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**Chapter 13, Section 2 Gravity and Motion**

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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

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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

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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

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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.

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Terminal Velocity Eventually the gravitational force pulling down on a falling object equals the force of air resistance pushing up.

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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.

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The forces are then balanced, the object quits accelerating and falls at a constant velocity = terminal velocity.

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**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.

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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.

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Vertical Motion Vertical motion is the motion that is perpendicular to the ground. Gravity causes vertical motion.

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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

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