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Free-Body Force Diagrams

Teacher/Instructor Matt Jones
Matt Jones

M.Ed., George Washington University
Dept. chair at a high school

Matt is currently the department chair at a high school in San Francisco. In his spare time, Matt enjoys spending time outdoors with his wife and two kids.

A free-body force diagram is used to show all the forces acting upon an object to predict the net force and ultimately the path of the object. Each force is drawn as vector arrow.

Let's look at some force body force diagrams. These are basically diagrams that show the force or forces that are acting on an object and they help us illustrate how these forces interact to move that object or not to move that object. Let's look at a couple of examples, let's take a book okay and if I have my book and I drop it there's obviously a force that's acting on right that's the force of gravity. But if I have my book and I hold it on a table okay gravity is still a force applied to that book. But the book is not moving right there's no change in the book. So there's another force that's acting on the book and that force is what we call a normal force. So if I drew a diagram of my book, I've got force down which I'm going to call fg force of gravity but I have another force acting up on that book and that's the normal force, that's the table applying equal force on that book so the book doesn't fall to the ground okay.

Let's look at a falling balloon, now the reason we're using a balloon is that I want to have something that's falling at a constant velocity okay. Because if it's just falling like that book we pretty much have just the force of gravity right okay. But that falling balloon with a constant velocity again has the force of gravity fg forcing it down but constant velocity means that it's not accelerating it's been moving at a constant velocity. So there's another force acting on it here and that is the force of the air friction or air resistance which is pushing up on it okay. So two forces on that falling balloon okay.

Let's take the book on the table and now let's move that book along the table. So I obviously have a force of acceleration which I'm applying to the book to cause it to move. Okay but as I'm moving it across the table there's another force pushing against and that force is the friction of the table okay. So it's applying the force back on that book okay. I also have a force applying downward pressure on there that's the force of gravity and then I have a force equal force going up which is the normal force, the force of the table being applied back up to the book. So see these are some of the ways we can use a diagram to illustrate all the different forces that interact on moving and stationary objects.