Gravity and Freefall

Gravity pulls on all objects on the Earth, trying to pull them to the center of the Earth. We measure the amount of pull on your body by gravity with a bathroom scale and get your weight. The pull of gravity depends on how far you are from the center of the Earth. –There is more gravity pulling on you at the equator than the top of Mount Everest. –You would weigh less on Mount Everest than in Florida!

As things fall, they increase their velocity. The beginning velocity of anything dropped is zero! After it is dropped, it increases velocity (speeds up!) The acceleration due to gravity ( g ) is 9.8 m/s 2.

Size Doesn’t Matter? In dropping an object, without a force to slow it down, the object will continue to accelerate at the rate of gravity. All objects will “try” to accelerate at the rate of gravity. In the right environment (no air – the moon, a vaccuum), a golf ball and a feather will drop at the same rate of gravity and hit the ground at the same time.

Checking Your Understanding!?! If a bowling ball falls for 3 seconds, what will the velocity be? V f -V o = a * t V o = 0 m/s V f - 0 m/s= 9.8m/s 2 * 3 s V f - 0 m/s= 29.4 m/s

Checking Your Understanding!?! If a ping pong falls for 30 seconds, what will the velocity be? V f -V o = a * t V o = 0 m/s V f - 0 m/s= 9.8m/s 2 * 30 s V f - 0 m/s= 294 m/s

Ping Pong Balls at the Speed of a Bullet? The last question for the ping pong ball makes no sense!! We know that a ping pong ball will not accelerate that fast! What prevents a ping pong ball from continuing to accelerate? Or any other object accelerating, for that matter?

Wind Resistance Wind or Air Resistance pushes objects in the opposite direction of their motion. The paper airplanes had wind resistance slowing them down after they were thrown. Falling objects also have wind resistance!

Size Does Matter? The larger an object is, the more wind resistance it creates. Heavier objects will have less impact due to wind resistance. You experience this while riding a bike. –If you scrunch down while riding, you feel less wind pushing you back –If you sit up tall, you feel more air pushing you backwards (wind resistance)

Equilibrium Eventually the pull of gravity downwards (trying to accelerate) is equal to the wind resistance pushing against falling. When this happens the two pushes are in equilibrium. Gravity will not increase the velocity Wind resistance will not decrease the velocity We will not get any more change in velocity!!

Terminal Velocity When an object reaches its maximum velocity caused by gravity, it is said to have reached terminal velocity. When the pull of gravity and wind resistance are equal, terminal velocity is reached. The terminal velocity of a human is 120 mph (approximately 196 ft/sec) The terminal velocity of an ant is about 16 ft/sec Larger masses and smaller volume create larger terminal velocities.

Checking Your Understanding!?! If the terminal velocity of a basketball is 25 m/s, how long will it take for basketball to reach its terminal velocity when dropped from a building? V f -V o = a * t V o = 0 m/s 25 m/s - 0 m/s= 9.8m/s 2 * s 25m/s / 9. 8m/s s to reach terminal velocity

Checking Your Understanding!?! How long will it take for a basketball thrown from the top of a building up into the air at 20 m/s to reach the terminal velocity of 25 m/s V f -V o = a * t V o = -20 m/s, V f = +25 m/s 45 m/s= 9.8m/s 2 * s 45 m/s / 9.8m/s s