displacement vector

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• February 9th 2010, 11:13 PM
hooke
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 ??
• February 10th 2010, 01:02 AM
Grandad
Hello hooke
Quote:

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
• February 10th 2010, 01:47 AM
hooke
Quote:

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 ?
• February 10th 2010, 01:53 AM
Grandad
Quote:

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

Yes.