the object is a particle, drawing a dot makes perfect sense. But even with a larger, funny-shaped object, that object still has a point in the middle called the center of mass. This is the place at which the forces appear to act. If forces didn't act at the center of mass, the object would start to rotate. So unless it's a rotating object, every arrow should point away from a dot. And they always go away, not towards. Mostly this is about neatness: if you point arrows toward the dot or mix the two together, it can be confusing and unclear.

Rule number 3: If an object is pressing on a solid, stationary surface, draw a normal force at 90 degrees to that surface.

If there wasn't a normal force, an object would go through the surface it's sitting on. In most cases, this will just be an object sitting on the ground or on a table, with a normal force pointing directly upwards. But if an object is on a slope, it's still 90 degrees to the surface. Or if you're pressing an object against the corner of a wall yourself, you might have two normal forces.

Rule number 4: The force of gravity (Fg) acts straight down.

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