# Homework Help: Calculate the velocity of the projectile

1. Aug 28, 2009

1. The problem statement, all variables and given/known data
A stone was thrown with initial velocity of 100 m/s. Calculate the velocity of the projectile at 10 m of height. (Consider g=10m/s^2, and neglect the effect of air molecules)

2. Relevant equations

where's the error in the following sollution?

3. The attempt at a solution
Vr= sqrt((Vx)^2+(Vy)^2)....(1) (Resultant velocity).
Vx= Vi cos(a)....(2)
Vy=Vi sin(a)+(-2gy)....(3)
sub. (2)& (3) in (1)
Vr = sqrt(Vi^2 (cos(a))^2+Vi^2 (sin(a))^2+(-2gy)); taking Vi as a common factor, and sqrt of both sides; we get
Vr = sqrt(Vi^2-2gy) then we proceed by applying the given data from the problem

2. Aug 28, 2009

### kuruman

Re: Projectiles

The error is right here

Vy=Vi sin(a)+(-2gy)

This equation is not dimensionally correct.

3. Aug 29, 2009

Re: Projectiles

Thanks Kuruman... so I believe it should be like this
Vy^2= Vi^2 (sin(a))^2 - 2gy.

4. Aug 29, 2009

### kuruman

Re: Projectiles

That's more like it.