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M1 Help :/

Okay so here's the question:

A particle P of mass 2kg is moving under the action of a constant force F newtons. When t=0, it has velocity (3i+2j)ms-1 and at time t=4s, P has velocity (15i-4j)ms-1. Find

(a) the acceleration of P in terms of i and j
- I have done this, it is (3i-1.5j)ms-2

(b) the magnitude of F
- Also done this, it's 6.708N

(c) the velocity of P at time t=6s

Now my problem lies here. I thought that since velocity = acceleration x time, I could do 6 x (3i-1.5j) but I cannot, since that gives me an incorrect answer.

I then thought I could use SUVAT with t=6, u=(3i+2j), a=(3i-1.5j) to find v with v=u+at. However, again, this gave me an incorrect answer.

If it is possible could someone please tell me
(a) Why my methods were wrong and
(b) How to obtain the correct answer?

Thank you so much!

Reply 1

roar558

v-u=at
v=at+u, you forgot to add the initial velocity

Reply 2

04bellot

no its velocity is theoriginal vestor time added to the acceleration times the time :/
i.e (3i+2j) + (3i-1.5j)ms-2*6 seconds

Reply 3

Insomniac_1901

i got (21i - 7j)m/s

have you got the answer?

Reply 4

Insomniac_1901

using v = u + at

v = (3i + 2j) + 6*(3i - 1.5j)

Reply 5

noobie93

Ahh yes that works now, thanks! :smile:

The answer is 21i-7j

Reply 6

bradford9

how did u work out part b)

Reply 7

Sinnoh

Original post by bradford9

how did u work out part b)

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