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A place-kicker must kick a football from a point 36.0 m (about 40 yards) from the goal. Half the crowd hopes the ball will clear the crossbar, which is 3.05 m high. When kicked, the ball leaves the ground with a speed of 20.6 m/s at an angle of 47.0° to the horizontal.(a) By how much does the ball clear or fall short (vertically) of clearing the crossbar? (Enter a negative answer if it falls short.)_____ m

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We would find the time at which the ball arrives the goal from the constant velocity model in the x direction. We have

xf = xi + vit

vit = xf - xi

t = (xf - xi)/vi

where

xf is the final position of ball

xi is the initial position of ball

vi is the initial velocity of the ball in the x direction

t is the time

From the information given,

xi = 0

xf = 36

vxi = vicosθ

θ = 47

v = 20.6

vxi = 20.6cos47

t = (36 - 0)/20.6cos47

t = 2.56 s

We would find the height of the ball at time of t = 2.56 from the particle under constant acceleration model in the y direction. We have

yf = yi + vyit + 1/2ayt^2

From the information given,

yf = 0 + 20.6sin47 * 2.56 + 1/2 * 9.81 * 2.56^2

yf = 38.569 - 32.145

yf = 6.42

The ball clears the crossbar by 6.42 - 3.05

= 3.27 m

User Dudi Konfino
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