Calculate the velocity of the projectile

Fadi A.Saif
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Homework Statement


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)


Homework Equations



where's the error in the following sollution?


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
 
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The error is right here

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

This equation is not dimensionally correct.
 


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


That's more like it.
 

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