There are a number of forces that act in a wide variety of cases and have been given specific names. Some of these, like friction and the normal force, are so common that we’re hardly aware of them as distinctive forces. It’s important that you understand how and when these forces function, because questions on SAT II Physics often make no mention of them explicitly, but expect you to factor them into your calculations. Some of these forces will also play an important role in the chapter on special problems in mechanics.
Weight
Although the words weight and mass are often interchangeable in everyday language, these words refer to two different quantities in physics. The mass of an object is a property of the object itself, which reflects its resistance to being accelerated. The weight of an object is a measure of the gravitational force being exerted upon it, and so it varies depending on the gravitational force acting on the object. Mass is a scalar quantity measured in kilograms, while weight is a vector quantity measuring force, and is represented in newtons. Although an object’s mass never changes, its weight depends on the force of gravity in the object’s environment.
For example, a 10 kg mass has a different weight on the moon than it does on Earth. According to Newton’s Second Law, the weight of a 10 kg mass on Earth is
This force is directed toward the center of the Earth. On the moon, the acceleration due to gravity is roughly one-sixth that on Earth. Therefore, the weight of a 10 kg mass on the moon is only about 16.3 N toward the center of the moon.
The Normal Force
The normal force always acts perpendicular (or “normal”) to the surface of contact between two objects. The normal force is a direct consequence of Newton’s Third Law. Consider the example of a 10 kg box resting on the floor. The force of gravity causes the box to push down upon the ground with a force, W, equal to the box’s weight. Newton’s Third Law dictates that the floor must apply an equal and opposite force, N = –W, to the box. As a result, the net force on the box is zero, and, as we would expect, the box remains at rest. If there were no normal force pushing the box upward, there would be a net force acting downward on the box, and the box would accelerate downward
Be careful not to confuse the normal force vector N with the abbreviation for newtons, N. It can be a bit confusing that both are denoted by the same letter of the alphabet, but they are two totally different entities.
Example
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A person pushes downward on a box of weight W with a force F. What is the normal force, N, acting on the box? | |
The total force pushing the box toward the ground is W + F. From Newton’s Third Law, the normal force exerted on the box by the floor has the same magnitude as W + F but is directed upward. Therefore, the net force on the box is zero and the box remains at rest.
Friction
Newton’s First Law tells us that objects in motion stay in motion unless a force is acting upon them, but experience tells us that when we slide coins across a table, or push boxes along the floor, they slow down and come to a stop. This is not evidence that Newton was wrong; rather, it shows that there is a force acting upon the coin or the box to slow its motion. This is the force of friction, which is at work in every medium but a vacuum, and is the bugbear of students pushing boxes across the sticky floors of dorm rooms everywhere.
