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M1 Suvat question June 2005 question 4

Hey.
I know for this question i need to get 2 different suvat equations then sole simultaneously but i cant find where i have made a mistake.

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Thanks.

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Reply 1

Notnek

Original post by SamuelN98

Hey.
I know for this question i need to get 2 different suvat equations then sole simultaneously but i cant find where i have made a mistake.

Attachment not found

Thanks.

For 'the next 1.2m', the initial velocity is not 'u' anymore since the particle has already been travelling for 0.8s and must have changed speed.

Instead, use the fact that P has travelled 8m in 2s with initial velocity 'u'.

(edited 8 years ago)

Reply 2

Zacken

Original post by SamuelN98

Hey.
I know for this question i need to get 2 different suvat equations then sole simultaneously but i cant find where i have made a mistake.

Attachment not found

Thanks.

For the second bit of the motion, you seem to have said let

u = u

but this is not true, you are considering motion in the last 6 metres, hence the initial speed is

u' = u + at = u + 0.8a

.

Urgh, nevermind. :facepalm:

Ninja-ed.

Reply 3

SamuelN98

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Thanks, that was really helpful.
I have another question...
I don`t understand why i cant use v=u+at for question (iii).

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Reply 4

Zacken

Original post by SamuelN98

I don`t understand why i cant use v=u+at for question (iii).

That's a kinematics question, you need to use calculus.

SUVAT equations are derived under the assumption that acceleration is constant. In this case, acceleration (and velocity) varies as a function of time, so you need to throw all thoughts of SUVAT out of your head when looking at those questions and focus on using calculus. You know how to maximise functions (*coughs* derivative) and you're given velocity as a function of time, so...