### Video: Recognizing Force Types

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Full Length Video Tutorial: Recognizing Force Types

#### Recognizing Force Types

Video Transcript

Introduction
First-time Physics students will at some point need to analyze an object and determine the forces that act upon it. Exactly how do you go about conducting such a force analysis? I'm Mr. H. and I have some answers for you.

Interaction Types
Forces are pushes or pulls that act upon an object as the result of its interaction with the surroundings. "Interactions" fall into two basic categories. First, there's the interaction occurring between two objects that don't physically touch. Gravity is an example of this. The earth and your body interact with each other even when you are not touching the earth. We call this a non-contact force or a field force. The repulsive force between two like-charged objects is also a non-contact force. Magnetic forces are a third common non-contact force.

The second interaction category occurs when two objects physically contact each other. When you sit in a chair, the chair and your  b - u - t - t are in contact with each other. There's a force resulting from the contact interaction between butt and chair. If you stand on the floor, your feet and floor interact, exerting a force on each other. This category of force is a contact force. Understanding that forces result from contact helps you to recognize what forces act upon an object.

Force Types
Falling within these two categories are different types of forces. I will discuss the details of six types. As a warning, different teachers may give these force types different names. One teacher's air resistance force is another teacher's drag force. Let your teacher be the final authority on what name to use. In this video, learn the skill of analyzing an object in order to identify the forces acting on it. This skill begins with asking two questions of the object:

1. Is there a planet, a charge, or a magnet near the object? (This allows you to recognize non-contact forces.)
2.  Is the object contacting anything in its surroundings?

Gravity Force
The answer to this first question is likely YES! Most analyses will involve objects located on or near Earth. The Earth and objects on or near it interact with each other; this interaction results in the Earth pulling down upon the object. We refer to this as a Gravity Force. Count on this force as being always present. It's our first type of force.

Tension Force
When an object is attached to a string, rope, cable, or wire, there is a tension forceacting upon the object as a result of its interaction with the string, rope, cable, or wire. This force occurs as long as the opposite end of the string is secured and the string is held tight. If the string isn't tight, there is no tension.

Normal Force
Normal forcesexist when there are two surfaces pressed against each other. The interaction between two pressed-together surfaces results in a force on each object. When sitting in a chair, the force of the chair surface pushing up on your butt surface is called a normal force. And when standing on the ground, the force of the ground pushing up on the surface of your feet is called a normal force. Even when you lean against a wall, there is a horizontal normal force resulting from the interaction with the wall. If the wall is removed, then ... .

Friction Force
Two surfaces sliding across each other interact and pull back on each other. This results in a type of force called sliding friction. A car skidding to a stop with locked wheels experiences sliding friction as a result of its interaction with the road surface. Friction can also occur when a stationary object is attempting to move across a surface, like the friction on this brick that is at rest on a wooden board titled at a mild angle. The friction force on a stationary, but attempting to move, object is called static friction

Air Resistance
Air resistance results from the interaction of an object moving relative to the surrounding air. The object could move through still air or the air could move past a still object. Either results in air resistance. While air resistance is large at high speeds, we would have to consider it to be an existing force (albeit small) whenever an object is moving relative to the surrounding air.

Applied Force
An applied force is the force of a person or thing pushing or pulling on an object. This contact interaction results in a force upon the object. Push a refrigerator across the kitchen floor; that's an applied force. Or lift a box upward; that's an applied force. This type of force is somewhat of a catch-all type that accounts for forces not accounted for by the other types. It's wise to remember that one teacher's applied force may be another teacher's normal force or propulsion force. The big idea is that you must account for all interactions of an object with its surroundings and assign a force type to any push or pull resulting from such interactions.

Conclusion
In Physics, everybody needs practice. We have some great interactive practice exercises on our website. You will find links to them in the Description section of this video. Give one a try to insure that you got this! Hey I'm Mr. H. Thanks for watching!

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