Force: Newton's Law of Motion

July 01, 2021

Important Question Answer (Chapter Revision); Ready to Test your knowledge

Q1. Why does the electric fan continue to rotate for sometime after the current is switched off.?

Ans. The fan continues to rotate due to inertia of motion, until the friction in bearings and that of air brings it to rest.

Q2. Is a train moving on a circular track, an inertial frame of reference.?

Ans. No

Q3. A trolley of mass 300kg carrying a sand bag of 25kg is moving at a uniform speed of 27km/h on a smooth track. After a while, sand begins to leak out of a hole on the trolleys floor at a rate of 0.05kg/s. What is the speed of the trolley after the entire sand bag is empty.?

Ans. As the trolley is moving uniformly on a smooth track, no external forces acts on the trolley bag system. Hence its speed will remain 27km/h even after the bag become empty through leakage of sand.

Q4. A Bob of mass 0.1kg hung from the ceiling of a room by a string 2m long is set into oscillation. The speed of the Bob at the mean position is 1m/s. What will be the trajectory of the Bob if the string is cut when the Bob is

(a) at one is its extreme positions

(b) at its mean position

Ans. (a) At the extreme position, the velocity of the Bob is zero. Hence, on cutting the string, the force of gravity vertical downward.

(b) At the mean position, the Bob has a horizontal velocity of 1m/s and is acted upon by the force of gravity vertical downward. The situation is same as at the highest point of projectile motion. Hence the Bob will fall down along a parabolic path.

Q5. Two bodies of masses M and m are allowed to fall from the same height. If the air resistance for each be the same, then will both the bodies reach the earth simultaneously.?

Ans. No. The forces acting on the body of mass M are : it's weight Mg and air resistance F. The net force is Mg - F.

Hence its acceleration a = Mg - F/ M = g - F/M.

Thus the acceleration of the body of larger mass will be greater and it will reach the earth earlier.

Q6. A nucleus is at rest in the laboratory frame of reference. It disintegrates into smaller nuclei. Show that the products must be emitted in opposite directions.

Ans. Let m1 and m2 be the masses of the products of disintegration and v1 and v2 their respective velocities. The total linear momentum of the system after disintegration is m1v1 + m2v2. Before disintegration, the nucleus was at rest and so linear momentum was zero. By conservation of linear momentum, we have

m1v1 + m2v2 = 0

v1 = -(m2/m1) v2

Since, v1 and v2 have opposite signs, the two parts of the nucleus fly off in opposite directions.

Q7.