Question

A particle P travels in a straight line through a fixed point O. Its distance, s metres, from O is given by s=t^3-9t^2+15t+40, where t seconds is the time after motion has begun.

Calculate the values of t when the acceleration has a magnitude of 9m/s^2.

so what I did was I differentiated twice to find the acceleration, then equated that to -9. I found only one answer, when there is two ><

Answer 1

I would have equated to +9

Answer 2

either way, i'd have only one answer

Answer 3

ok. let me look at it and get back to you

Answer 4

ohh wait you mean
equate to both -9 and 9

Answer 5

that'd give me both answers

Answer 6

...but why o-o

Answer 7

is it because when they say magnitude, it can mean both positive and negative?

Answer 8

yes. it says a magnitude of 9 m/sec^2 so that would mean in two directions + and -

Answer 9

ah cool thanks

Answer 10

gives two answers as you say

Answer 11

I get t=9/2 and 3/2

Answer 12

yeah those