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Unit 6 Chapter 6 Forces and Motion Gravity and Motion • 400 BC • Aristotle – Thought that the rate at which an object falls depends on its mass – Heavier things fall faster – Never tested this theory Gravity and Motion • 1589 • Galileo Galilei – Argued that Aristotle was wrong – Galileo said that mass doesn’t affect the rate of objects falling – Proved argument with the Leaning Tower of Pisa experiment • Stairwell demo Gravity and Motion • Acceleration due to gravity – Acceleration is the change in velocity over time – All objects on Earth accelerate toward Earth at a rate of 9.8 m/s2 – For every second you are in free fall, you speed up at a velocity of 9.8 m/s – We use the letter g as an abbreviation for this (9.8 m/s2) Gravity and Motion • Δv = g x t • Change in velocity equals acceleration due to gravity multiplied by time • To find velocity in free fall, multiply 9.8 times the number of seconds from start to finish Gravity and Motion • Air resistance – Opposes motion of objects falling – Flat paper vs crumpled, which falls faster? • Terminal velocity – As velocity increases, air resistance increases – Upward force of air resistance increases until it is equal to g – At this point, object stops accelerating, and falls at a constant velocity – terminal velocity Gravity and Motion • Terminal velocity is what keeps small things from hitting the ground extremely fast • Hail would normally fall at 500 mph without terminal velocity • Free fall – Motion of an object with only the force of gravity acting upon it Gravity and Motion • Feather vs lead • In normal circumstance, feather falls slower due to air resistance • In a vacuum, they fall at the same rate, since there is no air resistance • Free fall is how astronauts feel in outer space, because the force of gravity is very small • Are the astronauts weightless when floating? Gravity and Motion • Orbiting – an object traveling around another object in space • The moon orbits the Earth • The Earth orbits the Sun • The sun orbits around the galaxy Gravity and Motion • Most orbits are circular in motion • Objects in circular motion are always changing direction • Force that causes objects to move in circular motion is called centripetal force • Always moving toward the center of what you are orbiting Gravity and Motion • Projectile motion – Curved path an object follows when thrown – Has two components • Horizontal - outward straight • Vertical – downward straight – When two motions are combined, forms a curve Newton’s Laws of Motion • 1686 • Sir Isaac Newton – Came up with explanations of why things move the way the do – Three laws of motion Newton’s Laws of Motion • Newton’s 1st Law – An object at rest remains at rest, and an object in motion remains in motion at constant speed and in a straight line unless acted upon by an outside force – Something moving will move in a straight line forever – Something sitting still will stay sitting still forever – Unless some force stops or starts it moving Newton’s Laws of Motion • Slide a book across your desk • Does it stay moving forever? • What stops the book from moving? • Friction is an unbalanced force that opposes motion • Causes moving objects to slow down Newton’s Laws of Motion • Inertia • Newton’s 1st Law sometimes called Law of Inertia • Inertia is tendency to resist any CHANGE in motion • Larger the mass, the larger the inertia • Object at rest stays at rest because of inertia • Object moving stays moving because of inertia Newton’s Laws of Motion • Newton’s 2nd Law of Motion – The acceleration of an object depends on the mass of the object and the amount of the force applied – Acceleration depends on mass • Larger things accelerate slower – Acceleration depends on force • Larger force, larger acceleration – Written as F = ma Newton’s Laws of Motion • a = F/m • F = ma • Find the force of a 10 kg object being accelerated at 100 m/s2 • Find the acceleration of an object that was pushed with a force of 100 N and has a mass of 20 kg • How do you find the mass of an object if you know the force and the acceleration? Newton’s Laws of Motion • Newton’s 3rd Law – Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first – For every action there is an equal and opposite reaction – All forces act in pairs – Your butt pushes down on the chair, the Earth is pushing back up on your butt with the same force Newton’s Laws of Motion • Forces do not always act on the same object – Tires and road • Can sometimes be hard to see – A ball thrown through the air Momentum • Momentum – Depends on mass and velocity – Larger the mass, more momentum – Larger the velocity, more momentum • p = mv • Momentum = Mass x Velocity Momentum • What has more momentum, a 1300 kg car traveling at 25 m/s, or a 5000 kg truck traveling at 10 m/s? • p = mv Momentum • Law of Conservation of Momentum – Any time objects collide, the total amount of momentum stays the same – This applies whether the objects bounce off each other or stick together – Cars sticking together after a wreck, cue ball shot at 8 ball when playing billiards