Force in Physics: Definition & Examples

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  • 0:01 Definition of Force
  • 1:00 Forces at Work on a…
  • 1:40 Forces at Work on a…
  • 2:52 Another Example
  • 3:41 Lesson Summary
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Lesson Transcript
Instructor: Scott van Tonningen

Scott has a Ph.D. in electrical engineering and has taught a variety of college-level engineering, math and science courses.

Physical forces are all around us. Learn the defining characteristics of forces in physics, look at some examples, and test your understanding with a short quiz.

Definition of Force

I have just finished the annual task of raking leaves in my back yard. There were oak, maple, and ash leaves, but there were also leaves in my yard that I didn't recognize and probably came from farther away. How did they get here? All of the leaves got to my yard because of force. In this case, there were at least two forces operating on the leaves: gravity and the force caused by the wind. Gravity was due to an interaction between the earth and the leaves, pulling them toward the ground vertically, and the wind force was caused by an interaction between the air mass and the leaves, tending to blow them horizontally through the air.

From this quick observation, we conclude that there are three defining characteristics of force:

  • It starts with an interaction between two or more physical objects (which could include the air)
  • The interaction causes a push or pull to occur between the objects
  • The resulting push or pull has a direction associated with it

Forces at Work on a Moving Object

At first, there was movement due to these forces. When an individual leaf detached from the tree, it moved both vertically and horizontally, due to gravity and the wind, until it came to a stop. How fast did it move? Well, that depends, because there were other forces acting on the leaf as it traveled. For example, as the leaf moved downward due to gravity, there was also a small amount of resistance in the opposite direction (upward) provided by the air mass. We call this force drag, or air resistance. That's why you sometimes see leaves drift slowly to the ground. There may also have been a small amount of air resistance (drag) force working against the wind in a horizontal direction.

Picture of falling leaf with forces

Forces at Work on a Static Object

Let's say the leaf hit the ground and stayed where it landed. Were forces still there? Sure. Gravity was still pulling the leaf downward vertically, and the wind may still have been pushing on the leaf horizontally. But the leaf came to rest. Why? Because now different forces were involved. The ground was pushing back on the leaf vertically with an opposing force equal in magnitude to the force of gravity. In addition, the ground was interacting with the leaf to cause friction, a force that acted in a horizontal direction, so that the leaf didn't move even though the wind may still have been blowing on it.

Now let's picture the forces with the leaf on the ground:

Picture of leaf on ground with forces

Again, the leaf doesn't move because of the equal and opposite forces present. The leaf's journey and destination point out a couple of other things about force:

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