# Vector / mechanics problem

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#### mikev

##### New member
Hey guys, I'm really struggling with the last 4 marks of a question I'v been doing:

A particle moves in 2D with a vector velocity which at time t is ' v = (-0.4ti + 5j) m/s '.

At time t=0 the particle is at (3,2)

d) Find the time at which it crosses the y-axis

e) Show that the acceleration vector is perpendicular to the position vector at time t= square root of 15 seconds

I have calculated the acceleration vector to be '-0.4i', the position vector is 'r = (3-0.2t^2)i + (5t + 2)j', and the speed at time t = 10 seconds is 3 m/s. But, now I have no idea what to do for the next 2 parts, they're only worth 4 marks so it can't be too difficult I'm just not sure what to do.

Please could someone give me an idea of what to do here as it is the last part of my work and would be greatly appreciated! Any help would be amazing!

#### MarkFL

Staff member
Hello mikev,

Forum rule #6 states:

Do not cheat. Teachers expect questions that form part of an assessment that contributes towards the final grade of a student to be the work of that student and not the work of others. For that reason, MHB policy is to not knowingly help with such questions. If a question presents in such a way as to suggest that it falls in this category, a Moderator will Close the thread (with an explanation as to why the thread was closed). The original poster can always send a pm to the Moderator to discuss the situation but if the Moderator is unconvinced, then the thread will remain closed. When a Moderator is certain that a particular member is attempting to cheat, that member will be banned and the offending thread removed. Where possible, the institute at which the member studies at will also be notified and a copy of the thread provided. This may sound harsh but the fact is that MHB places a high premium on academic honesty and integrity.
You have indicated yourself that this question is for marks, which indicates that completion of this problem is part of your assessment or grade. Because you have stated this yourself, I don't feel you are trying to cheat, and if your instructor has given you permission to seek outside assistance, please feel free to send me a visitor message to that effect, and I will discuss reopening this topic with the other staff.

Until then I am directed by policy here to close this topic.

I would suggest you find a similar problem in your text book, and post it here, and someone will be glad to offer help with it.

I hope you can understand that we cannot knowingly give help with a problem that is intended by your instructor as an assessment of your knowledge and to be reflected in your final grade.

Best Regards,

Mark.

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