# Derive projectile equations. Please help!

1. Oct 23, 2011

### madeeeeee

I want to derive some physics equations from other equations.

a) How can I derive this formula:
R=Vo^2(sin(sα) / -a
from these two equations:
(1) x=(Vo cosα)t
(2) y=(Vo sinα)t + 1/2at^2

Things I know
- I think the way to do this is to isolate the variable t in equation (1) and substitute the t in equation (2).
- Is the identity 2sin(theta) cos(theta)= sin(2theta) useful?
-When the projectile returns to its original level y=0
Please help me understand how to do this.

b) From equation
y=yo + Vyo t + 1/2at^2

how can I show that:
h= -1/2a(tf)^2

If I measure h and tf and recall that
a=-g sin (theta)
than this equation can be used to derive an equation for g in terms of tf, h and theta.

Please help!

2. Oct 23, 2011

### LawrenceC

It would help if you defined your variables. What is sa? What is R? What is tf?

3. Oct 23, 2011

### madeeeeee

sorry its not sa its (2(alpha))
R is the range
tf is final time

so I did the part b and I got g=2h/(sin(theta)(tf)^2)....is this right? I still need help for the a)

4. Oct 23, 2011

### LawrenceC

Hint: Part a

Determine how long the projectile is in the air by determining how long it takes to reach its apex, then double it because it has to come down. You can get an equation for this time by using a little calculus on equation 2.

"Is the identity 2sin(theta) cos(theta)= sin(2theta) useful? "

Yes

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