Free Fall Motion

Falling objects accelerate The force of gravity causes falling objects to accelerate in the downward direction. Remember! Acceleration is a change in velocity. This falling apple’s speed increases as it falls.

Imagine there is no air resistance… Free Fall Falling Objects only pulled by gravity are said to be in free fall Imagine there is no air resistance… (Air resistance produces an upwards force, kinda like friction for falling things) An object moving under the influence of the gravitational force only is said to be in free fall.

https://youtu.be/5C5_dOEyAfk Drop a feather and a hammer on earth and the hammer reaches the floor far ahead of the feather. What about on the Moon? https://youtu.be/5C5_dOEyAfk We will ignore air resistance for the first part of the lesson.

Bowling ball Tennis ball Steel marble Wad of Paper http://www.watchframebyframe.com/watch/yt/zTWv1n97Mko Tennis ball http://www.watchframebyframe.com/watch/yt/lSFep-6Cv0A Steel marble http://www.watchframebyframe.com/watch/yt/EbDAQ7DUNQY Wad of Paper http://www.watchframebyframe.com/watch/yt/fm2Ct1dShHU

Free Fall: How Fast During each second of fall the instantaneous speed of the object increases by 9.8 meters for every second (per second). This gain in speed per second is the acceleration. We will round acceleration to 10 to make the math easier.

Acceleration due to gravity The acceleration of an object in free fall on the earth is -9.8 m/s2. We will round it to -10 m/s2 to make the math easier. Negative because it is in the down direction. We represent acceleration due to gravity with the letter “g.” a =g = 9.8 m/s2

How Fast: finding instantaneous speed of an object that started at rest. v = gt v represents speed/velocity. ΔV is final – initial but we can assume the initial is zero if we are dropping things. Therefore, the final velocity becomes the only one we need. g represents acceleration due to gravity (-9.8 m/s2). You may also see this in an equation as acceleration. Just remember for free fall it will mean gravity. t represents time the object is free-falling. We should look at the time it takes to fall or go up. Like throwing a ball. If it is doing both, then we would have double the time.

Rising Objects Rising objects decelerate at the same rate that falling objects accelerate. During the upward part of this motion, the object slows from its initial upward velocity to zero velocity, losing 9.8 m/s of speed per second. During the downward part of this motion, the object gains 9.8 m/s of speed per second.

Free fall equations d=vit + ½ g t2 d= ½ g t2 If the object is dropped then we can assume vi is zero and can be dropped from the equation. d= ½ g t2  

Free Fall An object is said to be in free fall if it is only under the influence of gravitational force. This object will fall at a rate of acceleration equal to 9.8 m/s2. No support or resistance force! Fg = 100 N

Free Fall Physicists consider air resistance to be negligible for heavier objects that fall near the surface of the Earth. Fg = 71.2 N Fg = 11 N Fg = 100 N

6.6 Free Fall Explained In Galileo’s famous demonstration, a 10-kg cannonball and a 1-kg stone strike the ground at practically the same time. This experiment demolished the Aristotelian idea that an object that weighs ten times as much as another should fall ten times faster than the lighter object.

4.8 Air Resistance and Falling Objects A feather and a coin accelerate equally when there is no air around them. Vacuum tube

4.8 Air Resistance and Falling Objects F gravity or weight is the only force How objects fall without air resistance?

Why? 4.8 Air Resistance and Falling Objects F gravity or weight is the only force How objects fall without air resistance? Objects accelerate equally. Why?

Why? 4.8 Air Resistance and Falling Objects F gravity or weight is the only force How objects fall without air resistance? Objects accelerate equally. Why? No atmosphere means no air resistance (no drag force), so surface area and weight make no difference. All objects fall at the same rate. Moon It’s just like on the __________________

4.8 Air Resistance and Falling Objects F gravity or weight is the only force How objects fall without air resistance?

Why? 4.8 Air Resistance and Falling Objects F gravity or weight is the only force How objects fall without air resistance? Objects accelerate equally. Why?

Why? 4.8 Air Resistance and Falling Objects F gravity or weight is the only force How objects fall without air resistance? Objects accelerate equally. Why? No atmosphere means no air resistance (no drag force), so surface area and weight make no difference. All objects fall at the same rate. Moon It’s just like on the __________________

6.6 Free Fall Explained g = weight/mass a = force/mass F stands for the force (or weight in newtons) m stands for the mass of the cannonball a is the rate of acceleration the weight-to-mass ratio is the same for these or any objects, which means that the acceleration rates are the same. 10x gravitational force or weight 1 kg rock 10 kg cannonball

Why? 6.6 Free Fall Explained Since the ratio of weight (F) to mass (m) is the same for the 10-kg cannonball and the 1-kg stone, they both fall at the same rate of acceleration. Why?

Why? 6.6 Free Fall Explained Since the ratio of weight (F) to mass (m) is the same for the 10-kg cannonball and the 1-kg stone, they both fall at the same rate of acceleration. Why? Weight-mass ratios for each are identical! It takes a larger force to keep the larger mass accelerating because a larger mass has more inertia.

6.6 Free Fall Explained All freely falling objects fall with the same acceleration because the ratio of weight to mass is the same for all objects. On Earth, it is 9.8 m/s2.

AIR RESISTANCE A resistance force caused by air molecules opposing the motion of an object as it moves through the air. A form of friction sometimes called drag.

6.7 Falling and Air Resistance Air resistance does not depend upon the weight of the object. 6.7 Falling and Air Resistance The amount of air resistance force an object experiences depends on the object’s speed and exposed surface area. 1. Speed The greater the speed, the greater the air resistance. 2. Exposed Surface Area The greater the surface area, the greater the air resistance.

AIR DENSITY Less drag Objects that fall through the atmosphere experience two forces weight downward air resistance or drag upward More drag As the object falls, its air resistance increases with speed, and the object’s rate of acceleration decreases until it is at constant velocity: That means that the object has reached terminal velocity! Balanced force

The speed and the exposed surface area 6.7 Falling and Air Resistance What two factors determine the air resistance force on an object? The speed and the exposed surface area

4.5 Free Fall: How Fast What is the acceleration of an object in free fall? 9.8 m/s2

4.6 Free Fall: How Far and How much time it takes to fall t = √2d/g

4.6 Free Fall: How Far 4.9 m 19.6 m 44.1 m 78.4 m 122.5 m

4.6 Free Fall: How Far For a falling object, how does the distance per second change? For each second of free fall, an object falls a greater distance than it did in the previous second.