NCERT Physics Class 11 Exemplar Ch 5 Laws Of Motion Part 5

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Q. 26. A mass of is suspended with thread AB (Fig.). Thread CD of the same type is attached to the other end of mass. Lower thread is pulled gradually, harder and harder in the downward direction so as to apply force on AB. Which of the threads will break and why?

Which thread will break

Which Thread Will Break

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Answer: because force on the upper thread will be equal to sum of the weight of the body and the applied force.

Q.27. In the above given problem if the lower thread is pulled with a jerk, what happens?

Answer: If the force is large and sudden, thread CD breaks because as CD is jerked, the pull is not transmitted to AB instantaneously (transmission depends on the elastic properties of the body).

Therefore, before the mass moves, CD breaks.

Q.28. Two masses of are suspended with help of massless inextensible strings as shown in Fig. Calculate when whole system is going upwards with acceleration

T1 and T2 on 5kg block

T1 and T2 on 5kg Block

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Q.29. Block A of weight rests on a frictionless inclined plane of slope angle (Figure). A flexible cord attached to A passes over a frictionless pulley and is connected to block B of weight W. Find the weight W for which the system is in equilibrium.

Forces on ramp

Forces on Ramp

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Q.30 A block of mass M is held against a rough vertical wall by pressing it with a finger. If the coefficient of friction between the block and the wall is μ and the acceleration due to gravity is g, calculate the minimum force required to be applied by the finger to hold the block against the wall?

Answer: If F is the force of the finger on the book, F = N, the normal reaction of the wall on the book. The minimum upward frictional force needed to ensure that the book does not fall is Mg. The frictional force Thus, minimum value of .