Description: Under the influence of gravity, objects experience constant acceleration.
Key Terms: Acceleration, velocity, Galileo, free fall, forces, gravity
Description: All objects on earth fall at an equal acceleration G = 9.8 meters/second. Change in distance can be calculated using G and time.
Key Terms: Change in time, velocity, acceleration, Galileo, equation
Description: All objects fall at the same rate, unless other forces like air exert a force on them. Air can cause light objects like feathers to fall more slowly.
Key Terms: Acceleration, free fall, force, wind
Description: A light weight and a heavy weight will fall at the same rate.
Key Terms: Changing speed, force, speedometer
Description: Objects that are light with a large area are slowed down by air resistance.
Key Terms: Mass, gravity, force, air resistance, normal force, Newton
Things fall because of the force of gravity. When a falling object is free of all restraints—no friction, with the air or otherwise—and falls under the influence of gravity alone, the object is in a state of free fall.
During each second of fall, the object gains a speed of 10 meters per second. This gain per second is the acceleration. Free-fall acceleration is approximately equal to 10 meters per second each second, or, in shorthand notation, (read as 10 meters per second squared). Note that the unit of time, the second, enters twice—once for the unit of speed and again for the time interval during which the speed changes.
In the case of freely falling objects, it is customary to use the letter g to represent the acceleration (because the acceleration is due to gravity). The value of g is very different on the surface of the Moon and on the surfaces of other planets. Here on Earth, g varies slightly in different locations, with an average value equal to 9.8 meters per second each second (9.8 m/s2 ). We round this off to in our present discussion to establish the ideas involved more clearly; multiples of 10 are more obvious than multiples of 9.8. Where accuracy is important, the value of 9.8 m/s2 should be used.
So far, we have been considering objects moving straight downward in the direction of the pull of gravity. How about an object thrown straight upward? Once released, it continues to move upward for a time and then comes back down. At its highest point, when it is changing its direction of motion from upward to downward, its instantaneous speed is zero. Then it starts downward just as if it had been dropped from rest at that height. During the upward part of this motion, the object slows as it rises. It should come as no surprise that it slows at the rate of 10 meters per second each second—the same acceleration it experiences on the way down.
It is a common observation that many objects fall with unequal accelerations. A leaf, a feather, or a sheet of paper may flutter to the ground slowly. The fact that air resistance is responsible for these different accelerations can be shown very nicely with a closed glass tube containing light and heavy objects—a feather and a coin, for example. In the presence of air, the feather and coin fall with quite different accelerations. But, if the air in the tube is removed by a vacuum pump and the tube is quickly inverted, the feather and coin fall with the same acceleration
