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Transcript
Forces Newton’s First and Second Laws • Newton’s 1st law of motion deals with inertia • An object at rest remains at rest, an object in motion maintains its velocity, unless acted upon by an outside force • Objects change their state of motion only when a net force is applied to the object • Inertia: the tendency of an object to maintain its state of motion. Inertia • The tendency of an object to maintain its state of motion • Inertia is related to the mass of an object – An object with more mass has more inertia – An object with less mass has less inertia • Which is harder to push? A couch or a car? – Which has the most inertia? Why? Inertia • Which is harder to stop? A train or a car traveling at 40 mph? – Why? • Use the concept of inertia to explain why seat belts are useful. • A pickup truck is at rest. A basketball is in the bed of the truck, near the cab. – What happens to the ball when the truck starts to move? – Why? Newton’s 2nd Law of Motion • Unbalanced forces cause an object to accelerate • Unbalanced forces produce a net force • Fnet = ma • Greater net force causes greater acceleration • Units of force – SI unit is the Newton – English unit is the pound Newton’s 2nd Law and Acceleration • Acceleration depends on mass and net force Fnet a m • Acceleration increases with more force • Acceleration decreases with more mass Gravity (12.2) • Weight and mass are not the same thing • Mass is a measure of matter • Weight is the force of gravity acting on a mass Weight • When an object falls, it experiences acceleration due to gravity, this is called free fall acceleration (g) • Weight = mass x free-fall acceleration • This is an application of Newton’s 2nd Law… F = ma or W = mg • Since weight is a force, units of weight are the same as units of force. Newtons (N) or pounds (lb) Weight and Mass • Weight of an object depends on the acceleration due to gravity • On Earth, W = mg Earth • WEarth = 66 kg x 9.8 m/s2 = 650 N ≈ 150 lb • On the Moon, W = mg Moon • W moon = 66 kg x 1.6 m/s2 =110 N ≈ 24 lb Gravity of Earth, Sun, Moon Law of Universal Gravitation • Gravity is a field force • It is always an attractive force between two masses • The force of gravity between two objects depends upon… • How big the masses are • How far apart they are Newton’s Law of Universal Gravitation • If the masses are bigger, the force of gravity between them is bigger • If the distance between the masses is larger, the force of gravity is smaller m1m2 F G 2 r Newton’s Law of Universal Gravitation Free Fall • When the only force acting on an object is gravity • In the absence of air resistance, all objects fall at the same rate… • Regardless of their masses Terminal Velocity • When air resistance acts on an object, it slows its rate of fall • Eventually the object will stop accelerating • Terminal velocity is the velocity of a falling object when the force of air resistance is equal to but opposite direction of force of gravity • In other words, the force of air resistance balances the force of gravity Are Astronauts Weightless? • No • Astronauts are in free fall • But the earth is curving away from them as they fall • So they don’t fall to Earth Projectile Motion • A projectile is a moving object that is under the influence of gravity only • Examples – Baseball, bullet, a hopping toad, a long jumper – NOT: powered rocket, airplane, etc. • Projectiles follow a parabolic path (trajectory) • Projectile motion has both vertical and horizontal components (parts) Projectile Motion has Horizontal and Vertical Components Projectile Motion