Free Fall

Free Fall• Only GRAVITY acting on object

– Air resistance is negligible

• Acceleration is ALWAYS 9.8 m/s/s downwards– Position may be positive, zero or negative – Velocity may be positive, zero or negative– Acceleration is always 9.8 m/s/s

downwards– 9.8 m/s/s is such well-known, constant that

it is given its own special letter: g

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Acceleration at the Top

• Velocity at top is zero

• What is the acceleration at the top?

Free Fall Preflight

• A ball is thrown vertically upward. At the top of its path its acceleration is

Acceleration of gravity is 9.8 m/s squared directed downward at all times.

acceleration is always g downwards. the acceleration of something in free fall is always -9.8 m/s/s

whenever an object is free falling its acceleration is always -9.8m/s2

The ball is no longer moving so it's acceleration is zero

acceleration is always zero at the top of the path because it is not moving.

77% responded -9.8 m/s/s23% responded 0

Free Fall• Drop heavy light objects• Mechanical Universe Clip

– Guinea/feather tube– Hammer/feather moon

ACT• The speed of an object in free fall

(Neglect Air Resistance)A. Always increases.B. is constant.C. Always decreases.D. May increase or decrease or be

constant.E. May increase or decrease but is never

constant.a = -g. Velocity becomes more and more negative. If v > 0, speed decreases. If v <= 0 speed increases.

Free Fall ACTS Fred throws a ball 30 mph vertically upward. Which of the following statements are true about the ball’s velocity and acceleration. (Let up be the positive direction) On the way up?

On the way down?

A) v < 0 B) v = 0 C) v > 0

A) a < 0 B) a = 0 C) a > 0

A) v < 0 B) v = 0 C) v > 0

A) a < 0 B) a = 0 C) a > 0

How Far

ACT Fred throws a ball 30 mph vertically upward and then catches it again at the same height he threw it from. What is the speed of the ball when he catches it? (Neglect air resistance)

1) v < 30 mph 2) v = 30 mph 3) v > 30 mph

vy2 = vy0

2 - 2g(y-y0)

vy2 = vy0

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Free Fall Example Fred throws a ball 30 m/s vertically upward. What is the maximum height the ball reaches? How long does it take to reach this height?

y = vo t + 0.5 a t2

y = (30 m/s)(3.1 s) + 0.5(-9.8 m/s2)(3.1s)2

= 46 m

v = v0 + a t

t = (v-v0) / a

= (0 – 30 m/s )/(-9.8 m/s2)

= 3.1 seconds

Dennis and Carmen are standing on the edge of a cliff. Dennis throws a basketball vertically upward, and at the same time Carmen throws a basketball vertically downward with the same initial speed. You are standing below the cliff observing this strange behavior. Whose

ball hits the ground first?

A. Dennis' ball B. Carmen's ball C. Same

vv00vv00

DennisDennisCarmenCarmen

HH

vvAA vvBB

ACT ACT

y = yo+vot + ½ a t2

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Dennis: 0 = H + vot - ½ g t2

Carmen: 0 = H - vot - ½ g t2

Dennis and Carmen are standing on the edge of a cliff. Dennis throws a basketball vertically upward, and at the same time Carmen throws a basketball vertically downward with the same initial speed. You are standing below the cliff observing this strange behavior. Whose ball is moving fastest when it hits the

ground?

A. Dennis' ball B. Carmen's ball C. Same

vv00vv00

DennisDennisCarmenCarmen

HH

vvAA vvBB

ACT ACT

Correct: v2 = v02 -2gy

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