2 . 0 0 k g balls are attached to the ends of a thin rod of length 5 0 . 0 c m and negligible mass. The rod is free to rotate in a vertical plane without friction about a horizontal axis through its centre . With the rod initially horizontal, a 5 0 . 0 g wad of wet putty drops onto one of the balls, hitting it with a speed of 3 . 0 0 m / s and then sticking to it. Through what angle (in degrees) will the system rotate before it momentarily stops?
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why angular momentum is conserved ????????????
due to law of conservation of angular mumentum
For the first instant, there is no net torque on this system. :D
yeah i understood that "first instant" thank you
Hey gravity will cause an external torque on the system about the pivot...so how can you conserve angular momentum?
Well, you see, initially at one instant the NET torque around the pivot by gravity is zero; and we are doing conservation of angular momentum only for that one INSTANT anyways, so we're good.
But when the putty has fallen,the centre of mass of the system gets deviated from the pivot.So gravity will apply a torque on the system.
@Kashif Ahmad – But by then you won't need to conserve ang. mom. anymore; by then your calculations will be of conserve. of energy instead.
i think its 1.3 degrees from center of mass
I think exact ans is 182.6°……
Actually this question has three parts. First ,to calculate the ω of the system ,second the ratio of the kinetic energy of the system after the collision to that of the putty wad just before.,.
And this has been left for you as a question ! The angular speed comes out to be 0 . 1 4 8 r a d / s e c and the ratio of kinetics energies is 0 . 0 1 2 3 And the third to what angle does the whole system rotates before momentarily coming to a stop.
As the rod rotates, the sum of its kinetic and potential energies is conserved. If one of the balls is lowered a distance h , the other is raised the same distance and the sum of the potential energies of the balls does not change. We need consider only the potential energy of the putty wad. It moves through a 9 0 o arc to reach the lowest point on its path gaining kinetic energy and losing gravitational potential energy as it goes. It then swings up through an angle θ losing kinetic energy and gaining potential energy, until it momentarily comes to rest. Take the lowest point on the path to be the zero of potential energy. It starts a distance 2 d above this point, so its initial potential energy is U i = 2 m g d .If it swings up to the angular position θ as measured from its lowest point, then its final height is ( 2 d ) ( 1 − cos θ ) above the lowest point and its final potential energy is u f = m g × ( 2 d ) ( 1 − cos θ ) The initial kinetic energy is the sum of that of the balls and wad: K i = 2 1 I ω 2 = 2 1 ( 2 M + m ) ( 2 d ) 2 ω 2 At it’s final position we’ve K f = 0 .Conservation of energy provides the relation: m g 2 d + ( 2 M + m ) ( 2 d ) 2 ω 2 = m g × ( 2 d ) ( 1 − cos θ ) When this equation is solved for cos θ ,the result is cos θ = − 2 1 ( m g 2 M + m ) ( 2 d ) × ω 2 On putting the values in S.I. units we get cos θ = − 0 . 0 2 2 6 Consequently, the result for θ is 9 1 . 3 o The total angle through which it has swung is 9 0 o + 9 1 . 3 o = 1 8 1 . 3 o which we can round off to nearest integer.
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The system of putty and balls gets some initial angular velocity ω , found by conserving angular momentum about pivot.The moment of inertia of the system about pivot is 4 2 M + m l 2 , where M is mass of ball, m is that of putty and l is length of rod
m v 2 d = ( 4 2 m + M l 2 ω ) .... (i)
Now, right ball and putty will come down and then rise to an angle θ above horizontal.
Using conservation of energy,
m g 2 l sin θ = 2 1 4 2 m + M l 2 ω 2 ... (ii)
From (i) and (ii), we get
θ ≈ sin θ = ( 2 M + m ) l g m v 2 in radians
Multiply by π 1 8 0 , and put values to get θ ≈ 1 . 3 ∘
So, it covers 1 8 0 + 1 . 3 = 1 8 0 . 3 ∘