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October 13, 2016 3.3 Using Newton's Laws • Which of Newton's laws explains why a skateboard moves backward when you step off of it? • Car crashes...wear your seatbelt! > designed to keep you from bouncing around in the car, and your seat to lengthen the impact time. This results in a smaller force on the passenger > Airbags do the same thing - lengthen the impact time for the passenger Newton's Second Law and gravitational acceleration • Anything that is dropped experiences acceleration due to the gravitational force. You can still use: F = ma • Air resistance - a friction-like force that opposes the motion of the objects that move through the air. > air resistance reacts in the direction opposite to the motion of an object moving through air > amount of air resistance depends on the size, shape, and speed of the object, as well as the properties of the air > air resistance is the reason feathers fall more slowly than pennies. > The more spread out an object will change air resistance Glenco Physical Science 2012 October 13, 2016 Glenco Physical Science 2012 Speed and Terminal Velocity • amount of air resistance increases as velocity increases • gravity causes objects to accelerate as they fall • terminal velocity - maximum speed an object will reach when falling through a substance (like air) > occurs when the upward air resistance force becomes large enough to balance the downward force of gravity and the net force on the object is zero > An object's terminal velocity depends on its size, shape, and mass October 13, 2016 Free fall - occurs when gravity is the only force acting upon an object • Satellites are in free fall around Earth Weightlessness • objects orbiting in a spacecraft are weightless and seem to float because they are falling around the Earth inside the spacecraft show a zero weight Glenco Physical Science 2012 October 13, 2016 Centripetal Force - a force exerted toward the center of a curved path • Anything moving in a circular path does so because centripetal force is accelerating it toward the center > Forces that can act as centripetal force – gravity – amusement park rides - the push of the wall on riders – friction between car tires and the road on a curve Glenco Physical Science 2012 October 13, 2016 Glenco Physical Science 2012 Law of conservation of momentum - if no external forces act on a group of objects, their total momentum does not change October 13, 2016 Rocket propulsion The hot gases that are created by burning rocket fuel exerts a force on these gases and causes them to escape out the back of the rocket October 13, 2016 Homework: P 92 # 22,23,25,27