mimiblake2
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Hi, I' really stuck on a part of my homework and was wondering if someone could explain it to me:

Particle P is moving so that its position vector at time, t is

r=(3e^2t)i+(2t^2-20t)jm

The first part was to find the vector for velocity which i got by differentiation, but then it asks:

Find the time at which the particle moves perpendicular to j and calculate the speed at this time.

I cant do the second part so if someone could show me how that would be great, thanks!
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DFranklin
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Do you know the condition for two vectors a and b to be perpendicular? (Also, do you know what a dot product is)?
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Sinnoh
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Perpendicular to j means that the j component of the velocity has to be equal to 0, so you can then solve for t.
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mimiblake2
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(Original post by Sinnoh)
Perpendicular to j means that the j component of the velocity has to be equal to 0, so you can then solve for t.
Thanks so much, I actually just worked that out and it's so simple my brain isn't awake yet I guess, aha thanks!!
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