Posted by James on Saturday, July 28, 2012 at 12:18pm.
3. A projectile is launched at 10.0 m/s in a direction 45.0 degrees above the horizon. It hits the ground 10.0 m below its initial position. What is its speed just before hitting the ground? Ignore air resistance.
The answer is supposed to be 17.2 m/s.

Physics  James, Saturday, July 28, 2012 at 12:21pm
I've been using:
v(f)^2 = v(i)sin(45)^2  2g(yy(i))
Then I use:
v = sqrt((v(f)^2 + v(i)^2)
However, doing this gives me 18.6 m/s instead of 17.2 m/s. Any help would be greatly appreciated.

Physics  James, Saturday, July 28, 2012 at 10:20pm
I've found that:
v(f)^2 = v(i)^2 2g(y  y(i))
gives me 17.2 m/s. However, this way doesn't account for the angle, so I don't think it's the right way to solve this.

Physics  ajayb, Sunday, July 29, 2012 at 1:18am
You've to consider the motion in vertical and horizontal directions separately.
A)Vertical (y) dir.=>
Vy^2 = Uy^2 + 2*(g)*s
= (10Sin45)^2 + 2*(9.8)(10)
= 50 + 196 = 246
So, Vy= 15.68 m/s
(Vy is the vertical component of the projectile when it hits the ground)
B) Horizontal (x)dir. =>
Vx= Ux = 10Cos45 = 7.07 m/s
Projectile's vel. just before it hits the gnd.=>
V = sqrt(Vy^2 + Vx^2)
= sqrt(246 + 50)
= 17.2 m/s
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