What Goes Up, Must Come Down!

Free Fall and Gravity

Acceleration Review 1. A biker is traveling at 12.0 m/sec and

speeds up to pass a car to 18.5 m/sec in 1.5 seconds. What is the biker’s acceleration?

2. A car accelerates from rest at a rate of 6.3 m/s2 for 4.3 seconds. What is the car’s final velocity?

What is Free Fall?An object is in free fall if it is

accelerating due to the force of gravity and no other forces are acting on it.A dropped baseball is in free fall as soon as it leaves your hand.

A ball thrown upward is also in free fall after it leaves your hand.

GalileoPerformed experiments to

convince others that the acceleration caused by gravity would be the same for all freely falling objects if there was no air to retard their motion.

He dropped two heavy metal balls together from the leaning tower. Although one weighed much more than the other, they reached the ground almost at the same time.

Acceleration Due to Gravity On Earth we

can’t fall without air resistance, but for normal situations the numbers are realistic.

When things fall they constantly accelerate.

Acceleration Due to Gravity If we can ignore air resistance, all objects accelerate at the same rate.

9.8 m/s2 = g (acceleration due to gravity on Earth at sea level)

Say What?When most of the air is

removed from a container, feathers and apples fall almost side-by-side, their speeds changing at almost the same rate.

If all the air was removed, they would accelerate downward at exactly the same rate.

Example of Calculating Free Fall Speed1. A cannon ball is dropped from a tall

building. Calculate the speed of the ball after 2.25 seconds.

Video Clipshttps://

www.youtube.com/watch?v=AYz_K3mwq6Ahttps://www.youtube.com/watch?v=_

mCC-68LyZMapp.discoveryeducation.com/search?

Ntt=free+fall

Using time to find distance:d = 1/2 g·t2

This formula only works for objects in free fall close to the surface of the Earth.

Check first example using this formula.

Examples:An apple falls from the top branch of a tree

and lands 1.38 seconds later. How tall is the tree?

Calculate the final speed and distance of free fall for a skydiver who waits only 4.17 seconds to open his parachute.

Air ResistanceThe opposing force on a moving

object due to the friction of air.Increases with speedThe size and shape of an object

affect the force of air resistance.Surface area to weight ratio.

Terminal VelocityThe maximum speed reached by an object

in free fall.The speed at which the forces of gravity

and air resistance are equal.After object reaches terminal velocity, its

speed is constant.Dependent on the ratio of an object’s

weight to its air resistance.