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# M1 help simple question watch

1. I feel like im going crazy, I know its simple but i cant see it.

This question PART C. Thanks.
I am making j = 0 and the book is saying make i=0 . why i and not j ? help appreciated.
Im thinking parallel to j is any line when y=0 and x= any value like the line x=3 is parallel to the y axis.

QUESTION:
The velocity, v m s–1, of a particle P at time t seconds is given by v = (1 – 2t)i + (3t – 3)j

(c) Find the value of t when P is moving (i) parallel to j,

ANSWER:
1− 2t = 0 ⇒ t = 0.5
t = 2/3, 0.67 or better

https://www.youtube.com/watch?v=8x7EiVrGTKI WORKED VIDEO
2. (Original post by kaya_lioness)
Im thinking parallel to j is any line when y=0 and x= any value like the line x=3 is parallel to the y axis.
You have this the wrong way around. The line x=3 contains the points where the x-value is 3 and the y-value can be any value.

Hence for your question, if you are moving parallel to j, then the velocity with respect to the i component shouldn't be changing.
3. Alternatively, you could think of it like this:

If the particle is moving parallel to j, then it is moving parallel to the y-axis (i.e. directly upwards/ downwards). If it's travelling vertically, then it's not moving horizontally at all. Therefore, the horizontal component of its velocity (i) equals 0

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