IIT JEE , AIEEE Forum - Physics , Mechanics

pinakinab

A body of mass 32 kg is suspended by a spring balance from the roof of a vertically operating lift & going downwards from the rest . At the instant the lift has covered 20 m & 50 m. the spring balance showed 30 kg & 36 kg respectively . Then what is the velocity of the lift

a)decreasing at 20 m. & increasing at 50 m.

b)increasing at 50 m. & decreaseing at 20 m.

c)continuously decreasing at constant rate throughout the

journey .

d)continuously increasing at constant rate throughout the

the journey .

master_purav

Increasing at 20 m and decreasind at 50 m...

kghedriu

It can b easily explained.....

let us assume that the acceleration of lift downwards be 'a' (note...'a' may b positive or negative)

now, drawing the free body diagram of the block gives

N=m(g+a)

where N is the normal force tht appears as the reading of the balance...

now, we know that mg=32

and, at 20m, m(g+a)=30 which is less than mg....i.e....at 20m, the lift is decelerating...so, its velocity is decreasing...

at 50m, m(g+a)=36 > mg....i.e the lift is accelerating..so, its velocity is increasing at this instant..

hope m clear..

Thank u..

Goutham..

master_purav

Hey Goutham,

This is a non inertial reference frame and the pseudo force on the object inside it willl act in the direction opposite to the direction of the acc of the frame.

So in this case the Net wt on the block would be m(g-a).

Am i right or not???

master_purav

For your FBD

Since the block is in equilibrium

N + ma = mg
N = m(g-a)

But in your solution you have considered N = m(g+a)!!!

abhijeet_0201

Increasing at 20 m and decreasind at 50 m.SAME AS MASTER PURAV.
it is bec when the lift is acc downwards(speed is inc) the effective wt of block will be less .hence the spring balance shows less reading .when speed is decreasing acc is -ve downwards(+ve upwards)
so the effective wt will be more.
plz check ur options (a) and (b) mean the same
if my sol is rite plz rate me.

abhijeet_0201

master purav is absolutely correct!!!!!!!!!

frenied

purav ur answer is quite correct.let me try to explain.
when the lift is not accelrating then kx acts upward and mg downward such that kx=32g.now at 20 m kx<32 g.that means some other force should also be acting in the direction along kx suxh that kx+F=mg thus lessening kx.this force is pseudo force of the lift and since it acts upward the lift must actually be accelerating downward.hence at this moment the velocity and acceleration being in the same direction velocity is increasing.in the other case kx>32 g.that is the total downward force is more than the weight.ie kx=mg+F.so pseudo force is acting in the downward direction and hence lift must be accelrating upward.again acceleration vector is opposite to velocity vector and hence velocity is decreasing at this moment.

kghedriu

hey....here is ma explanation....i think m correct....pls correct me if wrong..

Hey?.itz simple?.

-x doesnot imply a negative no

Eg, take x=-2, so, -x=2,a +ve no.

Similarly, here, a is either negative or +ve

See, acceleration upwards = -( accn down)

That was the concept I had used.

I mean, suppose it accelerates up with 2, m saying it decelerates down with 2

i.e if accn up = 3,

according to me, N=m(g+(-3))

according to u, N=m(g-3)

which are no different!

Hope I made maself clear.

Thank u

Goutham.

bhupesh

Dear
When the velocity is decreasing means a deceleration and when increasing means an acceleration the problem should be solved in side the lift i.e non inertial frame use pseudo force to see where its opposite to mg where its indirection with mg

deepti_hari

hi

according to me the right answer is

the velocity of lift is increasing at 20 m and decreasing at 50 m.

i analysed it in a way thinking that if we consider a man instead of the block the man loses when when the lift is moving downwards and gains weight when the lift is moving upwards.now in this situation the lift is moving downwards hence the block must lose weight if the velocity is increasing in downward direction and it must gain weight if the velocity is decreasing in the downward direction or in other words increasing in upward direction.hence according to me the when the distance covered by lift is 20m the block is is losing weight hence the velocity is increasing in given direction and gaining weight when it covered 50m hence the velocity is decreasing in the downward direction or increasing in upward direction.

this was my way of analysis.please let me know if there is any mistake

thanks for sparing time to read by explaination

bye.have a good day

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