Determine the minimum initial velocity v0 and the corresponding angle u0 at which the ball must be kicked in order for it to just cross over the 3-m high fence.



Answer :

The minimum initial velocity u = 58.3°

(v0) min = 9.76 m/s

Explanation:

Coordinate System: The coordinate system will be set so that its origin coincides with the ball’s initial position.

x-Motion: Here (v0) = v0 cos u x0 and x = 6 m Thus,                

 (+)     x = [tex]x_{0}[/tex] + (v0)xt

           6 = 0 + (v0 cos u)t

           t = 6/v0 cos u                                       (1)          

y-Motion: Here (v0)x = v0 sin u, ay = -g = -9.81m/s  and y0 = 0  

   ( A + c B)    y = y0 + (v0)y t +1/2 [tex]a_{y}[/tex][tex]t^{2}[/tex]

                      3 = 0 + v0 (sin u)t + 1/2 (-9.81) [tex]t^{2}[/tex]

                       3 = v0 (sin u)t - 4.905[tex]t^{2}[/tex]               (2)

Substituting Eq. (1) into Eq.(2) yields

        v0 = [tex]\sqrt[2]{\frac{58.86}{sin 2u - cos2 u} }[/tex]                                       (3)

From Eq. (3), we notice that is minimum when is

maximum. This requires [tex]\frac{df(u)}{du}[/tex]

[tex]\frac{df(u)}{du}[/tex] = 2 cos2u + sin2u = 0

tan2u = -2

2u = 116.57°

u = 58.28° = 58.3°

Substituting the result into Eq.(2), we have

(v0)min = [tex]\sqrt[2]{\frac{58.86}{sin 116.57° - cos2 58.28°} }[/tex]

(v0)min =  9.76 m/s

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