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Transcript
Newton’s Second Law Section 2 p. 556-562 Force and Acceleration When the motion of an object changes, the object is accelerating Acceleration occurs anytime an object speeds up, slows down, or changes direction. Newton’s Second Law The second law connects the net force on an object, its mass and its acceleration. The second law saysAn object acted upon by an unbalanced force will accelerate in the direction of the force a=Fnet/m Newton’s Second Law“Simplified” A moving object speeds up if the net force is in the direction of the motion. A moving object slows down if the net force is the direction opposite the motion A moving object turns if the net force is at an angle to the direction of motion. Example of Law Newton's Second Law of Motion Units of Force Force is measured in newtons (N) 1N=1 kg·m/s2 Gravity Gravity is a force Gravity is also the force that causes Earth to orbit the Sun and the Moon to orbit the Earth. What is gravity? The force of gravity exists between two objects that have mass. Gravity is always attractive and pulls objects between each other. Force of Gravity The force of gravity depends on the mass of objects and the distance between them. Weight Weight is the gravitational force exerted on an object. Your weight on Earth is the gravitational force between you and the Earth. W=m(9.8m/s2) Weight and Mass Weight and mass are different. Weight is a force and is measured in newtons. Mass is the amount of matter in an object and it does not depend on location. Weight will vary with location, but mass remains constant. The weight of a book on Earth and on Mars would be different, but mass would be the same on both planets. Calculating Acceleration Suppose you pull a 10kg box so that the net force on the box is 5N. What is the acceleration? a=Fnet/m a= 5N ÷10kg a=0.5m/s2 The box keeps accelerating as long as you keep pushing on it. Acceleration does not depend on how fast the box is moving. It depends only on the net force and mass of the box. This is an example of how Newton's Second Law works: Mike's car, which weighs 1,000 kg, is out of gas. Mike is trying to push the car to a gas station, and he makes the car go 0.05 m/s2. Using Newton's Second Law, you can compute how much force Mike is applying to the car. Answer = 50 newtons Practice A book with a mass of 2.0kg is pushed along a table. If the net force on the book is 1.0 N, what is the acceleration of the book? a=Fnet/m F a m Answer- 1.0 n ÷ 2.0 kg = 0.5 m/s2 Practice A baseball has a mass of 0.15kg. What is the net force on the ball if it has an acceleration of 40.0 m/s2? F a m Answer- 0.15kg x 40m/s2 = 6 N NASA Video on Second Law This NASA video segment explores how Newton's second law of motion applies to aerospace. Viewers watch an instructor at NASA's National Test Pilot School as he defines the second law and demonstrates how to calculate a person's mass using the law. There is also a discussion about how people experience different g forces at the top and bottom of a roller coaster hill. Footage of the instructor in a fighter jet illustrates what it means to pull 2 and 4 g. NASA Air Resistance Whether you are walking, running or biking, air is pushing against you. This push is air resistance. Air resistance is a form of friction that acts to slow any moving object down moving in air. Air resistance gets larger as an object moves faster. Air resistance depends on the shape of the object. Air Resistance (cont.) When air resistance force equals the weight, the net force on the object is zero. By Newton’s second law, the object’s acceleration is then zero, and its speed no longer increases. When air resistance balances the force of gravity, the object falls at a constant speed called terminal velocity. Terminal velocity is the fastest velocity with which an object falls. Terminal Velocity of a Penny Click on Penny to see video Terminal Velocity Video of Falcon 1.How fast is the fall of a typical skydiver? A normal skydiver in free-fall hits a speed of about 120 miles per hour (mph). 2. What materials are used in the lures the falconer, Franklin, makes to train his falcons? Franklin uses small cotton bags filled with lead, and topped with a piece of meat. 3. How long does it take the peregrine falcon, Frightful, to catch the lure? It takes Frightful only 43 seconds to capture the lure. 4. What was Frightful's top speed? Frightful was clocked at diving 242 mph. 5. Speed-skydivers adapt their clothing to emulate the peregrine falcon in free-fall. What clothes are designed to mimic the bird's feathers? The skydiver's pants expand near the ankle, allowing him much greater control in free-fall. 6. What was the top speed of Mark, the skydiver? Mark has hit speeds of just over 300 mph Examples By spreading larger sails on a sailboat, it increases the force of the wind against the sails The maximum speed of a skydiver is 120mph What happens to the velocity if you increase the size of the parachute? Center of Mass The center of gravity is the point in an object that moves as it all the object’s mass were concentrated at that point. The object moves as if the net force is being applied there. Load Transfer NASCAR corners are divided into three parts because the car's grip changes in different parts of a turn. The higher center of gravity in the new car challenges crew chiefs to minimize weight shift around a turn. Equipment like the sevenpost rig helps, but the ultimate test is on the track. NASCAR Principals at Work in Skateboarding Aerodynamics of Flight http://www.sciencekids.co.nz/videos /engineering/flightaerodynamics.htm l Make a Paper Airplane Learn how to make 10 great paper airplanes with free animated instructions Free Paper Airplane DesignsPrintable Templates