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Hi I apologies for simplistic question but I'm finding it difficult to understand. I'm doing simple GCSE Physics, the question at hand is in to parts.

a.) An object is moving at 24m/s. Calculate how long it will take to cover a distance of 6m.

Simple enough t = d/s, so t = 6/24, t= 0.25, t=0.25s

b.)It then decelerates at 3m/s2. What will it's velocity be after 3s?

I think this is correct, it decelerates by 3ms for every meter. So 3x3 = 9ms. 24 - 9 = 15ms.

If this is correct, im not sure what formula im using. I looked up the symbol for decelration, which is g. Ive not come across that yet in my material so im confused :(. Again apologies for the ignorance and stupidity. Any guidance would be appreciated.

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    The units of deceleration should be $m/s^2$, not $m/s$. If it is $3 m/s^2$ every second it loses $3 m/s$ of velocity. Please make sure to get the slashes in your velocities. The unit is $m/s$ as in meters per second, not $ms$ which could be milliseconds or meters-seconds. – 2017-01-25
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    Your units are all messed. For example, deceleration cannot have units of m/s. Do you mean it decelerated "to" 3 m/s or that it decelerated "by" 3 $m/s^2$ for a certain time. It think it's the latter.... – 2017-01-25
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    yes apologies thats my fault for missing that out. – 2017-01-25
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    As Ross Millikan points out, you really have to differentiate between ms and m$/$s. The first is milliseconds, and the latter is meters per second. (Meters-seconds would be m-s, typically, or perhaps just m s.) It appears that you are treating it correctly for this problem, as meters per second, but you'll have better luck getting people to help you if you use the units properly. – 2017-01-25

2 Answers 2

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For part b), you are using one of Newton's Laws of Constant Acceleration (Sometimes called the SUVAT equations): $$v=u+at$$ Where $u$ is the initial velocity, $a$ is the acceleration, $t$ is the time and $v$ is the final velocity.

Substituting the constants $a=-3 \text{ m/s}^{2}$, $u=24 \text{ m/s}$ and $t=3 \text{ s}$, we obtain the solution you have obtained: $$v=24 \text{ m/s}-3 \text{ s}\times 3\text{ m/s}^{2}=15 \text{ m/s}$$

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    You really should carry the units through your equation too, including the middle part of that last equation. – 2017-01-26
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    @Brick I've edited my answer. – 2017-01-26
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Ok i think ive worked it out. Rearranging the acceleration formula to find velocity.

$a = (v_f-v_i)/t$

so,

$at + u = (-3 \times 3) + 24 = 15$

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    Yes, that is correct. – 2017-01-25
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    Again - You need to keep track of the units here too. This expression cannot be correct without the units. – 2017-01-26