Question

Help @thomaster

Answer 1

A car travelling at 72km/h decelerates uniformly at 2m/s^2.
Calculate
(a)the distance it goes before it stops
(b)the time it takes to stop
(c)the distance it travels during the first and third seconds

Answer 2

@Mashy

Answer 3

This is a trivial problem
.. what are the initial data given?

Answer 4

Initial data?

Answer 5

to solve these problems.. u need the three equations of motion..

right? u know them?

Answer 6

yes yes

Answer 7

\[a(t)=-2\]
There's one!

Answer 8

yea.. so like he said..

a = -2 m/s^2..

what else do u know?

Answer 9

Oh lol
acceleration is -2km/h
initial velocity = 72km/h

Answer 10

-2m/s^2 sorry

Answer 11

yea ok.. so u need to convert it into m/s..

do u know the final velocity? (the car comes to stop)

Answer 12

and converting that 72km/h into m/s gives
20

Answer 13

yea final velocity is 0

Answer 14

ok.. so u have

u , v, and a
can u calculate t and S ?

Answer 15

)the distance it goes BEFORE it stops
means does it have any significance

Answer 16

BEFORE =
distance travelled till it stops..

its just a way of speaking in english.. not any significance.. :)

Answer 17

Oh , then it's super-duper trivial

Answer 18

now that i think about it.. this is really genuine doubt.. which students can have.. and we teachers tend to over look

thank you :)..

i ll keep this in mind while teaching :) :). for next time

Answer 19

yes , teachers tend to overlook
U a teacher??????

Answer 20

yup :D :D..

Answer 21

(c)the distance it travels during the first and third seconds

what should we do for this
distance travelled in 3 secons - distanc etravelled in 1 second

Answer 22

?

Answer 23

during first second


distance travelled in first second = distance travelled in two seconds - distance travelled in 1 second



in general.. distance travelled in nth second = distance travelled in total n seconds - distance travelled in (n-1) seconds

Answer 24

What is the difference , why can't we just take 1 second in our equation

Answer 25

1st second 2nd second or 3rd second.. all are 1 second time intervals..

so u need to know which 1 second interval we are talking about right?

Answer 26

but yea.. distance in 1 scond = distance in first second.. i agree

but distance in 2 second is not equal to distance in 2nd second

Answer 27

Why

Answer 28

sry for trouble though

Answer 29

u teach where btw

Answer 30

u can think

distance travelled in 2 seconds is.. total distance in 2 seconds interval

but distance travelled in 2nd second is distance travelled in the 2nd 1 second ka time interval..

lets take simple example

consider an object with zero speed.. and accelerates at 2m/s

then in 1second it goes s = 1/2 at^2 = 1m

in 2 second it goes s= 4m

in 3 second it goes s = 9m

in 4 second it goes s = 16 m.. and so on

but.. now if i ask.. distance travlled in 2nd second.. it is 4 - 1 = 3m.. (this is the distance coverd by the object in 2nd 1 second interval)

also distance in 3rd second = 9 - 4 = 5m.. (again.. this is distance travelled in the 3rd one second interval).. get it?

Answer 31

Oh i got the concept
But if the question was , the particle is at what position at 3 rd second then the answer is 9m right

Answer 32

u can't say.. AT 3rd second

cause 3rd second is an interval.. and in that interval.. the particle is travelling from 4m to 9 m..

so if u ask.. the position of the particle at the BEGINNING of 3rd second.. it is at 4m

and at the END of 3rd second.. it is at 9 m

Answer 33

This is confusing but i get it
btw where do u teach

Answer 34

ur a man

Answer 35

?

Answer 36

yea m a man :P.. i teach in mangalore..

Answer 37

grt , if u r searching for questions in physics
http://manishkumarphysics.in/kota/?page_id=271

Answer 38

(b)the time it takes to stop?
Do we use v=u+at

Answer 39

yes.. use that

Answer 40

thanks for the link.. is that ur site?

Answer 41

if that was my site i wouldn't have asked this question lol

Answer 42

that is for IIT preparation

Answer 43

haha lol.. ok xD

Answer 44

CAn we do the last part , i feel to discuss it

Answer 45

@Mashy

Answer 46

yes.. calculate..

Answer 47

i am not getting just the last part

Answer 48

fist second . u put t = 1 right?


for third second
find distance in 3 second, distance in 2 second.. then subtract

Answer 49

yes i made a math error

Answer 50

:P