Question

Please help... A lift,on its upward journey,starts from rest and accelerates for 5s until it reaches a velocity of 2m/s^-1.After this,it moves at a constant speed for 10s and then retards to to rest after another 5s.
Fir its downward journey,it again starts from rest and accelerates for 10s until it reaches a velocity of 3m/s^-1.Then it retards to rest after another 10s.Sketch the velocity-time graph for both the upward and downward journey made by the lift.
Calculate...

Answer 1

(i) its retardation during upward journey;
(ii) its retardation during downward journey;
(iii) what can you say about the areas under the two velocity-time graph.

Answer 2

eliasaab

Answer 3

zepdrix,thomaster

Answer 4

To call people; use @ before their name; like @sheelo73mughal :)

Answer 5

(i) its retardation during upward journey;
(ii) its retardation during downward journey;
Use -
acceleration = (final velocity - initial velocity) / time period

(iii) what can you say about the areas under the two velocity-time graph.
Area under the velocity time graph denotes displacement; so what's your guess ??

Answer 6

no guess

Answer 7

@LastDayWork

Answer 8

For (i) and (ii); you simply have to substitute the respective values in the formula.
Note that initial velocity for (i) will be zero; final velocity for (ii) will be zero.

As the question is asking for retardation; get magnitude from above ^^ and
(i) sign would be negative
(ii) sign would be positive

Now can you answer (i) and (ii) ??

Answer 9

ok i will try

Answer 10

but the time in upward journey is 5s and 10s so which time should we put in the formula

Answer 11

and how can we plot a graph

Answer 12

Sorry, I didn't realized what you were asking; my bad..
Use 5s

We can plot the graph later :)

Answer 13

yes i got(i)0.4
(ii)0.3
now can you tell me how can i plot the graph for it @LastDayWork

Answer 14

@eliasaab

Answer 15

can you help me @Loser66

Answer 16

help please its important

Answer 17

what does s^- in velocity mean?
the unit of velocity is m/s, why is yours m/s^- ?

Answer 18

its m/s power -1

Answer 19

@ybarrap

Answer 20

help

Answer 21

need your check @ybarrap

Answer 22

@Loser66 rest means zero velocity.

Answer 23

@Loser66 then that would mean infinite deceleration. Between 15 and 20 seconds it "retards to to rest", which means it decelerates to zero speed.

Answer 24

ok, I got you. I have problem with understanding hehehe.

Answer 25

@sheelo73mughal does all this make sense?

Answer 26

@sheelo73mughal
ybarrap drew the graph of downward journey (the triangle) by using the fact that the net displacement (area under the graph) for the lift would be zero (why?)

This also answers your question - "(iii) what can you say about the areas under the two velocity-time graph."