Classical Mechanics Answers

Q.3 A block of wood A of mass 10 kg is held on a rough horizontal table. An elastic string

connected to the block passes over a smooth pulley at the end of the table and then under a second smooth pulley carrying a body B of mass 5 kg as shown in Fig

The other end of the string is fixed to a point above the second pulley. When the 10 kg block is released, it moves with an acceleration of g/9. Determine the value of coefficient of friction between the block and the table.

https://imgur.com/a/4oSOLx2 (figure's image link)

Steep safety ramps are built beside mountain highways to enable vehicles with defective brakes to stop safely. A truck enters a 250-m ramp at a high speed v0, and travels 180 m in 6 s at constant deceleration before its speed is reduced to v0/2. Assuming the same constant deceleration, determine (a) the additional time required for the truck to stop, (b) the additional distance traveled by the truck.

The center of gravity of a 45gram meter stick is at the 49 cm. mark. The stick is part of a lever which has the fulcrum at the 73 cm. mark. Where must an 86gram object be hung in order to have equilibrium? Show solutions.

A body of mass 3M at rest explodes into two pieces of mass M and 2M. What is the ratio of the kinetic energy of M to that of 2M? (Ans:2)

A person of mass m stands on weighing scales in an elevator. The elevator is accelerating upwards with acceleration a. The reaction force from the scales on the person is R. What is the reading on the scales? (Ans: R)

A body that started from rest moves with constant acceleration in a straight line. After travelling a distance d the speed of the car is v. What is the distance travelled when the speed of the car was v/2? (Ans: d/4)

An electric train develops a power of 1.0 MW when travelling at a constant speed of 50 ms-1. The net resistive force acting on the train is (Ans: 20 kN).

My work: P=FV, F=P/V, F=2x10^-6

A machine lifts an object of weight 1.5 x 103 N to a height of 10 m. The machine has an overall efficiency of 20%. The work done by the machine in raising the object is: (Ans: D)

A. 3.0 x 103 J

B. 1.2 x 104 J

C. 1.8 x 104 J

D. 7.5 x 104 J 

Which one of the following objects is in equilibrium? (Could someone give reasoning on the answer?)

A. A stone trapped in the tread of a rotating tyre. B. An air molecule as a sound wave passes through the air. C. A steel ball falling at constant speed through oil. D. An electron moving through a metal under the action of a potential difference.

Ans from sheet: D