Homework Help: Projectile on horizontal surface

1. Aug 26, 2012

Darth Frodo

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

A particle is projected with a speed of 98 m s−1 at an angle A to the horizontal.
The range of the particle is 940·8 m. Find
(i) the two values of A

3. The attempt at a solution

U = 98cosAi + 98sinAj

i direction

s = ut
940.8 = 98cosAt

j direction

s = ut + (0.5)at$^{2}$
0= 98sinAt - (0.5)gt$^{2}$
98sinA = gt/2
t = 20sinA

i direction
940.8 = (20sinA)(98cosA)
94.8 = (2sinAcosA)g
94.8 / g = sin2A

And I end up with a math error. Help please.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Aug 26, 2012

TSny

Oops.

3. Aug 26, 2012

Darth Frodo

Ooops, sadly it doesn't solve the problem.

94.08 = 2sinAcosA(9.8)
94.08 / 9.8 = sin2A
9.6 = sin2A

math error

4. Aug 26, 2012

TSny

This equation doesn't follow from 940.8 = (20)sinAcosA(98)

5. Aug 26, 2012

Darth Frodo

AHHH!! Stupid error!! Thanks!!