tamsinaribena
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Hola, basically I'm doing M1 and I kind of hate maths at the moment so bare with me if I ask any ridiculous really obvious questions.

I've got an old exam question from 2003 & our teacher gave us all the final answer for it but I'm confused as to how she got it.

5. A particle P moved with constant acceleration (2i-3j)m/s^-2. At time t seconds, it's velocity is v/m/s^-1. When t=0, v=-2i+7j.

a. Find the value of t when P is moving parallel to the vector i. (I've done this bit & got it correct. The answer is t= 2 and a third seconds.)

b. Find the speed of P when t=3. (this is the bit I'm ridiculously stuck on. I don't understand how to get to the answer, is there a formula or something I should know about that and I'm just being a bit dense?)

c. Find the angle between the vector j and the direction of motion of P when t=3. (Haven't yet tried this, but I suck at maths so all help would be appreciated.)

Thank you for your help in advance, even if you have to roll your eyes at my stupidity please help me
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ztibor
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(Original post by tamsinaribena)
Hola, basically I'm doing M1 and I kind of hate maths at the moment so bare with me if I ask any ridiculous really obvious questions.

I've got an old exam question from 2003 & our teacher gave us all the final answer for it but I'm confused as to how she got it.

5. A particle P moved with constant acceleration (2i-3j)m/s^-2. At time t seconds, it's velocity is v/m/s^-1. When t=0, v=-2i+7j.

a. Find the value of t when P is moving parallel to the vector i. (I've done this bit & got it correct. The answer is t= 2 and a third seconds.)

b. Find the speed of P when t=3. (this is the bit I'm ridiculously stuck on. I don't understand how to get to the answer, is there a formula or something I should know about that and I'm just being a bit dense?)

c. Find the angle between the vector j and the direction of motion of P when t=3. (Haven't yet tried this, but I suck at maths so all help would be appreciated.)

Thank you for your help in advance, even if you have to roll your eyes at my stupidity please help me
The velocity vector in time of t, is sum of two components vectors.
1. THe velocity in t=0 which is -2i+7j and remain constant
2. The velocity from acceleration which is parallel with the vector
of acceleration and the value of this after t time is t*a ->2ti-3tj
Adding together, the velocity vector at the time of t
v=(2t-2)i+(7-3t)j
For t=3 sub in t 3
For the magnitude of the velocity
|v|=V(2t-2)^2+(7-3t)^2

For angle between j (y direction) and the vector consider that
vy/v=cos (alpha)
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tamsinaribena
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(Original post by ztibor)
The velocity vector in time of t, is sum of two components vectors.
1. THe velocity in t=0 which is -2i+7j and remain constant
2. The velocity from acceleration which is parallel with the vector
of acceleration and the value of this after t time is t*a ->2ti-3tj
Adding together, the velocity vector at the time of t
v=(2t-2)i+(7-3t)j
For t=3 sub in t 3
For the magnitude of the velocity
|v|=V(2t-2)^2+(7-3t)^2

For angle between j (y direction) and the vector consider that
vy/v=cos (alpha)
Thank you so much! You really helped & now I've finally got all my hw done, thanks again.
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