A ball tossed vertically upward from the ground next to a building passes the bottom of a window 1.8 s after being tossed and passes the top of the window 0.20 s later. The window is 2.0 m high from top to bottom. What was the ball's initial velocity? The unit vector j^ is directed upward. How far is the bottom of the window from the launch position? How high does the ball rise above the launch position?

Respuesta :

The ball takes 0.2 sec to travel the height of the window, which is 2 m.

 Then the speed of the ball at that time was

[tex]v = \frac{d}{t}[/tex]

[tex]v = \frac{2}{0.20}[/tex]

[tex]v = 10\ \frac{m}{s}[/tex]

We know that:

[tex]v = v_0 -gt[/tex]

Where [tex]v_0[/tex] is the initial velocity.

So:

[tex]10 = v_0 -9.8(2)[/tex]

[tex]10 + 9.8(2) = v_0[/tex]

[tex]v_0 = 29.6\ m / s[/tex]

Then we have the equation for the position as a function of time.

[tex]r = r_0 + v_0t - \frac{1}{2}gt^2[/tex]

Where

[tex]r_0[/tex] = initial position

r = position as a function of time

[tex]v_0[/tex] = initial velocity

g = acceleration of gravity

t = time.

If the ball is thrown from the ground then:

[tex]r_0[/tex] = 0 m

We want to find now the distance between the window and the ground.

When the ball reaches the bottom of the window t = 1.8s

So:

[tex]r = 0 + 29.6(1.8) - 0.5(9.8)(1.8)^2[/tex]

[tex]r = 37,404[/tex] m

The window is 37,404 m high

Finally, to know how high the ball rises we must know at what moment the vertical velocity of the ball is zero.

[tex]v = v_0 -gt\\\\0 = v_0 -gt\\\\gt = v_0\\\\t = \frac{29.6}{9.8}[/tex]

[tex]t = 3.02\ s[/tex]

Now we replace t in the position equation

[tex]r = 0 + 29.6(3.02) -0.5(9.8)(3.02)^2[/tex]

[tex]r = 44.70\ m[/tex]

The ball reaches up to 44.70 m in height.