Velocity and acceleration of an army tank

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Homework Statement

An army tank on a mountainside fires its gun horizontally. The shell strikes the ground at a horizontal distance of 6000 m away from the tank. As the vertical component of the shell’s striking velocity is vy=200 m/s, how soon after it is fired does the shell strike the ground? (g=10 m/s2)

Homework Equations

v^2=2ax (1)
x=at^2/2 (2)

The Attempt at a Solution

40000=2*a*6000
then find t with (2) but why is it the component vy?

 

[NascentOxygen]

Homework Statement

An army tank on a mountainside fires its gun horizontally. The shell strikes the ground at a horizontal distance of 6000 m away from the tank. As the vertical component of the shell’s striking velocity is vy=200 m/s, how soon after it is fired does the shell strike the ground? (g=10 m/s2)

Homework Equations

v^2=2ax (1)
x=at^2/2 (2)

The Attempt at a Solution

40000=2*a*6000
then find t with (2) but why is it the component vy?

It isn't. If you use an equation involving vertical acceleration, then the other variables in the equation will be vertical displacement, vertical velocity, etc.

Your mistake is that you mixed a vertical velocity and a horizontal distance in the one equation!