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Forces of Motion Chapter 2 Gravity and Motion • Aristotle 400 B.C the rate at which and object falls depends on its mass All object fall with the same acceleration • Galileo questioned Aristotle’s idea. • He proved mass does not affect the rate of falling objects Acceleration Due to Gravity • The same for all objects • While gravity is greater b/t objects with large mass • Larger mass object require more force to fall at higher speeds Accelerating at a Constant Rate • All falling object have the same acceleration • Thus gravity= g • agravity = g = 9.8m/s2 = 9.8 m/s/s • It is important to point out that 9.8m/s2 down is NOT gravity, it is the acceleration DUE TO GRAVITY Math Break • What is the penny’s velocity after it has fallen for 2 seconds? Solution v = gt 9.8 m/s/s x 2s = 19.6 m/s Math • The Penny hits the ground in 4.5 seconds. What is its final velocity? Solution • 9.8 m/s x 4.5 s = 44.1 m/s Air Resistance Slows Down Acceleration • Drop two Sheet of Paper one balled up and one flat • What happened? • Why? Answer • Fluid friction that opposes motion in air – *Remember Fluid friction includes gases • This fluid friction is called air resistance – Occurs b/t the surface of the falling object and the air that surrounds it Elephant and a Feather • Who will fall the fastest and why? Air Resistance Depends • Size • Shape The force of air resistance The net force Not 0 the object accelerates down – The force of gravity pulls the object downward as the force of air resistance pushes it upward The force of gravity rate would be 9.8 m/s w/o air resistance Acceleration Stops At terminal Velocity • Terminal Velocity- the constant velocity of a falling object when the size of the upward force of air resistance matches the size of downward force of gravity ( net = 0) • http://wsgfl2.westsussex.gov.uk/aplaws/intergames/sec_science/ParachuteGame.swf • Terminal velocity can be helpful how Free fall Occurs When there is No Air Resistance • Free Fall- gravity is the only force acting • Example in a vacuum Orbiting Objects Are in a Free Fall • No Objects are weightless why? • Because as long as an object has mass then some gravitational pull (force) will exist Orbiting • Moving Forward • Ex: When Space Ship orbits the Earth • Freefall Projectile Motion and Gravity Newton’s 1st Law • Objects remain at rest or constant speed unless acted on by an unbalanced force • Object at Rest • Object in Motion Inertia • The tendency to resist change Friction is often the opposing Force • Friction gets in the way of Newton's law Newton’s 2nd Law • The acceleration of an object depends on mass and force applied • a=F/m or F=ma Math Break 1 • What is the acceleration of a 7 kg mass if a force of 68.6 N is used to move it toward the Earth? (1 N = 1Kg x m/s/s) Solution • a= f / m • 68.6 N / 7 kg =9.8 m/s2 Math Break 2 • What force is necessary to accelerate a 1,250 kg car at a rate of 40 m/s/s? Solution 2 • F=ma • 1,250kg x 40 m/s/s = 50,000 N 2nd Law • Mass • Force • acceleration • Acceleration Newton’s 3rd Law • For every action, there is an equal and opposite reaction. – Forces come in pairs Break • 1. While driving down the road, a firefly strikes the windshield of a bus and makes a quite obvious mess in front of the face of the driver. This is a clear case of Newton's third law of motion. The firefly hit the bus and the bus hits the firefly. Which of the two forces is greater: the force on the firefly or the force on the bus? Answer • Trick Question! Each force is the same size. For every action, there is an equal ... (equal!). The fact that the firefly splatters only means that with its smaller mass, it is less able to withstand the larger acceleration resulting from the interaction. Besides, fireflies have guts and bug guts have a tendency to be splatterable. Windshields don't have guts. There you have it. • 4. In the top picture (below), Kent is pulling upon a rope which is attached to a wall. In the bottom picture, the Kent is pulling upon a rope which is attached to an elephant. In each case, the force scale reads 500 Newtons. Kent is pulling ... • • a. with more force when the rope is attached to the wall. • b. with more force when the rope is attached to the elephant. • c. the same force in each case. • Kent is pulling with 500 N of force in each case. The rope transmits the force from Kent to the wall (or to the elephant) and vice versa. Since the force of Kent pulling on the wall and the wall pulling on Kent are action-reaction force pairs, they must have equal magnitudes. Inanimate objects such as walls can push and pull. Momentum Depends • Mass and Velocity • Property of moving objects • Ex: Which is harder to stop a train or car Law of conservation of momentum • Any time two or more objects interact, they my exchange momentum, but the total amount of momentum stays the same • Ex: Bowling • When bowling the bowling balls momentum is transferred to the pins Calculating • • • • Momentum (p) Mass (m) Velocity (v) The units of momentum are kg·m/s Question • Calculate the momentum of an 80kg basketball player driving to the basket with a constant velocity of 8 m/s Solution • p=mv • p = (80kg) (8 m/s)