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: Use unique (different) names (be sure to name each force with a different name).Point forces in the correct direction (the head of the arrow points to where the force acts.Replace surfaces with forces (floor, hand, and objects touching it become arrows).Add coordinate frame (which way is positive x and positive y?). Remember the rules from section 2.2 still apply: Pushing or pulling on an object becomes an applied force with the arrow pointing to or from (pushing or pulling) the location where the pushing or pulling occurs. The floor becomes a normal force arrow and a frictional force arrow. When modelling a single object using an FBD, you are simplifying a complex problem into specific forces using arrows and an object floating in space. In this section, first we will learn how to do a FBD for a part, then we look at how to model a system of multiple objects. if we wanted to make a FBD of you and me high-5’ing, you would apply the force from your hand onto my hand, not at my center of mass. In a rigid body FBD, you have to be precise about pointing the head of the force arrow to the location where it applied. The biggest difference between a particle and rigid body FBD is where the force is applied. Following what we learned in Section 2.2 on particle Free-Body Diagrams (FBDs), this section will expand on that for rigid bodies.
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