Kinematic question from giancoli

[nobelium102]

Homework Statement

Suppose you adjust your garden hose nozzle for a hard stream of water. You point the nozzle vertically upward at a height of 1.5 m above the ground (the hose is 1.5 m off the ground). When you quickly move the nozzle from the vertical you hear the water striking the ground next to you for another 2.0 s. What is the water speed as it leaves the nozzle?

Homework Equations

?

The Attempt at a Solution

my attempt s = ut+1/2 at^2
-1.5 = u(2) + 1/2(-9.8)(4)
u = 9.05 m/s

and it is right
but the question is that why is the acceleration still -9.8m/s^2 when the water is going up and then falling down?

 

[Delphi51]

why is the acceleration still -9.8m/s^2 when the water is going up and then falling down?

The acceleration is always downward so it is negative. It means the upward speed decreases steadily at a rate of 9.8 m/s every second. Initially the upward speed is 9.05 m/s. After 1 second, it is 9.05-9.81 = -.76 m/s. That is just after the maximum height where the speed is zero. After two seconds, the upward speed is -.76 - 9.81 = -10.57 m/s, which is the speed just before hitting the ground.

 

[nobelium102]

oh man i didn't understand at first but i think i kind of get it

you are saying that acceleration has decreased so much that it became negative direction?
right?
hope this is right

 

[Delphi51]

No, the acceleration has not decreased. It is constant at -9.81.
The velocity decreases steadily at 9.81 m/s per second.

I would recommend you carefully read all of this page:
http://en.wikiversity.org/wiki/Motion_-_Kinematics#Acceleration

 

[rwisz]

I can explain the reason for the acceleration remaining constant at -9.8m/s^2. This is due to the force of gravity acting on the water as it moves in a vertical direction. According to Newton's laws of motion, an object in motion will continue to move in the same direction and with the same speed unless acted upon by an external force. In this case, the external force acting on the water is gravity, which causes it to accelerate towards the ground at a rate of -9.8m/s^2. Therefore, even though the water is initially moving upwards, it will eventually slow down and fall back down towards the ground due to the acceleration caused by gravity. This is why the acceleration remains constant throughout the motion of the water.