Riannnne
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a particle p is projected from origin with velocity 12i+24j. the particle moves freely under gravity
find
a) the position vector of P after 3 second
for this I got 36i+27.9j (which is correct)

I'm havinga problem with part b)

find the speed of P after 3 seconds.

the answer to this is 13ms-1. I have seen the solution to this part but I just don't understand how they got to that answer....please help
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Sir Cumference
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(Original post by Riannnne)
a particle p is projected from origin with velocity 12i+24j. the particle moves freely under gravity
find
a) the position vector of P after 3 second
for this I got 36i+27.9j (which is correct)

I'm havinga problem with part b)

find the speed of P after 3 seconds.

the answer to this is 13ms-1. I have seen the solution to this part but I just don't understand how they got to that answer....please help
Please post your attempt or let us know if you don't know where to start.
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dextrous63
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You seem to know how to apply suvat correctly as you have got part (a) correct.
Use your skills to find the velocity vector after 3 seconds, and then calculate the magnitude of this to find the speed.
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Riannnne
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(Original post by Sir Cumference)
Please post your attempt or let us know if you don't know where to start.
what I did was

I resolved taking the upward vertical direction as +ve

u=24 v=? a=-9.8 t=3

v the final velocity being the velocity/speed when t=3

subbed it into the s=ut+0.5at^2 formula

and I got -5.4ms-1

is this incorrect ????
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the bear
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you can use v = u + at

where u = 12i+24j

and a = 0i - 9.8j

and t = 3
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Sir Cumference
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(Original post by Riannnne)
what I did was

I resolved taking the upward vertical direction as +ve

u=24 v=? a=-9.8 t=3

v the final velocity being the velocity/speed when t=3

subbed it into the s=ut+0.5at^2 formula

and I got -5.4ms-1

is this incorrect ????
Yes this is correct (I'm assuming you meant v=u+at and not the formula you posted)?

But you've just found the vertical component of the velocity. You also need the horizontal component and together they make up the velocity of the particle. Then you'll need to find the magnitude of the velocity which is the speed.
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Riannnne
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(Original post by Sir Cumference)
Yes this is correct (I'm assuming you meant v=u+at and not the formula you posted)?

But you've just found the vertical component of the velocity. You also need the horizontal component and together they make up the velocity of the particle. Then you'll need to find the magnitude of the velocity which is the speed.
thank you so much! I really appreciate your help
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Sir Cumference
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(Original post by the bear)
you can use v = u + at

where u = 12i+24j

and a = 0i - 9.8j

and t = 3
For some reason, textbooks/teachers always seem to separate into components for questions in this topic. I personally prefer to keep it all together like you have done.
Last edited by Sir Cumference; 4 days ago
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the bear
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(Original post by Sir Cumference)
For some reason, textbooks/teachers always seem to separate into components for questions in this topic. I prefer your way and more students should do it like this because it's faster.
:five:
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13SJIqbal
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Keep in your i 's and j 's
V=u+at
Vx=12+0t
Vy=24+(-g)t
and take the magnitude
Last edited by 13SJIqbal; 4 days ago
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dextrous63
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(Original post by Sir Cumference)
For some reason, textbooks/teachers always seem to separate into components for questions in this topic. I prefer your way and more students should do it like this because it's faster.
Guess it comes down to the way they were taught themselves.
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Riannnne
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(Original post by the bear)
you can use v = u + at

where u = 12i+24j

and a = 0i - 9.8j

and t = 3
thank you
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