Module 3—Effects of Force on Velocity
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Introduction to Free-Body Diagrams
In previous applications of Newton's second law, you solved problems where only one force was acting. It is not uncommon to have multiple forces acting on a single mass at one time.
For example, when a train is travelling along the tracks, the engine produces an applied force; a frictional force resists the engine force; a gravitational force (or weight) pulls down; and the ground provides a supportive, upward force (or normal force). All of these forces are illustrated here.
weight: a measure of the force of gravity on an object
normal force: the perpendicular force that a surface exerts on an object with which it is in contact
free-body diagram: a drawing of a system with forces acting on it
In physics, this diagram will be turned into a free-body diagram, where all forces originate from the object’s approximate centre of mass but maintain their relative directions and magnitudes.
Before you explore free-body diagrams in detail, you will first investigate the common kinds of forces that should be included in such analyses. Common forces you will need to know include the following:
- Weight (
) is the force that results from the action of gravity on matter. According to Newton's second law, if F = ma, then W = mg, where g is the acceleration due to gravity. Weight (force in newtons) should not be confused with mass (kilograms), although the terms are commonly mixed up. Weight is an expression of the pull of gravity on an object. In an environment where there is no gravity, an object would be weightless. Mass, however, is a measure of the amount of matter in an object (not to be confused with volume, which is the space an object occupies) and will not change under varying gravitational forces.
- Normal force (
) is one component of the force that a surface exerts on an object with which it is in contact, namely the component that is perpendicular to the surface. When the surface of contact is horizontal, the normal force is equal in magnitude but opposite in direction to the weight of the upper object. The word normal in free-body diagrams is a synonym for perpendicular or right-angled.
- Friction (
) is the force that opposes the relative motion or tendency of such motion between two surfaces that are in contact. Friction is parallel to the surface and is directed opposite to motion.
- Applied force (
) is a general term used to describe a force generated from a person, motor, or other object.
Normal Force and Weight
Normal force is a supporting force that acts between two surfaces that are in contact. For example, the railway tracks that a train travels on exert an upward force on the wheels of the train, preventing it from falling into the tracks and the ground below. Determining the normal force requires an application of Newton's second law.
Watch and Listen
Complete this tutorial on the normal force and friction.
Try This
TR 1. The animation and associated text (viewed by clicking the T on the animation) will either provide the answers directly or will provide the basic information that needs to be applied to find the answers for the following questions.
- What is the significance of the word normal in normal force?
- A block is being pressed against a vertical wall by means of an applied force. The arrow represents the applied force. In what direction is the normal force exerted by the wall on the block?
- Along what surface does the normal force act to support an object?
- When a book is resting on a table, the normal force exerted by the table on the book depends on the weight of the book. What is the physical mechanism that causes the normal force to be smaller in one case and larger in another? In other words, what is different about the table and/or the book such that the normal force is smaller for one book and larger for another book?
- Is there an expression for the normal force that one object exerts on another, like the expression for the gravitational force (e.g.,
) that one object exerts on another? If so, what is this expression? If not, how can one calculate the normal force in a given situation?
- What is the physical origin of the normal force? For example, what enables a table to push upward on a book that is resting on the table?
- Describe the relationship between the size of the normal force and the amount of compression in the upper layers of the table molecules when various weights are placed on a table.