Good Problems From Newton's Laws | CYU 24, MCQ 2 & 10 | Pathfinder

Pathfinder | Newton's Laws Of Motion | Check Your Understanding 24 | CYU 24
A disc of radius R inclined at an angle with the horizontal is evenly covered with a thin layer of sand. Coefficient of friction between the sand and the disc is u. The disc is made to rotate about its central axis of symmetry with gradually increasing angular velocity. The angular velocity is increased so gradually that tangential acceleration of any of the sand particle is practically negligible. What is the angular velocity of the disc, when 7 fraction of the sand has fallen off the disc? Acceleration due to gravity is g.
Pathfinder | Newton's Laws Of Motion | Multiple Choice Question | MCQ 2
2. In the setup shown a thread is taut between a nail A on a wall and a nail B on an inclined plane as shown in the figure. You can change the length of the thread by changing location of the nail B. A bead that can slide on the thread without friction is released from the nail A. What should the length of the thread be so that the bead reaches the nail B in shortest time?
(a) h
(c) h sec (0.50)
(b) hcose (d) hcos@sec (0.50)
Pathfinder | Newton's Laws Of Motion | Multiple Choice Question | MCQ 10
10. In the setup shown, the string is inextensible and light, there is no friction at the axles of the pulleys but friction between the string and the pulley wheels is sufficient to prevent slipping. All the pulleys are light except the pulley P, which has whole of its mass m concentrated in its axle. Initially the block B of mass M is on the ground and the block A is held at rest. How much minimum mass should the block A have, so that after releasing it, the block B starts rising?
(a) 0.5M
(b) 0.5M - m
(c) Whatever mass the block A has, the block B will not rise.
(d) Pulley P moves downwards without rotation of its wheel while wheels of all the other pulleys rotate.

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[@A-man-z3i]

How can T/M and T/mA deduce each other?

[@Phyx_IITM]

Yeah I didn't pay attention to that while recording man a+b is not 2g . It doens't matter for the current problem tho as both a and b become g

[@sarthaksingh5614]

at 3:40 a+b will not be equal 2g because (mA)- mass of block A and (m)- mass of pulley are not the same.. even though we will still get the answer intuitively.. and man your approach for second problem was pure beauty

[@Phyx_IITM]

Oo yeah didn't notice that while recording, good thing it doesn't matter in the end.

[@manojrawat4435]

this channel is pure gem 🎉

[@shlokdave6360]

The calculations in the first questions seem to be wrong. The limiting condition for the block B to lift is 2T = Mg. Based on the string and pulley constraints, T.ap - T.aa + 2T.ab = 0, where ab = a of B, ap = a of P and aa = a of a. Since ab = 0 for the limiting case of B just lifting up, acceleration of mass A = acceleration of pulley P. Using the FBD for A and P, m'g - T = m'a and mg + T = ma. Eliminating a from the two equations gives m(m'g - T) = m'(mg + T) or (m + m') T = 0. Since m + m' can not be 0, T must be 0. If T = 0, there is no upward force on B to lift it up. Therefore, B can not be lifted.
This is an example of Trivial Tension arising from the "output" of the pulley P connected to its "input". The system as a whole can never deliver any power. Hence no mass in place of B can be lifted no matter what is hanging in place of A.

[@infinite1759]

👍 great

[@Mithun2917_]

I absolutely love the way u explain stuff my humble requests
1) please make playlists chapter wise (for advanced)
Example: Sbt questions Pathfinder questions and test series/AITS questions of Kinematics make one playlist as Kinematics Advanced Questions
2) Please Bring something for Jee Mains Physics too, like solving Important questions of Kinematics for jee mains in that add AITS questions,tricky PYQs etc

[@peacefighterpro682]

Crazy solution for second one 😮

[@matrixdem1t0r93]

for the first one can we not just say that t=0 and that it is in freefall, great video btw. I didnt do rotation yet but understood almost everything

[@Siddhesh_JEE]

What you do broo❤

[@ab5kv671]

Very nice

[@PRIME_RAKSHIT]

Pls solve my doubt 3:45
Why mass of block a(ma)= (m) mass of block p

[@princerathore3717]

At 18.05 can this circle have diameter is equal to AB and this circle have smallest radius

[@deepikasah-hz5wg]

modern physics ka koi playlist nhi aisa qq

[@malhardongre1948]

at 3:47 eq1 + eq2 cant be equal to 2g as at this stage of problem we dont know whether the denominator of T is same or not although good explaination

[@jaykugosenpai9393]

I am sorry but why will the net torque about the axle be zero in the first question?!

[@NilanjanHalder-lp9vd]

Torque= I.Alpha , I=0 so torque= 0

[@lordvader-jn3gz]

Inertia is zero so torque is zero

[@letsrockwithvansh1508]

Bhaiya please apni videos ko playlisy mein arrange krdo... Bht mushkil ho rhi h 🙏

[@Phyx_IITM]

Mene pf nlm me daala he bhai.

[@letsrockwithvansh1508]

@@Phyx_IITM bht saari videos playlist mein nhi h bhaiya