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# Help with mechs 1 watch

1. Hiya..

I am stuck with one question below.. I've done parts a-c and having a bit of problem with d. I hope you can help me this as it seems I cannot find the right answer (which is 10.5) . Thanks..

[In this question, the unit vectors i and j are due east and due north respectively.]
A particle P is moving with constant velocity (–5i + 8j) m s–1. Find

(a) the speed of P,
(b) the direction of motion of P, giving your answer as a bearing.

At time t = 0, P is at the point A with position vector (7i – 10j) m relative to a fixed origin O. When t = 3 s, the velocity of P changes and it moves with velocity (ui + vj) m s–1, where u and v are constants. After a further 4 s, it passes through O and continues to move with velocity (ui + vj) m s–1.

(c) Find the values of u and v.
(d) Find the total time taken for P to move from A to a position which is due south of A.
2. presumably you get u = +2, v= -3.5

You then need to be at a position where the I is the same as at A, ie I = +7

So the time to do that will be 3 for the first velocity, 4 seconds to get to the origin and then whatever it takes to get to +7I.
3. oh.. easy easy easy!... i got 3.5s for origin to 7i but didn't realise that i have to add 3s and 4s.. thanks!

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