When an object is released near the earth’s surface, it starts falling towards the earth’s center as a result of the earth’s gravitational pull on the object. This is true whether the object is released from rest (dropped) or if it is thrown (up or down, sideways — whatever!). We will see a very different behavior of the object when it is thrown, however, if that object is a crumpled-up piece of paper or if it is a rock, even if they are thrown in exactly the same way. The reason for the difference in motion of these two objects after being thrown is that the presence of the air or a slight breeze can greatly affect the motion of the paper, while the rock moves in about the same way whether the air is present or not. (A rock falling through air and one falling through a vacuum show just about the same behavior. Indeed, a rock falling through vacuum and a crumpled-up piece of paper falling through vacuum fall in exactly the same way!)
If the motion of an object near the earth’s surface is virtually unaffected by the presence of the earth’s atmosphere (that is, air resistance hardly affects its motion at all), and the object’s motion is predominantly determined by the earth’s gravitational pull, then we say that the object is undergoing free-fall motion.
Any object undergoing free-fall motion at the earth’s surface has an acceleration which has a direction pointing directly towards the center of the earth. This very special acceleration (to us here on Earth, at least!) is called the acceleration due to gravity, and is given the special symbol g. The magnitude of the acceleration due to gravity at the earth’s surface is
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The only thing new in this lecture is the fact that, under the conditions for free-fall motion described above, we now automatically know the acceleration of an object. (Note that this acceleration does not depend on how heavy the object is!! Does this make any sense to you? It may or may not now, but it should later on in this course.) Since this is a constant acceleration, the kinematic equations must still apply. We can therefore apply our kinematic expertise to solving free-fall problems!
For your convenience, the kinematic equations have been listed for you again in the next section.