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<p>Forces are given many names, such as push, pull, thrust, lift, weight, friction, and tension. Traditionally, forces have been grouped into several categories and given names relating to their source, how they are transmitted, or their effects. The most important of these categories are discussed in this section, together with some interesting applications. Further examples of forces are discussed later on.</p>
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<h2>Normal Force</h2>
<p><strong>Weight</strong> (also called force of gravity) is a pervasive force that acts at all times and must be counteracted to keep an object from falling. You definitely notice that you must support the weight of a heavy object by pushing up on it when you hold it stationary, as illustrated in the (a) part of the figure below. But how do inanimate objects like a table support the weight of a mass placed on them, such as shown in the (b) part of the figure below?</p>
<p>When the bag of dog food is placed on the table, the table actually sags slightly under the load. This would be noticeable if the load were placed on a card table, but even rigid objects deform when a force is applied to them. Unless the object is deformed beyond its limit, it will exert a restoring force much like a deformed spring (or trampoline or diving board).</p>
<p>The greater the deformation, the greater the restoring force. So when the load is placed on the table, the table sags until the restoring force becomes as large as the weight of the load. At this point the net external force on the load is zero. That is the situation when the load is stationary on the table. The table sags quickly, and the sag is slight so we do not notice it. But it is similar to the sagging of a trampoline when you climb onto it.</p>
<div class="wp-caption alignnone" style="width: 400px"><img alt="A person is holding a bag of dog food at some height from a table. He is exerting a force F sub hand, shown by a vector arrow in upward direction, and the weight W of the bag is acting downward, shown by a vector arrow having the same length as vector F sub hand. In a free-body diagram two forces are acting on the red point; one is F sub hand, shown by a vector arrow upward, and another is the weight W, shown by a vector arrow having the same length as vector F sub hand but pointing downward. (b) The bag of dog food is on the table, which deforms due to the weight W, shown by a vector arrow downward; the normal force N is shown by a vector arrow pointing upward having the same length as W. In the free-body diagram, vector W is shown by an arrow downward and vector N is shown by an arrow having the same length as vector W but pointing upward." height="400" src="https://data.ulearngo.com/assets/228ad064-ddee-41ab-bb71-dc6170cc662e" width="400"/><p class="wp-caption-text">(a) The person holding the bag of dog food must supply an upward force \({\mathbf{\text{F}}}_{\text{hand}}\) equal in magnitude and opposite in direction to the weight of the food \(\mathbf{\text{w}}\). (b) The card table sags when the dog food is placed on it, much like a stiff trampoline. Elastic restoring forces in the table grow as it sags until they supply a force \(\mathbf{\text{N}}\) equal in magnitude and opposite in direction to the weight of the load.</p></div>
<p>We must conclude that whatever supports a load, be it animate or not, must supply an upward force equal to the weight of the load, as we assumed in a few of the previous examples. If the force supporting a load is perpendicular to the surface of contact between the load and its support, this force is defined to be a <strong>normal force</strong> and here is given the symbol \(\mathbf{\text{N}}\). (This is not the unit for force N.)</p>
<p>The word <em>normal</em> means perpendicular to a surface. The normal force can be less than the object’s weight if the object is on an incline, as you will see in the next example.</p>
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<h3 data-type="title">Common Misconception: Normal Force (N) vs. Newton (N)</h3>
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<p>In this section we have introduced the quantity normal force, which is represented by the variable \(\mathbf{\text{N}}\). This should not be confused with the symbol for the newton, which is also represented by the letter N.</p>
<p>These symbols are particularly important to distinguish because the units of a normal force (\(\mathbf{\text{N}}\)) happen to be newtons (N). For example, the normal force \(\mathbf{\text{N}}\) that the floor exerts on a chair might be \(\mathbf{\text{N}}=\text{100 N}\).</p>
<p>One important difference is that normal force is a vector, while the newton is simply a unit. Be careful not to confuse these letters in your calculations! You will encounter more similarities among variables and units as you proceed in physics. Another example of this is the quantity work (\(W\)) and the unit watts (W).</p>
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Normal Force

Physics is a science discipline concerned with the nature and properties of matter and energy. Physics topics include mechanics, heat, light and radiation, sound, electricity, magnetism, the structure of atoms, and so on.

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Discover the development of the force concept, explore Newton&#8217;s first, second, and third laws of motion, and learn about problem-solving strategies and further applications in this tutorial on dynamics and the four basic forces.


Normal Force

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