Free Fall Motion Questions вЂў Does every object fall the same? вЂў Should every object fall the same? вЂў What is a fluid? вЂў Do we live in a fluid? вЂў What is the composition of our atmosphere? вЂў Make a table showing the differences between the Earth and Moon. AIR DENSITY 4.5-4.6 Free Fall Falling Objects Imagine there is no air resistanceвЂ¦ An object moving under the influence of the gravitational force only is said to be in free fall. 4.5-4.6 Free Fall The acceleration of an object in free fall is 9.8 m/s2. Neglecting air resistance! 4.8 Air Resistance and Falling Objects Drop a feather and a hammer on earth and the hammer reaches the floor far ahead of the feather. What about on the Moon? http://history.nasa.gov/40thann/videos.htm 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 How objects fall without air resistance? F gravity or weight is the only force 4.8 Air Resistance and Falling Objects How objects fall without air resistance? Objects accelerate equally. Why? F gravity or weight is the only force 4.8 Air Resistance and Falling Objects 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 ItвЂ™s just like on the __________________ F gravity or weight is the only force 4.5 Free Fall: How Fast During each second of fall the instantaneous speed of the object increases by an additional 9.8 meters per second. This gain in speed per second is the acceleration. 4.5 Free Fall: How Fast v = gt v represents both speed and velocity. g represents acceleration due to gravity (9.8 m/s2) t represents time the object is free-falling 4.5 Free Fall: How Fast 9.8 m/s 19.6 m/s 29.4 m/s 39.2 m/s 49 m/s 9.8 m/s x t Free Fall вЂў 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 вЂў Physicists consider air resistance to be negligible for heavier objects that fall near the surface of the Earth. DonвЂ™t worry about air when making calculations! Fg = 71.2 N Fg = 100 N Fg = 11 N 4.5 Free Fall: How Fast 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. The object is accelerating because its velocity is changing. How much does its speed decrease each second? 4.8 Air Resistance and Falling Objects How objects fall without air resistance? F gravity or weight is the only force 4.8 Air Resistance and Falling Objects How objects fall without air resistance? Objects accelerate equally. Why? F gravity or weight is the only force 4.8 Air Resistance and Falling Objects 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 ItвЂ™s just like on the __________________ F gravity or weight is the only force 6.6 Free Fall Explained In GalileoвЂ™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. 6.6 Free Fall Explained g = weight/mass F stands for the force (or weight in newtons) m stands for the mass of the cannonball a is the rate of acceleration a = force/mass вЂў the weight-to-mass ratio is the same for these or any objects, which means that the acceleration rates are 10x gravitational force the same. or weight 1 kg rock 10 kg cannonball 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? 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. AIR RESISTANCE вЂў A resistance force caused by air molecules opposing the motion of an object as it moves through the air. вЂў A form of friction sometimes called drag. 6.7 Falling and Air Resistance Air resistance does not depend upon the weight of the object. The amount of air resistance force an object experiences depends on the objectвЂ™s speed and exposed surface area. 1. Speed The greater the speed, the greater the air resistance. 2. Surface Area (exposed or frontal) The greater the surface area, the greater the air resistance. Another variableвЂ¦ вЂў How would the composition or density of an atmosphere influence air resistance? Composition of Air Helium EarthвЂ™s atmosphere Same concentration, different masses AIR DENSITY Less drag More drag 6.7 Falling and Air Resistance What two factors determine the air resistance force on an object? The speed and the exposed surface area 6.7 Falling and Air Resistance The air resistance force an object experiences depends on the objectвЂ™s _______ and ___________ _______. 4.5 Free Fall: How Fast think! How fast would a free-falling rock be moving 4.5 seconds after it drops from rest? How about 8 seconds after it is dropped? 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 For each second of free fall, an object falls a greater distance than it did in the previous second. 4.6 Free Fall: How Far These distances form a mathematical pattern: at the end of time t, the object starting from rest falls a distance d. 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 think! An apple drops from a tree and hits the ground in one second. What is its speed upon striking the ground? What is its average speed during the one second? How high above ground was the apple when it first dropped? 4.6 Free Fall: How Far think! An apple drops from a tree and hits the ground in one second. What is its speed upon striking the ground? What is its average speed during the one second? How high above ground was the apple when it first dropped? Answer: 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.

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