You know how to find the weight of an object using its mass and the acceleration of gravity. The weight is also known as the force of gravity on an object. If that were the only force acting on the object, the object would be in free fall. Most of the time, though, the weight is balanced with another force, especially if the object is sitting still on a surface. So far, the free body diagrams have shown these forces as the force of the surface on the object. Watch this video to see what scientists call this force.
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Narration |
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The definition of Normal Force is on the screen. The narrator talks about its meaning. Below the definition is a table drawn in black and white with a blue book sitting on top of it. |
We know that all objects on earth experience the force of gravity. But if that was the only vertical force that these objects experienced, they would always be accelerating down or they'd always be in freefall. So we know there has to be some force that counteracts the force of gravity. And that force is the normal force. We'll often see it abbreviated F sub n. The normal force is the force that a surface exerts on an object to counteract the force of gravity. To help us understand that, let's think of the example of a book lying motionless on a table. |
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The narrator draws a green arrow going downward from underneath the book. He labels it F-sub-g. He then draws a green arrow going upwards from the top of the book and labels it F-sub-n. |
Now we know that the force of gravity is being exerted on this book and that's a downward force like that, F sub g. But we also know that the book is an equilibrium. So there must be an equal in magnitude force pointing upwards. And that force is the normal force. And it's exerted on the book by the table. |
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Next to the table the narrator writes in red the word perpendicular as he explains what is meant by normal in physics. |
Now we use the term normal in our normal daily vocabulary to mean something different. But in physics and math, normal means perpendicular. And that's because the normal force is always perpendicular to the surface that's exerting it. In this case, the table is horizontal. So the normal force that it exerts is vertical-- perpendicular to the surface. To help us better understand this, let's look at an example. |
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The narrator reads out loud the new problem showing on the screen. The narrator describes a free-body diagram for the problem and draws it as he describes it. |
A 91 kilogram student is sitting on the floor. What is the normal force that the floor exerts on the student? As always, let's begin by drawing a free body diagram. We'll draw our student as a dot here. We know that the downward force of gravity is acting, F sub g, as well as a counter-balancing normal force, F sub n. |
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In red the narrator writes the variables to be used and the steps to figure out the force of gravity. |
We know that the students mass, n, is equal to 91 kilograms. We know that the force of gravity, F sub g, that's equal to mass times the acceleration of gravity. So in this case, that's 91 kilograms times negative 9.81 meters per second squared. That gives us that the force of gravity acting on this student is negative 893 newtons. Negative because it's pointed downward. |
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In blue the narrator writes down the steps for net force and finishes solving the problem. |
Now let's think about the net force acting on the student, sigma F. Because the student is in a state of equilibrium, we know that the net force acting on them is zero newtons. And we also know that the net force is the sum of these two forces acting on them. There is the upward normal force plus the downward force of gravity. If we add 893 newtons to both sides of this equation, we get that the normal force is equal to 893 newtons. Equal in magnitude, but opposite in direction as the force of gravity. |
Question
Can the normal force ever be greater than or less than the object's weight?
Absolutely! In cases where you want to slow an object down as they descend, you would have to apply a greater force upward than the force of gravity on the object. In cases where you want to speed up an object on its way down, but not as fast as gravity alone would do, you would apply a force up that is smaller than the force of gravity.
