Question

A placekicker 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 20.0 m/s

at an angle of 53.0° to the horizontal. (a) By how much

does the ball clear or fall short of clearing the crossbar?

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

while falling?

Answer 1

a)

* Vertical velocity of football is

V = 20.0 m/s Sin 53

=15.97 m/s

* Horizontal velocity of football is

u = 20.0 m/s Cos 53

= 12.04 m/s

Football moves with constant horizontal velocity So, time take to travel 36.0 m is

t = 36.0/u = 36.0 ,m/12.04 m/s = 2.99s

Vertical heightof ball at time t = 2.99s in

h = Vt + 1/2at²

=Vt + 1/2gt²

= 47.75 - 43.81

= 3.94 m

ball clear the crossbar by 3.94 m - 3.05 m

= 0.89 m

b) Time taken to reach maximum height is

T = V/g = 1.63 s

Time taken to cross the cross bar is 2.995 so, the ball approach the croosbar while falling.