Thread: 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 ??

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. .

Grandad

Originally Posted by Grandad

Hello hookeRead the question: Find the time, not Find the times. .

Grandad

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

Originally Posted by hooke

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

Yes.