# Thread: Distance - velocity - acceleration

1. ## Distance - velocity - acceleration

Hi! I need help with this problem, it's really confusing me...

The displacement s metres from point O of a particle travelling along a straight line through O, after t seconds, is given by

s= 2..8t^3-t^4 , 1<t<3
Question: Calculate the value of t when the particle reaches O.

Thanks!

2. Hello !
Originally Posted by thegirlwhoneedshelp
Hi! I need help with this problem, it's really confusing me...

The displacement s metres from point O of a particle travelling along a straight line through O, after t seconds, is given by

s= 2..8t^3-t^4 , 1<t<3
Question: Calculate the value of t when the particle reaches O.

Thanks!
If I understood correctly your question, that's what you have to do :

Find t such that s=0, because when the particle reaches point O, the displacement from point O will be 0 !

3. Originally Posted by Moo
Hello !

If I understood correctly your question, that's what you have to do :

Find t such that s=0, because when the particle reaches point O, the displacement from point O will be 0 !
So I have to write 2.8t^3-t^4=0 and isolate t, how do I do that?

4. Originally Posted by thegirlwhoneedshelp
So I have to write 2.8t^3-t^4=0 and isolate t, how do I do that?
Factorise by t^3 :

$2.8t^3-t^4=t^3 \cdot (2.8-t)$

Note that a product of numbers is equal to zero if and only if at least one of its factor is equal to 0.

So either $t^3=0$, either $2.8-t=0$

5. Oh! and how do I find the displacement, in m, whenthe particle comes to rest in the same problem?

6. Originally Posted by thegirlwhoneedshelp
Oh! and how do I find the displacement, in m, whenthe particle comes to rest in the same problem?
Hmmm Okay, I'm lost on it, I don't understand what you've said

Is it possible for you to restate that or to say it in French ?

7. Lol! I mean that I have to find the distance between the point O and the particle when the velocity is 0... I would know how to find the time but not this...

8. Originally Posted by thegirlwhoneedshelp
Lol! I mean that I have to find the distance between the point O and the particle when the velocity is 0... I would know how to find the time but not this...
Ok, so it looks like you're able to find the velocity (by differentiating).
Solve for t in velocity=0.
You'll get a certain time. Substitute it in the displacement formula to get the distance ! It's just logic, so think a little

9. Oh yeah! I forgot I could do that! lol! I¨m just tired^^
Thanks!