How to Calculate Position at 8 Seconds Using Kinematic Equations?

[rodjav305]

Homework Statement

I have put the diagram in an attachment. I need to find position at 8 seconds.

Homework Equations

X = ½ at² + vt + x

The Attempt at a Solution

-34m + (-3m/s)(8.0s)+ ½ (-4.0m/s²)(1.0s)² = -165 meters

 

[rl.bhat]

You have to find the position and the velocity at the end of 2 s and then find position at the end of 8 seconds. ( i.e. in the remaining 6 s.)

 

[rodjav305]

How would I go upon doing that? A couple of people told me to use two equations

 

[rl.bhat]

Up to 2 s,

x1 = xo + vo*t1 + 1/2*a*t1^2

v1 = vo + a1t1

For remaining 6 s,

x2 = x1 + v1*t2 + 1/2*a2*t2^2

 

[rodjav305]

hmmmm this is what i just did for x2

x2= (-42m) + (-4m/s)(6.0s)+ 1/2(-4.0m/s^2)(6.0s)^2 = -66m + (-72m) = -138m

It still gives me a wrong answer...can you explain where i went wrong?