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Chapter 13, Section 2 Gravity and Motion Pages 376 - 382 Falling Objects • Galileo - all objects fall to the ground at the same rate (9.8 m/s/s) because the acceleration due to gravity is the same for all objects. • Velocity of falling objects: ∆v = g x t g = 9.8 m/s/s t=falling time Example • 1. If a stone at rest is dropped from a cliff and it takes 3 seconds to hit the ground, what is the stone’s downward velocity when it hits the ground? • ∆v = g x t • ∆v = 9.8 m/s/s x 3s • ∆v = 29.4 m/s Practice • A penny at rest is dropped from the top of a stairwell. What is the penny’s velocity when it hits the ground after falling for 4.5 seconds? • ∆v = g x t • ∆v = 9.8 m/s/s x 4.5 s • ∆v = 44.2 m/s Air Resistance • Air resistance is a type of fluid friction that opposes the motion of objects moving through the air. • The amount of air resistance on an object depends on its size and shape. Terminal Velocity • Eventually the gravitational force pulling down on a falling object equals the force of air resistance pushing up. Free Fall • An object is in free fall only if gravity is pulling it down and no other forces are acting on it. • Free fall can only happen in a vacuum or in space. • The forces are then balanced, the object quits accelerating and falls at a constant velocity = terminal velocity. Projectile Motion and Gravity • Projectile motion is the curved path an object follows when thrown or propelled near the surface of the Earth. • Has two components - horizontal and vertical. Horizontal Motion • Horizontal motion is the motion that is parallel to the ground. • When you throw a ball, your hand gives the ball its horizontal motion. Vertical Motion • Vertical motion is the motion that is perpendicular to the ground. • Gravity causes vertical motion. Orbiting and Gravity • An object is orbiting when it is moving around another object in space. • Centripetal force is the unbalanced force that makes an object move in a circular path around another object. • Gravity provides the centripetal force on the planets and their moons that keeps them in orbit