# Horizontal velocity for projectile motion

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1. Oct 14, 2015

### Michael Ngo

1. The problem statement, all variables and given/known data
There was a plane with the height of 1 km above the ground. The plane is moving horizontally 200 m/s and can fire a bullet with the velocity of 400 m/s. At one time, the the gun form an angle of 37(degree) and fired the bullet.

I was confused with the horizontal (x-axis) velocity of the bullet. Does the velocity of the plane firing the gun also relevant or does it not ?

2. Relevant equations
The equation for the horizontal velocity of the bullet should be :
Vx = Vo. Cos37

3. The attempt at a solution
So it should be
Vx = Vo. Cos37 = 319.45 m/s

If the plane is moving 200 m/s horizontally. Does it add up with the horizontal velocity of the bullet? Like for say the total horizontal velocity of the bullet is 319.45 m/s + 200 m/s = 519.45 m/s .

Can someone explain it to me ? Sorry for asking such trivial question

2. Oct 14, 2015

### toboldlygo

This is about relative motion, right? So it depends on your frame of reference. According to the plane's frame of reference, the bullet would be moving 319 m/s, but according to someone on the ground, the bullet would be moving at 519 m/s.

3. Oct 14, 2015

### Michael Ngo

Oh, i see. So its relative. Thank you sir