NEET Physics : System of Particles and Rotational Motion

A coil has resistance 30 O and inductive reactance 20 O at 50 Hz frequency. If an AC source 200 V, 100 Hz, is connected across the coil, the current in the coil will be

• 4.0 A

• 8.0 A

• 
• 

A closely wound solenoid of 2000 turns and area of cross -section 1.5 x 10^-4 m² carries a current of 2.0 A. It is suspended through its centre and perpendicular to its length, allowing it to turn in a horizontal plane in a uniform magnetic field 5 x 10^-2 T making an angle of 30^o with axis of  the solenoid. The torque on the solenoid will be

• 3 x 10^-3 N-m

• 1.5 x 10^-3 N-m

• 1.5 x 10^-2 N-m

• 1.5 x 10^-2 N-m

A thin circular ring of mass M and radius r is rotating about its axis with constant angular velocity ω. Two objects each of mass m are attached gently to the opposite ends of a diameter of the ring. The ring now rotates with angular velocity given by

• (M +2m)ω /2m

• 2Mω / M + 2m

• (M +2m)ω / M

• (M +2m)ω / M

From a circular disc of radius R and mass 9M, a small disc of mass M and radius R/3 is removed concentrically. The moment of inertia of the remaining disc about an axis perpendicular to the plane of the disc and passing through its centre is

• 40MR²/9

• MR²

• 4 MR²

• 4 MR²

Point masses m₁ and m₂ are placed at the opposite ends of a rigid rod of length L and negligible mass. The rod of length L and negligible mass. The rod is to be set rotating about an axis perpendicular to it. The position of point P on this rod through which the axis should pass so that the work required to set the rod rotating with angular velocity ω₀ is minimum is given by 

A small mass attached to a string rotats on frictionless table top as shown. If the tension is the string is increased by pulling the string causing the radius of the circular  motion to decrease by a factor of 2, the kinetic energy of the mass will

• remain constant

• increase by a factor of 2

• increase by a factor 4 

• increase by a factor 4 

A rectangular coil of length 0.12 m and width 0.1 m having turns of wire is suspended vertically in a uniform magnetic field of strength 0.2 Wb/m².The coil carries a current of 2 A. If the plane of the coil is inclined at an angle of 30^o with the direction of the field, the torque required to keep the coil in stable equilibrium will be

• 0.15 Nm

• 0.20 Nm

• 0.24 Nm

• 0.24 Nm

A gramophone record is revolving with an angular velocity ω. A coin is placed at a distance r from the centre of the record. The static coefficient of friction is μ. The coin will revolve with the record if

• r = μgω²

• 
• 
• 

Under the influence of a uniform magnetic field, a charged particle moves with constant speed v in a circle of radius R. The time period of rotation of the particle

• depends on v and not on R

• depends on R and not on v

• is independent of both v and R

• is independent of both v and R

A circular disk of moment of inertia I_t is rotating in a horizontal plane, about its symmetry axis, with a constant angular speed. Another disk of moment of inertia I_b is dropped coaxially onto the rotating disk. Initially, the second disk has zero angular speed. Eventually, both the disks rotate with constant angular speed.The energy lost by the initially rotating disc due to friction is