Lecture 05 Free Fall and Apparent Weight l

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Lecture 05: Free Fall and Apparent Weight l Free Fall l Apparent Weight

Lecture 05: Free Fall and Apparent Weight l Free Fall l Apparent Weight

Free Fall l Only force acting on object is GRAVITY èNewton’s 2 nd Law

Free Fall l Only force acting on object is GRAVITY èNewton’s 2 nd Law » Fg = ma » -mg = ma » a = -g l SF = ma (force of gravity is mg down) (thus acceleration is g downwards) Acceleration is ALWAYS g downwards èPosition may be positive, zero, or negative èVelocity may be positive, zero, or negative èBUT acceleration is always g downwards èKinematics Equations: Ø y = y 0 + vy 0 t – ½ g t 2 Ø vy = vy 0 – g t Ø vy 2 = vy 02 – 2 g (y-y 0) y x

Free Fall Example l Fred throws a ball 30 m/s vertically upward. How long

Free Fall Example l Fred throws a ball 30 m/s vertically upward. How long does it take to hit the ground 2 meters below where he released it? è y = y 0 + vy 0 t – ½ g t 2 è (0 m) = (2 m) + (30 m/s) t – ½ (9. 8 m/s 2) t 2 Ø y = y 0 + vy 0 t – ½ g t 2 Ø vy = vy 0 – g t Ø vy 2 = vy 02 – 2 g (y-y 0) è Use quadratic formula to solve for t! t = 6. 19 s or -. 06 s • Only 6. 19 s makes physical sense.

Apparent Weight l Recall: SF = m a èConsider person accelerating in an elevator.

Apparent Weight l Recall: SF = m a èConsider person accelerating in an elevator. FN èDraw FBD èApply SF = m a » FN – mg = ma » FN = m ( g + a ) Fg l Apparent weight is normal force from scale or floor. l Note: in free fall a = -g so N = 0. 33

Apparent Weight Example l You are standing on a scale inside an elevator. Normally

Apparent Weight Example l You are standing on a scale inside an elevator. Normally you weigh 600 N, but the scale reads 720 N. What is your acceleration? è è First, note that your mass is (600 N)/(9. 8 m/s 2) = 61. 2 kg. Second, use: FN = m ( g + a ). » » (720 N) = (61. 2 kg) ( 9. 8 m/s 2 + a ) a = +1. 96 m/s 2 Your acceleration is 1. 96 m/s 2 UP.

Summary of Concepts l Free Fall èOnly force is gravity èAcceleration is 9. 8

Summary of Concepts l Free Fall èOnly force is gravity èAcceleration is 9. 8 m/s 2 down l Apparent Weight èIf object is accelerating in vertical direction weight appears different èAccelerating up increases apparent weight èAccelerating down decreases apparent weight