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Inside a smooth fixed hollow cylinder of radius R, three identical smooth cylinders of radius r are placed with their axes parallel to axis of the fixed cylinder as shown. Then the equilibrium cannot exist unless R<nr. The value of n is

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Important Questions on Laws of Motion

MEDIUM
A mass of 10 kg is suspended vertically by a rope from the roof. When a horizontal force is applied on the rope at some point, the rope deviated at an angle of 45° at the roof point. If the suspended mass is at equilibrium, the magnitude of the force applied is g=10 m s-2
MEDIUM
A mass of 10 kg is suspended by a rope of length 4 m, from the ceiling. A force F is applied horizontally at the mid-point of the rope such that the top half of the rope makes an angle of 45° with the vertical. Then F equals: (Take g=10 m s-2 and the rope to be massless)
MEDIUM
A block starts moving up an inclined plane of inclination 30° with an initial velocity of v0. It comes back to its initial position with velocity v02. The value of the coefficient of kinetic friction between the block and the inclined plane is close to 11000, The nearest integer to I is :
EASY

The sum of the three vectors in the figure below is zero. The magnitude of OC and OB is,

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MEDIUM
A block of mass 10 kg is in contact against the inner wall of a hollow cylindrical drum of radius 1 m. The coefficient of friction between the block and the inner wall of the cylinder is 0.1. The minimum angular velocity needed for the cylinder to keep the block stationary when the cylinder is vertical and rotating about its axis will be  g=10  m s-2,
HARD

The pulleys and strings shown in figure are smooth and of negligible mass. For the system to remain in equilibrium, the angle θ should be:

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HARD
An elevator in a building can carry a maximum of 10 persons, with the average mass of each person being 68 kg . The mass of the elevator itself is 920 kg and it moves with a constant speed of 3 m/s . The frictional force opposing the motion is 6000 N . If the elevator is moving up with its full capacity, the power delivered by the motor to the elevator g=10m/s2 must be at least:
MEDIUM

For a free body diagram shown in the figure, the four forces are applied in the 'x' and 'y' directions. What additional force must be applied and at what angle with positive x-axis so that the net acceleration of body is zero?

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EASY

A particle moving with velocity V is acted by three forces shown by the vector triangle PQR. The velocity of the particle will
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HARD
A block of weight 100 N is suspended by copper and steel wires of same cross-sectional area 0.5 cm2 and, length 3 m and 1 m, respectively. Their other ends are fixed on a ceiling as shown in figure. The angles subtended by copper and steel wires with ceiling are 30o and 60o, respectively. If elongation in copper wire is ΔlC and elongation in steel wire is ΔlS, then the ratio ΔlCΔlS is x1.Find the value of x.
[Young's modulus for copper and steel are 1×1011N m-2 and 2×1011Nm-2 respectively]

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EASY

A block of mass 200 g is kept stationary on a smooth inclined plane by applying a minimum horizontal force F=x N as shown in figure.

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The value of x=_____.

HARD
On heating water, bubbles being formed at the bottom of the vessel detatch and rise. Take the bubbles to be spheres of radius R and making a circular contact of radius r with the bottom of the vessel. If r << R, and the surface tension of water is T, value of r just before bubbles detatch is :
(density of water is ρ w )

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MEDIUM

A mass of 10 kg is suspended vertically by a rope of length 5 m from the roof. A force of 30 N is applied at the middle point of rope in horizontal direction. The angle made by upper half of the rope with vertical is α=tan-1x×10-1. The value of x is _____ .

(Given, g=10 m s-2)

EASY

A 90 N mass is hung on a rope tied between two poles as shown in the figure. The tension T1 and T2 in the two parts of the rope are (in N)

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MEDIUM

Four forces are acting at a point P in equilibrium as shown in figure. The ratio of force F1 to F2 is 1:x, where x=_____ .

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EASY

A block of 3 kg is attached to a string whose other end is attached to the wall. An unknown force F is applied so that the string makes an angle of 30° with the wall. The tension T is : (Given g =10 m s2)

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MEDIUM
A block of mass M slides down on a rough inclined plane with constant velocity. The angle made by the incline plane with horizontal is θ. The magnitude of the contact force will be :
HARD
A 60HP electric motor lifts an elevator having a maximum total load capacity of 2000 kg. If the frictional force on the elevator is 4000 N, the speed of the elevator at full load is close to : 1 HP=746 W, g=10 m s-2
MEDIUM
Two masses m1=5 kg and m2=10 kg, connected by an inextensible string over a frictionless pulley, are moving as shown in the figure. The coefficient of friction of horizontal surface is 0.15. The minimum weight m that should be put on top of m2 to stop the motion is:

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MEDIUM
A rod of weight W is supported by two parallel knife edges A and B and is in equilibrium in a horizontal position. The knives are at a distance d from each other. The centre of mass of the rod is at distance x from A . the normal reaction on A is: