# Position of an object after a certain amount of time?

## Homework Statement

Your location is at the origin. When an object passes you, it is traveling 5 m/s in the −z direction. If the object's mass is 200 kg, and the net force on the object remains constant at ‹ 0, 0, −30 › N, what is the position of the object 20 seconds after it passes you?

## Homework Equations

The Change in momentum = Force * deltat

## The Attempt at a Solution

I solved for the impulse, and got the vector, <0,0,-600>N. This is equal to the final momentum minus the initial momentum. The initial momentum is <0,0,-8>m/s * 200kg. And the final momentum is pf = vf * 200kg. I solved for vf, using vf = vi + Fnet*deltat, but I am stuck.

I keep getting the wrong answer, and I am unsure of where I've went wrong and what I should do next. Thanks.

Related Introductory Physics Homework Help News on Phys.org
gneill
Mentor

## Homework Statement

... what is the position of the object 20 seconds after it passes you?

...

## The Attempt at a Solution

I solved for the impulse, and got the vector, <0,0,-600>N. This is equal to the final momentum minus the initial momentum. The initial momentum is <0,0,-8>m/s * 200kg. And the final momentum is pf = vf * 200kg. I solved for vf, using vf = vi + Fnet*deltat, but I am stuck.

I keep getting the wrong answer, and I am unsure of where I've went wrong and what I should do next. Thanks.
You're looking for the final position, not the final velocity. Consider using a different kinematic equation, one that relates distance traveled to force applied to a mass (acceleration).

So I'll use:
deltax = vi*t + 0.5*a*t^2.
I plug in the proper values, and I don't get the correct answer. I am unsure of where I am going wrong. The initial position is, <0,0,0>, so I don't need to worry about that.

gneill
Mentor

Ok,
xf = xi + vi*t + 0.5*a*t^2
xf = <0,0,0>m + <0,0,5>*20 + 0.5*(<0,0,-30>N/200kg)*(20s^2)

gneill
Mentor
Check your velocity vector against what the original problem states.

So, the velocity would be <0,0,-5> since it is in the -z direction?

gneill
Mentor
So, the velocity would be <0,0,-5> since it is in the -z direction?
What do you think?