Module 3—Effects of Force on Velocity
Lesson Summary
As you worked through this lesson, you should have developed answers to these questions:
- How does friction affect moving objects?
- How does friction affect making an object move?
- How do free-body diagrams, vector analyses, and Newton's second law help solve moving-object problems?
There are many different kinds of forces that can act on a single mass. A free-body diagram is used to illustrate and understand how multiple forces acting on a single mass contribute to the net force and acceleration of that mass. The following common forces will be used in free-body analysis.
- An object's weight is the force that results from the action of gravity on matter. According to Newton's second law, if F = ma, then weight is defined by W = mg, where g is the acceleration due to gravity.
- The normal force is a supporting force that acts between two surfaces in contact. On a flat, horizontal surface, the normal force can be determined by an application of Newton's second law (F = mg) since it opposes all of the object's weight.
- Friction is the force that opposes the relative motion or tendency of such motion of two surfaces that are in contact. The mathematical expression for friction depends on whether the mass is moving (kinetic) or stationary (static).
If an object slides, it experiences a kinetic frictional force given by 
. If an object is at rest, a static frictional force resists motion. The maximum static frictional force is given by 
.
Lesson Glossary
coefficient of kinetic friction: ratio of friction force to normal force once two objects in contact stop moving as one object
coefficient of static friction: ratio of the maximum friction force to normal force while two objects in contact move as one object
free-body diagram: a drawing of a system with forces acting on it
kinetic friction: the type of friction that an object is subject to after it is in motion
normal force: the perpendicular force that a surface exerts on an object with which it is in contact
static friction: the friction between two objects that are in contact but are not moving
weight: a measure of the force of gravity on an object