# Kinematics problem: x(t) --> v(t) --> a(t)

1. Sep 14, 2015

### ilanmichaeli

1. Problem statement:
an object moves along the x axis so it's position at any given moment is
x(t) = −t3(to the third power) + 6t − 12 m.
a. what is it's velocity at any given moment
b. what is it's acceleration
c. at what moment does the object stops

2. Relevant equations
s=vt+1/2at2

3. The attempt at a solution
due to the weird expersion they gave i couldn't even think about any reasonable thing to do, but i tried nonetheless to solve for it but in vein... i'll be grateful for some assistance

Last edited by a moderator: Sep 14, 2015
2. Sep 14, 2015

### Staff: Mentor

That equation applies to motion with constant acceleration. Not to this problem.

Hint: Calculus.

3. Sep 14, 2015

### Staff: Mentor

That's not the correct Relevant Equation. You are given position as a function of time x(t), and are asked to find the velocity v(t) and acceleration a(t). What operation do you use to get v(t) from x(t)? And what operation do you use to get a(t) from v(t)?

Also, I'll add some descriptive words to your thread title. "kinematics problem" is not very descriptive of this problem...

EDIT -- Beaten to the punch by Doc Al again!

4. Sep 14, 2015

### ilanmichaeli

thanks for your help and regarding to what you've said i would use v=s/t for velocity and a=v/t for acceleration. in fact i did try to divide that expression by time (using v=s/t) but it just didn't work out for me unless i'm doing it wrong.. :(.

5. Sep 14, 2015

### Staff: Mentor

Those only work for constant velocity and constant acceleration, respectively.

How about using the hint I gave?

6. Sep 14, 2015

### ilanmichaeli

if i still don't know any calculus i don't have any hope for solving this problem? because that's the case unfurtunately

7. Sep 14, 2015

### Staff: Mentor

Oh well. How did you get this problem if you haven't had calculus?

8. Sep 14, 2015

### Staff: Mentor

It sure looks like a Calculus problem to us. What class is this from?

EDIT -- Dang it, dang it, dang it!

9. Sep 14, 2015

### ilanmichaeli

it's from a collection of problems of kinematics, up to this question i could solve everything they asked because there wasn't any calculus involved, but from this one and on every thing looks kinda messy :)

10. Sep 14, 2015

### Staff: Mentor

LOL!

11. Sep 14, 2015

### Staff: Mentor

Yeah, I suspect the problems went from the basics up to the more advanced. Time to learn some calc!

12. Sep 14, 2015

### ilanmichaeli

thanks anyway for your time. both of you.

13. Sep 14, 2015

### SteamKing

Staff Emeritus
What's weird about the expression showing position as a function of time?

What's the relationship between the change in location, x(t), w.r.t. time and the velocity? What's the relationship between the change in velocity, v(t), w.r.t. time and the acceleration?

You should have learned the definitions of velocity and acceleration by now. The SUVAT equations are special cases where acceleration is a constant.