If instead, the book were being pulled by a string, the image would be the same but the applied force and frictional force would change direction (because friction always opposes motion).Ī free body diagram is a tool used to solve engineering mechanics problems. any applied force at the point of application, such as your hand pushing on the book (blue arrow).the gravitational force acting at the center of mass (pink arrow).the frictional force running along the bottom surface between the book and table (yellow arrow).the normal force on the bottom of the book (green arrow).To model a book being pushed across the table, you would apply the following forces at the following locations (see image below) Applied forces (and moments), such as distributed loads, motors, pushing on an object, tension, etc.In application, you set the direction of the frictional force to match if it is pushing or pulling. Spring force is often shown as negative because the force acts in the opposite direction of the motion traveled.Friction always opposes motion, a fact that will be very important in your dynamics class.This makes it a shear force, which we’ll look at in Chapter 6. Frictional forces act parallel to the plane between the two surfaces.Normal forces always act perpendicular to the surface, so if the ground is at an angle, then the normal force acts 90 degrees from that angle (perpendicular).

This is also because we generally know the total mass of something and where that occurs on an object (often at the geometric center), so we concentrate the force of gravity at this center of mass. Because we don’t want a million little arrows on the object, we sum the effect of gravity at the center of mass. Gravity acts on every particle in an object.Here are some tips to keep in mind about each of the forces:

Rigid body free body diagrams