Forces and Motion Chapter 6

Gravity and motion Section 6:1

Gravity and Falling Objects Background 400 BCE, Aristotle thought that the rate an object falls depends on its mass. He never tested this idea 1500’s Galileo Galilee questioned Aristotle's idea and argued that the mass doesn’t matter Galileo changed the way people thought

Gravity and Acceleration Objects fall at the same rate because acceleration due to gravity is the sam for all objects Acceleration depends on force and mass Heavier objects are harder to accelerate because they have more mass Extra mass balances out the gravitional forces

Acceleration due to gravity Acceleration is the rate at which velocity changes over time All objects on Earth accelerate toward Earth at 9.8m/s2 Every second an object falls, its downward velocity increases 9.8 m/s

Velocity of falling objects Change of velocity of a falling objects V = G x t Velocity = gravity x time G = 9.8m/s2 Change in velocity = final velocity – initial velocity Example A penny is dropped for the top of a tall stairway. What is the penny’s velocity after 2 seconds? After 4.5 seconds

Air Resistance and Falling Objects Force that opposes motion though air Depends on the object’s size, shape and speed Examples Flat vs. crumpled piece of paper

Acceleration Stops at Terminal Velocity Terminal velocity is the constant velocity of a falling object when the forces of the air resistance is equal in magnitude and opposite in direction as gravity As an object falls, speed and air resistance increase Upward air resistance increases until it is equal to the downward pull of gravity This equals a net force of 0 and the object stops accelerating Terminal velocity is a good thing Hailstones normally travel at a velocity between 5-40 m/s Without air resistance = 350 m/s

Free Fall Free fall occurs when there is no air resistance Only gravity is working on the objects Only place with no outside force ( air resistance) is space or a vaccuum

Orbiting objects are in free fall Astronauts float because they are in free fall Gravitational force is very small because the distance is very large They still have mass Gravity will attract, if slightly, so technically they are NOT weightless

Orbiting When one object is traveling around another object in space 2 motions are present Shuttle moves forward and it is in free fall Makes a curved path Shuttle is falling while in orbit

Centripetal Force Moon or other planets in orbit travel in circular path Any object in circular path in constantly changing directions Unbalanced forces need to change directions therefore the unbalanced force is in circular motion Centripetal force is an unbalanced force that causes objects to move in a circular path

Centripetal means “toward center” Gravity is the centripetal force that keeps planets in orbit

Projectile Motion and Gravity Curved path that an object follows when thrown, launched or otherwise projected near the Earth’s surface Examples frog leaping, water sprayed by sprinklers, arrow shot by archer Projectile motion has 2 componets Horizontal motion Vertical motion

Horizontal Motion Force to put an object in forward motion baseball Gravity doesn’t affect horizontal motion constant

Vertical motion Gravity pulls objects down 9.8m/s2 Gravity gives vertical motion Aim is caused by downward acceleration