# Thread: Finding deceleration

1. ## Finding deceleration

Hey I am doing some exam questions from a mechanics textbook and I am really confused I was just wondering if anyone could help me out The question is:

A train is uniformly retarded from 35ms to 21ms over a distance of 350m.

How do I find the deceleration?

And how do I find the total time to decelarate from 35ms with this retardation?

2. Originally Posted by andythefox
Hey I am doing some exam questions from a mechanics textbook and I am really confused I was just wondering if anyone could help me out The question is:

A train is uniformly retarded from 35ms to 21ms over a distance of 350m.

How do I find the deceleration?

And how do I find the total time to decelarate from 35ms with this retardation?
u = 35 m/s

v = 21 m/s

x = 350 m

a = ?

Substitute into an appropriate formula for uniform straight line motion (I assume you have been taught them).

3. but then don't you need time?

what formula would i use?

4. Originally Posted by andythefox
but then don't you need time?

what formula would i use?
It's expected that you're familiar with the formulae for uniform straight line motion. Surely you've been taught them if you're attempting questions like this one.

The relevant formula for this question is $2ax = v^2 - u^2$.

5. what is x?

6. it owuld help me if you did this question and i then can see if we have got the same answer, also the textbook i have has the answers in the back so i can check there aswell

7. Originally Posted by andythefox
it owuld help me if you did this question and i then can see if we have got the same answer, also the textbook i have has the answers in the back so i can check there aswell
x is displacement.

The formula might be written in your textbook or class notes as $2as = v^2 - u^2$ (I assume you've actually looked in your textbook ....?)

Surely you can substitute some numbers into a formula and get an answer. I'm not going to do it - I've already done enough.

And what would help you more is going back and actually revising this material. You obviously have a textbook that is asking these questions - in that textbook are the formulae that you're expected to know.

8. Originally Posted by andythefox
it owuld help me if you did this question and i then can see if we have got the same answer, also the textbook i have has the answers in the back so i can check there aswell
Post you solution so we can see what you have tried and where you are going wrong

CB