Free Fall Honors Physics Date: September 16

Engagement Why is easy to step off a curb, but very scary (and dangerous) to step off the top of building?

Objectives Describe the motion of an object in free fall and how it is different from motion we have previously studied Describe the motion of an object thrown straight up and allowed to fall until it hits the ground Evaluate how speed and the distance play a part in a falling object at any time after an object is dropped from rest, when air resistance is negligible

Intro to Free Fall Questions What is free fall? What is an example of something in free fall? Facts When the only thing affecting an object is gravity (there is no air resistance) Dropping something (not throwing it down) Throwing something up and allowing it to fall back down

A little background on gravity A free-falling object is an object that is falling under the sole influence of gravity. Gravity can be measured as an acceleration which is the result of the central attractive force that matter exerts on other matter. This is because all accelerations are the result of force (we will learn more about this). All objects exert gravity on each other; however, the gravity we experience is the gravity of the Earth. Gravity is constant on Earth; therefore, the acceleration is constant (does not change) when an object is in free fall even though the velocity is changing!

Gravity Questions What is gravity What might be the units for gravity? Why? What is the value for gravity? Facts The acceleration that an object has when in free fall m/s 2 Because gravity is measured as an acceleration Actual = 9.8m/s 2 For class = 10 m/s 2

More about gravity Questions What type of motion have we studied so far? (v=d/t) (a=v/t) What type of motion do we witness with free fall? Facts -Linear, horizontal motion - Linear, vertical motion

How Does Gravity Work? What is happening to the velocity? Why? As the ball falls the velocity is increasing because gravity has more time to act. At each point, gravity remains 10 m/s 2 t = 0 s v = 0 m/s t = 2 s v = 20 m/s t = 1 s v = 10 m/s t = 3 s v = 30 m/s t = 4 s v = 40 m/s t = 5 s v = 50 m/s

How Does Gravity Work? Now you try to complete the figure! Explain the figure!!! t = _______ v = _______ t = _______ v = _______ t = _______ v = _______ t = _______ v = _______ t = _______ v = _______ t = _______ v = _______

How does gravity work? Consider what happens when an object is thrown straight up and falls back down.

How Does Gravity Work? Question Explain what happens when an object is thrown straight up and allowed to fall back down. Facts Moving up- the velocity is decreasing because the object is moving against gravity. Falling Down- the object gains speed at the same rate it was slowed down because gravity is constant?

How gravity works? Question What will happen if you drop 2 different objects at the same time from the same height? Why? Facts] They will hit the ground at the same time because… Gravity is constant and is not dependent on the mass or weight of an object.

Check for Understanding If you drop a feather and a coin at the same time in a vacuum tube with no air resistance, which will reach the bottom of the tube first? Neither, they will both hit the ground at the same time!

What affects free fall? Question How does distance play a part in a falling object at any time after an object is dropped from rest? Facts The higher (more distance) an object falls, the more time gravity has to act; therefore, the faster the object will hit the ground.

Let’s go back to the engagement Questions Why is easy to step off a curb, but very scary (and dangerous) to step off the top of a building? Facts A curb is only a small distance from the ground; therefore, gravity acts only for a small time and velocity is not greatly gained. From a building, the distance is greater, giving gravity more time to act, which results in a greater velocity when hitting the ground- Ouch!

Check for understanding A freely falling object starts from rest. After falling for 1 second, it will have a speed of about a. 5 m/s. b. 10 m/s. c. 20 m/s. d. 40 m/s. a. 5 m/s. b. 10 m/s. c. 20 m/s. d. 40 m/s.

Check for Understanding A ball is thrown upwards and caught when it comes back down. In the absence of air resistance, the speed of the ball when caught would be a.less than the speed it had when thrown upwards. b.more than the speed it had when thrown upwards. c.the same as the speed it had when thrown upwards. a.less than the speed it had when thrown upwards. b.more than the speed it had when thrown upwards. c.the same as the speed it had when thrown upwards.

Check for Understanding When a basketball player jumps to make a shot, once the feet are off the floor, the jumper's acceleration a.varies with body orientation. b.is usually greater for taller players (but not always). c.depends on all the above. d.is g; no more, no less. a.varies with body orientation. b.is usually greater for taller players (but not always). c.depends on all the above. d.is g; no more, no less. Remember, Gravity is constant on Earth; therefore, the acceleration is constant (does not change) when an object is in free fall even though the velocity is changing!

Check for Understanding A ball is thrown straight up. At the top of its path its instantaneous speed is a.0 m/s. b.about 5 m/s. c.about 10 m/s. d.about 20 m/s. a.0 m/s. b.about 5 m/s. c.about 10 m/s. d.about 20 m/s.

Let’s look at some equations d = v o t + (1/2)gt 2 v f 2 = v o 2 + 2gd v f = v o + gt Questions How do we calculate when an object is in free fall? Facts We use three equations when an object is in free fall

Free Fall vs. Other Equations Questions Why can’t we use v = d/t or a = v/t for free falling objects? Facts Because neither equation takes gravity into consideration

Homework  Review Free Fall Notes and Reading!  Try practice problems 9, 10, and 13  Have lab notebook next class.