# Thread: displacement vector

1. ## displacement vector

A particle is tossed vertically upwards from a point O . At time t seconds later , the position vector of the particle from O is given by r=(12t-5t^2)j m . Find the time when the particle is at least 4m above O .

shouldn't it be done liek this :

12t-5t^2=4 , solving gives t=2 or t=2/5

but the answer given is 1.6 s ??

2. Hello hooke
Originally Posted by hooke
A particle is tossed vertically upwards from a point O . At time t seconds later , the position vector of the particle from O is given by r=(12t-5t^2)j m . Find the time when the particle is at least 4m above O .

shouldn't it be done liek this :

12t-5t^2=4 , solving gives t=2 or t=2/5

but the answer given is 1.6 s ??
Read the question: Find the time, not Find the times. $2-0.4 = 1.6$.

Grandad

3. Originally Posted by Grandad
Hello hookeRead the question: Find the time, not Find the times. $2-0.4 = 1.6$.

Grandad
oh , so is it how long the particle would remain at least 4m away from O , to rephrase this ?

4. Originally Posted by hooke
oh , so is it how long the particle would remain at least 4m away from O , to rephrase this ?
Yes.