A force F=$a\hat{i}$$+3\hat{j}$$+6\hat{k}$ is acting at a point r=$2\hat{i}$-$6\hat{j}$-12$\hat{k}$. The value of α for which angular momentum about origin is conserved is

(1) -1

(2) 2

(3) zero

(4) 1

A solid cylinder of mass 50 kg and radius 0.5 m is free to rotate about the horizontal axis.A massless string is wound round the cylinder with one end attached to it and other hanging freely.Tension in the string required to produce an angular acceleration of 2 rev/ s² is

(a)25N

(b)50N

(c)78.5N

(d)157N

The ratio of the accelerations for a solid sphere (mass m and radius R) rolling down an incline of angle  θ without slipping and slipping down the incline without rolling will be:
1. 5:7
2. 2:3
3. 2:5
4. 7:5

A small object of uniform density rolls up a curved surface with an initial velocity v'. It reaches up to a maximum height of 3v²/4g with respect to the initial position. The object is

(1) ring

(2) solid sphere

(3) hollow sphere

(4) disc

When a mass is rotating in a plane about a fixed point, its angular momentum is directed along:

Two persons of mass 55 kg and 65 kg respectively, are at the opposite ends of a boat.The length of the boat is 3.0m and weighs 100 kg.The 55 kg man walks up to the 65 kg man and sits with him.If the boat is in still water the centre of mass of the system shifts by 

(1)3.0m                           

(2)2.3m

(3)zero                             

(4)0.75m

A solid cylinder of mass 3kg is rolling on a horizontal surface with velocity 4 $m{s}^{-1}.$ It collides with a horizontal spring of force constant 200 $N{m}^{-1}$. The maximum compression produced in the spring will be

1. 0.5 m

2. 0.6 m

3. 0.7 m

4. 0.2 m

A circular platform is mounted on a frictionless vertical axle. Its radius is R = 2m and its moment of inertia about the axle is 200 kg m². Initially, it is at rest. A 50 kg man stands on the edge of the platform and begins to walk along the edge at a speed of 1 m s^–1 relative to the ground. The time taken by the man to complete one revolution is:
1. $\mathrm{\pi } \sec$                 
2. $\frac{3\mathrm{\pi }}{2}\sec$
3. $2\mathrm{\pi } \sec$               
4. $\frac{\mathrm{\pi }}{2}\sec$

The moment of inertia of a uniform circular

disc is maximum about an axis perpendicular

to the disc and passing through

(1) B                   

(2) C

(3) D                   

(4) A

Three masses are placed on the x-axis:

300 g at origin, 500 g at x= 40 cm and 400g

at x=70 cm. The distance of the center of

mass from the origin is 

(1) 40 cm                   

(2) 45 cm

(3) 50 cm                   

(4) 30 cm