Chapter 2 Forces and Newton’s Laws

Free Body Diagrams

“A picture is worth a thousand words” in physics when solving problems based on Newton’s Laws. Usually, if you can at least draw all the forces in a free body diagram correctly, you should be able to solve more than half of the problems.

There are a variety of different forces that can be categorized into two groups based on whether or not the force results from the contact of two interacting objects.

Contact Forces	Non-Contact Forces
Frictional	Gravitational Force
Tension	Electrical Force
Normal	Magnetic Force
Air Resistance
Applied
Spring

When drawing a free body diagram, you must first find the appropriate contact forces, all of which occur at a contact point. Afterwards, find the non-contact forces in the system, which include gravitational, electrical, and/or magnetic forces.

If you draw a circle around the object in need of analyzing, you can easily see all the forces in and out of the circle’s boundary, which are the external forces to be included in the net force calculation. All the forces that stay inside the circle are internal forces, which are not part of the net force calculation.

Examples:

1. A book is at rest on a tabletop. Draw a free body diagram of the forces acting on the book.

2. Draw a free body diagram for (a) a block being pushed on a horizontal surface at a constant velocity and (b) a block sitting motionless on an inclined plane with an angle of ∝.

(a)(b)