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M1 south-east direction position vectors

Relative to a fixed origin O , the horizontal unit vectors i and j are pointing due east and due north, respectively. A particle P is moving with constant acceleration of (0.25 0.5 i j − ) 2 ms− . It is initially observed passing through the point with position vector (2 3 i j − ) m with velocity of (i j + 3.5 ) 1 ms− .

a) Find an expression for the velocity of P , t s after it was first observed.

b) Determine an expression for the position vector of P , t s after it was first observed.

c) Calculate the distance of P from the origin O , when P is moving in a southeastern direction.

Hi, I have done the first two parts of this question and got the right answer which is (2+t+0.125t^2)i + (-3+3.5t-0.25t^2)j . For part c, since its a south-east direction, surely the i components will switch signs and this new vector will be equal to the j vectors and you solve for j. For this I got t= 35.8s however the right answer should be t=18. Thanks. ( this video shows an example of how to tackle it http://www.examsolutions.net/a-level...5&solution=7.4

Original post by coconut64

"moving in a southeastern direction" means the velocity will be in that direction, not the position necessarily.

Reply 3

coconut64

Original post by ghostwalker

"moving in a southeastern direction" means the velocity will be in that direction, not the position necessarily.

But exam solution explains that its the position vector in the video though... Thanks.

Reply 4

ghostwalker

Original post by coconut64

But exam solution explains that its the position vector in the video though... Thanks.

Video talks about position vectors, but don't know what the original question was that it's trying to answer.

Reply 5

coconut64

Original post by ghostwalker

Video talks about position vectors, but don't know what the original question was that it's trying to answer.

http://www.examsolutions.net/a-level-maths-papers/Edexcel/Mechanics/Mechanics-M1/2011-June/paper.php its question 7 of this paper. thanks

Reply 6

ghostwalker

Original post by coconut64

http://www.examsolutions.net/a-level-maths-papers/Edexcel/Mechanics/Mechanics-M1/2011-June/paper.php its question 7 of this paper. thanks

Yes, that paper talks about the position being north or north-west, which is completely different to moving (i.e. the velocity being) in a particular direction.