 # Chapter 13, Section 2 Gravity and Motion

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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

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