Acceleration is a vector quantity, (i.e., it has a direction and magnitude!) that measures any change in velocity. That means speeding up, slowing down, and turning. Most people use the term only to mean speeding up, but when slowing down, we are applying negative acceleration to the movement. This is a very key concept for everything else we will learn, so keep in mind that: Acceleration is any change in velocity and that includes speeding up, slowing down, and turning.

A good short definition for acceleration is the rate of change in velocity. Suppose that at a certain instant a car has velocity (don't forget that velocity has direction!) v_i, and that at a later time t, its velocity is v_f, in this context the average acceleration of the car is a = (v_f-v_i)/t

Note: people normally use bold font to note vectors. Also we call v_i initial velocity and v_f final velocity.

You probably have an intuitive understanding of acceleration. You can feel changes in speed and direction. That's why fast cars and roller coasters are so much fun. Speeding around those tight turns and curves, or accelerating from 0 to 60 miles per hour in a few seconds causes your body to be pushed and pulled as the vehicle changes velocity and your body is forced to catch up.

So if you are stopped at a red traffic light, with v_i=0, and after 60 seconds you are at 12m/s then your average acceleration in this period was 12 m/s².

Galileo (1564-1626), considered by many the father of modern science, was the first to perform experiments on free fall. He pointed out that experiments should be conducted in a controlled way, and he designed many experiments to accurately time the fall of various objects of different masses. Based on these experiments he realized that the mass of an object does not influence the way it falls.

The bottom line here is that all falling objects are subjected to the same acceleration pulling them towards the Earth: gravity. In the same way an engine accelerates a car, gravity accelerates objects when they fall. The magnitude of the velocity of a falling body is shown in the graph below. If you calculate the "rate of change" of the velocity in any time interval in this plot you will see that the rate of change is g=9.8 m/s², the acceleration due to gravity.

 

 

Observe that the velocity-time data above reveal that the object's velocity is changing by 10 m/s each consecutive second. That is, the free-falling object has an acceleration of 10 m/s/s.

Assuming that the position of the free-falling ball dropped from a position of rest is shown every 1 second, then the velocity of the ball can be shown to increase as depicted in the diagram at the right.

(NOTE: The diagram is not drawn to scale - it would not take more than two seconds for a ball to drop from shoulder height to toe height.)

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Now it may be a good time to check out other pages. Let's go see what happens when elephants and feathers fall on: http://www.geocities.com/Athens/Academy/9208/efff.html

After this trip, there is a worksheet to fill about free fall: Worksheet4

I guess now we all understand what is going on whe things fall in ONE direction. What about baseballs and cannon balls???? These things undergo what we call projectile motion.

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The motion of a projectile (a ping-pong ball, a baseball, a cannonball, a rocket, a stone, etc...) can be "separated" into two motions. One up and down, and one left to right, just like when we were describing path and trajectory in the North-South or East-West direction. In other words, projectile motion is a combination of vertical free-fall motion and horizontal motion at a constant speed. The blue dots show the trajectory of the ball, while the red one, its free fall motion, accelerated by gravity. The green dots show the constant velocity motion in the horizontal direction.