# Homework Help: Throw a Pebble from a Pyramid

1. Sep 12, 2009

### joemama69

1. The problem statement, all variables and given/known data

A pebble is thrown off the side of a pyramide perpindicular to its slope. Find the vertical distance h under the assumption the drag forces are negligible.

2. Relevant equations

3. The attempt at a solution

vx=vcos(90-Q) Q = theta
vy=vsin(90-Q)

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x = vxt; therefore t = $$\frac{x}{v_x}$$ = $$\frac{x}{v_{x}cos(90-Q)}$$

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tanQ=$$\frac{y}{x}$$; therefore y = xtanQ

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y=yo + vyt - $$\frac{1}{2}$$gt2

y= $$\frac{xv_{x}sin(90-Q)}{v_{x}cos(90-Q)}$$ - $$\frac{9.8}{2}$$($$\frac{x}{v_{x}cos(90-Q)}$$)2

y= xtan(90-Q) - $$\frac{4.8x^{2}}{v^{2}cos^{2}(90-Q)}$$

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Set both y's equal to each other

xtanQ = xtan(90-Q) - $$\frac{4.8x^{2}}{v^{2}cos^{2}(90-Q)}$$

tanQ = tan(90-Q) - $$\frac{4.8x}{v^{2}cos^{2}(90-Q)}$$

now im gonna solve it for x

x = $$\frac{v^{2}cos^{2}(90-Q)[tanQ-tan(90-Q)]}{-4.8}$$

and now i pluf it into y = xtanQ

y = $$\frac{v^{2}tanQcos^{2}(90-Q)[tanQ-tan(90-Q)]}{-4.8}$$

seems a little to messy, did i miss somethig

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2. Sep 13, 2009

### rl.bhat

It is right. But you can put it in a better form.
write tan(90 - Q) = cot Q
cos(90 - Q) = sin Q

3. Sep 13, 2009

### ehild

Take care, the slope goes downwards so y = -xtanQ. And 9.8/2 =4.9.

ehild

4. Sep 13, 2009

### joemama69

-tanQ = tan(90-Q)-$$\frac{4.9x}{v^{2}cos^{2}(90-Q)}$$

x = $$\frac{v^{2}cos^{2}(90-Q)}{4.9}$$[tan(90-Q) + tanQ]

x = $$\frac{v^{2}sin^{2}Q}{4.9}$$[cotQ + tanQ]

y = -$$\frac{v^{2}tanQsin^{2}Q}{4.9}$$[cotQ + tanQ]

so there is nothin left to do with this one

5. Sep 13, 2009

### rl.bhat

In the last step put cotQ = 1/tanQ and simplify.

6. Sep 13, 2009

### joemama69

y = $$\frac{-v^{2}sin^{2}Q[1 + tan^{2}Q]}{4.9}$$

7. Sep 13, 2009

### rl.bhat

1 + tan^2θ = sec2θ