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# Finding the time when the speed of an accelerating particle is declared? watch

1. I'm pretty terrible at all this SUVAT mechanics stuff - but here's what I know!

"A particle moves with constant acceleration (-0.4i + 0.2j)ms2. Initially it has a velocity of (4i + 0.5j)ms1. Unit vectors i and j are directed east and north respectively.

How will I find the time when the speed of the particle is 5ms?

Thanks!
2. Find an expression for the velocity as a function of time. Use this to find an expression for the speed (squared) as a function of time. (It's easier to find the square of the speed than the speed). Then find t such that the speed^2 = 25. (As then the speed = 5).
3. (Original post by MarcusThePotato)
I'm pretty terrible at all this SUVAT mechanics stuff - but here's what I know!

"A particle moves with constant acceleration (-0.4i + 0.2j)ms2. Initially it has a velocity of (4i + 0.5j)ms1. Unit vectors i and j are directed east and north respectively.

How will I find the time when the speed of the particle is 5ms?

Thanks!
Speed is a scalar quantity so you need to get the magnitudes of your vectors then work with for the time.
4. (Original post by RDKGames)
Speed is a scalar quantity so you need to get the magnitudes of your vectors then work with for the time.
You can only "find magnitudes, then do v=u+at" if the (initial) velocity vector and acceleration vector are parallel.
5. (Original post by DFranklin)
You can only "find magnitudes, then do v=u+at" if the (initial) velocity vector and acceleration vector are parallel.
Well my mind went completely blank there..

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