Video: Free Fall

We provide the transcript below to those who for whatever reason would find the written words to be preferred over in addition to the actual video. 

Free Fall

  
Introduction
If you're taking Physics, the term free fall is eventually going to drop. What exactly is free fall? And how is free fall motion described? I'm Mr. H and I have some answers for you.
 

Defining Free Fall
Objects in free fall are moving through the air under the sole influence of gravity. Other forces are either non-existent or small enough to be considered insignificant. Gravity is the sole influencer. I will be discussing two types of free fall - the motion of an object dropped from rest position and the motion of an object thrown vertically upward from ground level.
 

The Acceleration Caused by Gravity
Gravity causes objects to accelerate - to speed up and slow down. Since the force of gravity is directed downward, it causes objects to accelerate downward. At locations near Earth's surface, this acceleration is a constant value. In fact, its value is 9.8 m/s/s, or approximately 10 m/s/s. As an object rises and falls, it slows down and speeds up, but the acceleration value remains constant. This means that for each second of motion, a free-falling object always changes its speed by the same change amount - by approximately 10 m/s.
 

The Velocity Vector
As described by the dot diagram, as an object rises, it gets slower. It decreases its speed. This is consistent with a downward acceleration. As it falls downward, it gets faster or increases its speed; again, a downward acceleration. Velocity is a vector; it is a speed with a direction. We can represent these ideas by a velocity vector diagram. Each arrow represents a velocity vector. The length of the arrow indicates the speed; the direction of the arrow represents the direction of the velocity vector. 
 
When we express these ideas by numbers, we often use a - to indicate downward. Don't think of - as less than 0. Think of it as a direction: downward. This table shows how velocity changes over the course of time. For every 1 second of time change, there is a -10 m/s velocity change. (Pause) This table is for an object dropped from an elevated position. What if the object is thrown upward from the ground?
 
An upward-thrown object decreases its velocity value as it rises, by 10 m/s each second. As it falls, the value of velocity increases by 10 m/s each second. The direction of the velocity is in the direction of motion (always, for free fall or for any motion). So velocity is directed up while rising and down while falling. 
 

Hang Time
An important location on the trajectory is the highest position where the velocity is momentarily 0 m/s. If thrown upward from the ground at 60 m/s, it takes 6 seconds to reach this highest point ... another 6 seconds to fall back to the ground. Its in the air for a total time of 12 seconds.
 

Predicting t and v
It's worth noting that values of  t and v are predictable. Knowing the velocity at any time allows you to predict the velocity at every time. Suppose you know the velocity is +30 m/s at 3.0 seconds. The you know its 10 m/s slower 1 second later, and another 10 m/s slower 1 second after that, and so forth. And you know it was moving 10 m/s second faster 1 second earlier. Using an acceleration of 10 m/s/s, down, you know the entire history of t and v.
 
Finally, it's worth noting that there are two times at which an object has a speed of 20 m/s. One of these times is when its rising ... 2 seconds before the peak. The other time is when it is falling ... 2 seconds after the peak. Wherever the object has the same height, it also has the same speed. At one time, it's moving up; at the other time, it's moving down. 
 

Conclusion
In the Description section of this video, you will find links to some awesome interactive exercises on our website. The best way to ensure understanding of free fall is to apply the concepts you're trying to understand. So give one of them a try and make sure you got this. I'm Mr. H. Thanks for watching!  
 
 
 

 

 

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