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A semicircular portion of radius r is cut from a uniform rectangular plate as shown in the figure. The distance of centre of mass C of remaining plate from point O is:

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Important Questions on Centre of Mass, Momentum and Collisions

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Look at the drawing given in the figure which has been drawn with ink of uniform line-thickness. The mass of ink used to draw each of the two inner circles and each of the two line segments is m. The mass of the ink used to draw the outer circle is 6 m. The coordinates of the centres of the different parts are: outer circle (0,0); left inner circle (-a, a), right inner circle (a ; a), vertical line (0,0) and horizontal line (0,-a). The y-coordinate of the centre of mass of the ink in this drawing is

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A radioactive nucleus initially at rest decays by emitting an electron and neutrino at right angles to one another. The momentum of the electron is 3.2×10-23 kg m s-1 and that of the neutrino is 6.4×10-23 kg m s-1. The direction of the recoiling nucleus with that of the electron motion is, 

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A particle of mass 1 kg is thrown vertically upwards with a speed of 100 m/s . After 5 s, it explodes into two parts. If one part of mass 400 g comes back with speed 25  m/s, then the speed of other part just after explosion is
Take g=10 m s-2.
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A mass m moves with a velocity v and collides inelastically with another identical mass at rest. After collision, the 1st mass moves with velocity v3 in a direction perpendicular to the initial direction of motion. Find the speed of the 2nd  mass after collision:

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A mass of 10 gm moving horizontally with a velocity of 100 cm sec-1 strikes the bob of a pendulum and sticks to it. The mass of the bob is also 10 gm. The maximum height to which the system can be raised is g=10 m sec-2

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A solid iron ball A of radius r collides head-on with another stationary solid iron ball B of radius 2r. The ratio of their speeds just after the collision (e = 0.5) is :
HARD
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Two balls, having linear momenta p1=pi^ and p2=-pi^, undergo a collision in free space. There is no external force acting on the balls. Let p1' and p2' be their final momenta. The following option(s) is(are) NOT ALLOWED for any non-zero value of p,a1,a2, b1, b2,c1and c2
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Assertion: A sphere of mass m moving with speed u undergoes a perfectly elastic head-on collision with another sphere of heavier mass M at rest (M>m), then the direction of the velocity of the sphere of mass m is reversed due to collision [no external force acts on a system of two spheres]

Reason: During a collision of spheres of unequal masses, the heavier mass exerts more force on lighter mass in comparison to the force which lighter mass exerts on heavier mass.