# Projectile Motion - Finding initial velocity

You need to send a bowling ball up exactly $205$ meters so someone at the top of the Washington Monument can take a picture of it "hanging" in space. The bowling ball will be shot from a mortar tube from a platform $25$ meters above the ground. You can determine what the initial velocity is because the technician will put in enough black powder to give the bowling ball whatever initial velocity you tell him to give you. So....

(1) What initial velocity ($v_o$) will cause the bowling ball to "freeze" in mid-air at exactly $205$ meters above the ground?

(2) How fast will the bowling ball be moving right before it hits the ground?

Using The formula $s(t) = s_0 + v_0(t) + .5gt^2$

$s_0 = 25$ and $t = 18$ sec and $g = 10$ m/sec$^2$

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What have you already tried? –  anorton Feb 11 '13 at 2:04
are we ignoring air resistance? –  robjohn Feb 11 '13 at 2:06
This belongs on physics, where it must be a duplicate. –  Ross Millikan Feb 11 '13 at 2:09
I am doing a calculus problem actually. Yes we are ignoring air resistance. I have tried The formula $s(t) = s_0 + v_0(t) - .5gt^2$ where $s_0 = 25$ and $t = 18$ sec and $g = 10$ m/sec –  Sally Feb 11 '13 at 2:11
@RossMillikan: it is a physical problem, but I wouldn't call it a physics problem. It looks like many problems in a first year calculus course. –  robjohn Feb 11 '13 at 2:13

Use conservation of energy.

At 25 m above the ground, energy = kinetic + potential. Kinetic = $(1/2) m v_0^2$. Potential = $m g h$. $m$ = mass of ball, $g$ = acceleration due to gravity, $h$ = 25 m.

At $H$ = 205 m above the ground, the ball is changing direction, so the velocity is...? And therefore the kinetic energy is...? The potential energy is $m g H$. Now set the total energy at 25 m and 205 m equal to each other. You should have only one unknown after cancellation.

At the ground, the potential energy is...?

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Can you help me solve it using The formula s(t) = s not + v not(t) + .5gt^2 –  Sally Feb 11 '13 at 2:17
Looks like someone else already did that. Please specify these things first, as we are not mind readers. –  Ron Gordon Feb 11 '13 at 2:43

You need the ball to rise $180$ meters, so it should start at the speed it would have if it fell $180$ meters. Do you know formulas for the velocity and distance covered under gravity?

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I have tried The formula s(t) = s not + v not(t) + .5gt^2 s not = 25 t = 18 sec g = 10m/sec –  Sally Feb 11 '13 at 2:17
@Sally: what is s not? $g$ is not 10 m/sec-the numeric value is closer to $9.8$ (probably close enough) and the units are m/sec^2 –  Ross Millikan Feb 11 '13 at 2:21
@Sally: Please use LaTeX. "v not" is confusing when you mean $v_0$. –  robjohn Feb 11 '13 at 2:24
K I am new to the site. I am using 10m/sec for round numbers.I just want to get the idea of how to do it. –  Sally Feb 11 '13 at 2:28

This is probably a better fit for Physics.SE...

I'm going to assume you are ignoring air resistance, as there's no Differential Equations tag.

## Part 1

We have from conservation of energy: $$\Delta K + \Delta U_g = 0$$ $$\frac{1}{2}m\left(v_f^2-v_0^2\right) + mg(y_f-y_0) = 0$$ Dividing by $m$ and plugging in values: $$\frac{1}{2}\left(-v_0^2\right) + g(205\text{m}-25\text{m}) = 0$$ Simplify and solve for $v_0$: $$\frac{1}{2}\left(v_0^2\right) = g(205\text{m}-25\text{m})$$ $$v_0 = \sqrt{2g(205\text{m}-25\text{m})}$$ $$v_0 = \sqrt{2(9.80)\text{m/s^2}(180\text{m})}$$ $$v_0 = \sqrt{3528}\frac{\text{m}}{\text{s}}\approx 59.39\frac{\text{m}}{\text{s}}$$

## Part 2

The ball will fall $205$ meters. Again, conservation of energy: $$\Delta K + \Delta U_g = 0$$ $$\frac{1}{2}m\left(v_f^2-v_0^2\right) + mg(y_f-y_0) = 0$$ Dividing by $m$ and plugging in values: $$\frac{1}{2}\left(v_f^2\right) + g(-205\text{m}) = 0$$ $$v_f^2 = 2g(205)\text{m}$$ $$v_f = \sqrt{9.80\frac{\text{m}}{\text{s}^2}(410)\text{m}}$$ $$v_f \approx 63.39\frac{\text{m}}{\text{s}}$$

## With Kinematic Equations

I guess this is supposed to be solved with the kinematic equations: $$s = s_0 + v_0t - \frac{gt^2}{2}$$ $$v(t) = v_0 - gt$$ (With $g$ being the magnitude of gravity ($9.80\frac{\text{m}}{\text{s}^2}$))

The first step is to solve for the time it takes to reach the top. The easiest way is to use the equation: $$v(t) = v_0 - gt$$ Thus, we have: $$0 = v_0 - gt_{top}$$ $$t_{top} = \frac{v_0}{g}$$ Now we plug this in to the other equation: $$s = s_0 + v_0t - \frac{gt^2}{2}$$ $$s_{top} = s_0 + v_0\left(\frac{v_0}{g}\right) - \frac{g\left(\frac{v_0}{g}\right)^2}{2}$$ $$205 = 25 + \frac{v_0^2}{g} - \frac{v_0^2}{2g}$$ $$180 = \frac{2v_0^2-v_0^2}{2g}$$ $$180 = \frac{v_0^2}{2g}$$ $$v_0^2 = 180(2g)$$ $$v_0 = \sqrt{360\cdot9.80\frac{\text{m}^2}{\text{s}^2}} \approx 59.4 \frac{\text{m}}{\text{s}}$$

The falling speed is very similar, so I will not work it out for you.

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Can you help me solve it using The formula s(t) = s not + v not(t) + .5gt^2 –  Sally Feb 11 '13 at 2:18
@Sally: shouldn't that be $-.5gt^2$? –  robjohn Feb 11 '13 at 2:19
@Sally Done. :) –  anorton Feb 11 '13 at 2:33

s(t) = -5t-squared + v-naught t + 25 v(t) = -10t + v-naught setting v-naught = 0 yields v-naught = 10t

therefore s(t) = -5t-squared + 10t-squared + 25 205 = 5t-squared + 25 180 = 5tt 36 = tt thus t = 6 seconds

t = -b/(2a) thus max occurs at 6 seconds and total trip is 12 seconds. s(t) = -5tt + 60t + 25 will yield a maximum height of 205 feet at the 6 second mark.

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