Handle the putty wad with care!

The 2.00 k g 2.00 kg balls are attached to the ends of a thin rod of length 50.0 c m 50.0 cm 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 50.0 g 50.0 g wad of wet putty drops onto one of the balls, hitting it with a speed of 3.00 m / s 3.00 m/s and then sticking to it. Through what angle (in degrees) will the system rotate before it momentarily stops?


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The answer is 181.

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2 solutions

Discussions for this problem are now closed

Jatin Yadav
Apr 29, 2014

The system of putty and balls gets some initial angular velocity ω \omega , found by conserving angular momentum about pivot.The moment of inertia of the system about pivot is 2 M + m 4 l 2 \dfrac{2M+m}{4} l^2 , where M M is mass of ball, m m is that of putty and l l is length of rod

m v d 2 = ( 2 m + M 4 l 2 ω ) mv \dfrac{d}{2} = \bigg(\dfrac{2m+M}{4} l^2 \omega\bigg) .... (i)

Now, right ball and putty will come down and then rise to an angle θ \theta above horizontal.

Using conservation of energy,

m g l 2 sin θ = 1 2 2 m + M 4 l 2 ω 2 mg \dfrac{l}{2} \sin \theta = \dfrac{1}{2} \dfrac{2m+M}{4} l^2 \omega^2 ... (ii)

From (i) and (ii), we get

θ sin θ = m v 2 ( 2 M + m ) l g \theta \approx \sin \theta = \dfrac{mv^2}{(2M+m)lg} in radians

Multiply by 180 π \dfrac{180}{\pi} , and put values to get θ 1. 3 \theta \approx 1.3^{\circ}

So, it covers 180 + 1.3 = 180. 3 180 +1.3 = 180.3^{\circ}

why angular momentum is conserved ????????????

Naman Negi - 7 years, 1 month ago

due to law of conservation of angular mumentum

Suleman Sadiq - 7 years, 1 month ago

For the first instant, there is no net torque on this system. :D

Fan Zhou - 7 years, 1 month ago

yeah i understood that "first instant" thank you

Naman Negi - 7 years, 1 month ago

Hey gravity will cause an external torque on the system about the pivot...so how can you conserve angular momentum?

Kashif Ahmad - 7 years, 1 month ago

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.

Fan Zhou - 7 years, 1 month ago

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 - 7 years, 1 month ago

@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.

Fan Zhou - 7 years, 1 month ago

i think its 1.3 degrees from center of mass

David Andrian - 7 years ago

I think exact ans is 182.6°……

Amit Sharma - 7 years, 1 month ago
Abhishek Singh
Apr 24, 2014

Actually this question has three parts. First ,to calculate the ω \omega 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 0.148 r a d / s e c 0.148 rad/sec and the ratio of kinetics energies is 0.0123 0.0123 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 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 90^{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 \theta 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 d 2 \frac{d}{2} above this point, so its initial potential energy is U i = m g d 2 U_{i}= \frac{mgd}{2} .If it swings up to the angular position θ \theta as measured from its lowest point, then its final height is ( d 2 ) ( 1 cos θ ) \left(\frac{d}{2}\right)(1-\cos\theta ) above the lowest point and its final potential energy is u f = m g × ( d 2 ) ( 1 cos θ ) u_{f} =mg \times \left(\frac{d}{2}\right)(1-\cos\theta ) The initial kinetic energy is the sum of that of the balls and wad: K i = 1 2 I ω 2 = 1 2 ( 2 M + m ) ( d 2 ) 2 ω 2 K_{i} = \frac{1}{2}I\omega^{2}=\frac{1}{2}(2M+m)\left(\frac{d}{2}\right)^{2} \omega^{2} At it’s final position we’ve K f = 0 K_{f}=0 .Conservation of energy provides the relation: m g d 2 + ( 2 M + m ) ( d 2 ) 2 ω 2 = m g × ( d 2 ) ( 1 cos θ ) mg\frac{d}{2}+(2M+m)\left(\frac{d}{2}\right)^{2} \omega^{2}= mg \times \left(\frac{d}{2}\right)(1-\cos\theta ) When this equation is solved for cos θ \cos \theta ,the result is cos θ = 1 2 ( 2 M + m m g ) ( d 2 ) × ω 2 \cos\theta=-\frac{1}{2}\left(\frac{2M+m}{mg}\right)\left(\frac{d}{2}\right) \times \omega^{2} On putting the values in S.I. units we get cos θ = 0.0226 \cos\theta=-0.0226 Consequently, the result for θ \theta is 91. 3 o 91.3^{o} The total angle through which it has swung is 9 0 o + 91. 3 o = 181. 3 o 90^{o}+91.3^{o}=\boxed{181.3^{o}} which we can round off to nearest integer.

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