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# What was the ball's initial speed?

A spring gun at ground level fires a golf ball at an angle of 45 degrees. The ball lands 10 m away.

a) What was the ball's initial speed?

b) For the same initial speed, find the two firing angles that make the range 6 m.

Recall that the Ideal Projectile Motion Equation is

r=(vo*cos(theta))ti+((vo*sin(theta)t-1/2*g*t^2)j.

b) theta=1/2*arcsin(3/5),

theta=pi-1/2*arcsin(3/5).

And is arcsin the same thing as sin^-1?

### 3 Answers by Expert Tutors

Robert J. | Certified High School AP Calculus and Physics TeacherCertified High School AP Calculus and Ph...
4.6 4.6 (13 lesson ratings) (13)
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a)

Horizontal direction: vo cos45 t = 10 ......(1)

Vertical direction: vo sin45 t - (1/2)g t2 = 0 ......(2)

Since sin45 = cos45,

10 = (1/2)gt2 ==> t = √(20/g)

Plug in (1),

vo√(10/g) = 10

Solve for vo,

vo = √(10g)

b)

Horizontal direction: vo cosθ t = 6 ......(1)

Vertical direction: vo sinθ t - (1/2)g t2 = 0 =>Since t ≠ 0,  t = 2vo sinθ/g......(2)

Plug in (2) in (1),

vo2sin2θ/g = 6 => 10sin2θ = 6, since vo2 = 10g.

sin2θ = 3/5

θ = (1/2)arcsin(3/5)

or

2θ = pi - arcsin(3/5)

θ = pi/2 - (1/2)arcsin(3/5)

Yes, arcsin(x) = sin-1(x)

Grigori S. | Certified Physics and Math Teacher G.S.Certified Physics and Math Teacher G.S.
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For the vertical component (v y) of the initial speed(v 0) we can write:         vy = v0 sin θ, where θ = 450 = √2/2.

For the horizontal component we have: v x = v0 cos θ. We have following equations for the vertical and horizontal motions:

v y = gt, g = 9.8 m/sec 2 .  Or v0 sin θ = gt. Thus,  t = v0 sin θ/g

In the neantime          L = 2vxt = 2v0t cos θ..

Substitute t into last expresion. We will obtailn:    L = 2V02 sinθ cosθ/g. Using trigonometric identity

2 sinθ cosθ = sin2θ = sin 90 0 = 1  we then can wriite

v02 = gL    and therefore     v 0 = sqrt(9.8 x10) = 9.9 m/sec

For two other angles when L = 6m you can write

sin 2θ = gL/v02 = 9.8x6/98 = 0.6

Two angles are  18.43 0 and 71.57 0 . They are complimentary angles.