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FREE FALL What did the video show? In the absence of air resistance, all objects fall – regardless of mass or shape – fall at the same rate. Video So, on the moon or in a vacuum – a feather and a hammer fall at the same rate. FREE FALL Free fall is vertical (up/down) motion of an object where gravity is the only major force. (This means there is little air resistance) Freely falling objects have constant acceleration towards earth, represented by the symbol g. g = 9.81 m/s2 towards Earth. Make the motion graphs of a ball thrown up in the air.. AIR RESISTANCE In the absence of air resistance, all objects fall at the same rate! Why? Because all objects are subject to the same gravitational force. That’s what we saw in the video. Air resistance is a force that acts opposite gravity. This depends on the shape of object (think parachutes!). When air resistance occurs, and object will have a terminal velocity – a maximum velocity – instead of accelerating continuously Without air resistance, rain drops would fall hard enough to cause serious damage! TEST YOUR UNDERSTANDING Read the problem and think – 2 minutes. Then, show 1 for Akira, 2 for Burt, 3 for Cataline, or 4 for None of them. SOLVING FREE FALL PROBLEMS We solve free fall problems using the same equations and procedures as other acceleration problems. Just keep in mind a few things … a always equals 9.8 m/s2 An object’s velocity is 0 m/s at the top of its path A ‘dropped object’ has zero vi IMPORTANT HINTS So long for as problem an object is thrown solving !!! from the same height at which it lands, the path up and the path down are symmetrical Time up = time down Velocity of launch = -velocity of landing FREE FALL PROBLEMS – WE DO Sally throws a ball upward with initial speed of 20 m/s. How high will it go? How long will it take for the ball to come back? FREE FALL PROBLEMS – WE DO Sally throws a ball upward with initial speed of 20 m/s. How high will it go? How long will it take for the ball to come back? Givens: vi = 20 m/s g = - 9.8 m/s2 at the top v = 0 Unknowns: t=? y=? Use vf = vi + at to find t = 2 sec Use x = vi t + 20 m 2 to find x = When talking about vertical distance, we often use y instead of x! FREE FALL PROBLEMS – WE DO Mrs. Radja, hovering in a helicopter 2.0 X 102 m above our school suddenly drops her pen. How much time will the students have to save themselves? What is the velocity/speed of the pen when it reaches the ground? Givens: vi = 0 m/s (dropped) g = 9.8 m/s2 x = vi t + vf = vi + at 2 Unknowns: t=? vf = ? t = 6.4 sec Vf = -63 m/s FREE FALL PROBLEMS – YOU DO 1. A coin is tossed vertically upward. (a) What happens to its velocity while it is in the air? (b) Does its acceleration increase, decrease, or remain constant while it is in the air? 2. A pebble is dropped down a well and hits the water 1.5 s later. Determine the distance from the edge of the well to the water’s surface. 3. Stephanie hits a volleyball from a height of 0.8 m and gives it an initial velocity of 7.5 m/s straight up. a) How high will the volleyball go? b) How long will it take the ball to reach its maximum height? FREE FALL PROBLEMS – YOU DO 1. a) The velocity is upward and decreases until it reaches zero, then the velocity is downward and increases until it lands. b) Constant. 2. Vi = 0, a = -9.8 m/s2, t = 1.5 sec find x 11 m down 3. Vi = 7.5, xi = 0.8 m, vf = 0, a = -9.8 m/s2 find xmax and time until xmax vf2 = vi2 + 2ax x max = 2.9 m + 0.8 m = 3.7 m vf = vi + at t = 0.77 sec x = vi t + 2