A placekicker must kick a football from a point 36.0 m (about 39.0 yd) from the

Question

A placekicker must kick a football from a point 36.0 m (about 39.0 yd) from the goal, and the ball must clear the crossbar, which is 3.05 m high. When kicked, the ball leaves the ground with a velocity of 22.6 m/s at an angle of 50.0° to the horizontal.

(a) By how much does the ball clear or fall short of clearing the crossbar?
---Select--- The ball falls short The ball clears the crossbar by ____m

(b) Does the ball approach the crossbar while still rising or while falling?

The ball approaches the crossbar while falling.The ball approaches the crossbar while rising.

Explanation / Answer

a)

Time taken for the ball to clear the crossbar = 36/(22.6*cos(50 deg)) = 2.48 s

Using the equation of motion :

s = ut + 0.5*at^2

So, height reached , s = 22.6*sin(50 deg)*2.48 + 0.5*(-9.8)*2.48^2

= 12.8 m

So, the ball falls short by = 36 - 12.8 = 23.2 m <----- answer

b)

Let highest point achieved be H

So, at this point,the height from the ground be h

So, using the equation of motion : v^2 = u^2 + 2as

So, 0^2 = (22.6*sin(50 deg))^2 + 2*(-9.8)*h

So, h = 15.3 m

As the height it is at when the ball clears the crossbar is 12.8 m , so it must be falling while the ball approches the crossbar.

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