Rod and particle

Consider a rod of mass M M and length L L is free to rotate about an horizontal axis O passing through its center. A particle of mass m falls from point P which is at a height h from Q and collides elastically with rod at a distance L 4 \frac L4 form O . If the rod makes an angle α \alpha with horizontal before collision. Then find the angular velocity of rod just after collision.

Details And Assumptions

  1. The Rod is at rest before collision
  2. M = 2.5 kg
  3. L = 4 m
  4. m = 2 kg
  5. h = 5 m
  6. α = 37 \alpha = {37}^{\circ}
  7. Take g = 10 m / s 2 g= 10 m/s^2


The answer is 5.98.

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3 solutions

Neelesh Vij
Feb 14, 2016

Let v 0 v_0 be the velocity of particle before collision. And ω \omega the angular velocity of road after collision and v x v_x and v y v_y the component of particle just after collision along and perpendicular the rod respectively

From the equation of e e along the line of impact:

e = 1 = l 4 × ω v y 0 4 5 × v 0 e = 1 = - \displaystyle \dfrac{\frac l4 \times \omega - v_y}{ 0 - \frac 45 \times v_0}

l 4 × ω v y = 4 5 × v 0 \rightarrow \frac l4 \times \omega - v_y = \frac 45 \times v_0

Now conserving angular momentum form the point where rod is attached to the axis:

m × 4 v 0 5 × l 4 = m v y × l 4 + I ω m \times \frac {4v_0}{5} \times \dfrac {l}{4} = mv_y \times \frac l4 + I \omega , where I I is the moment of inertia of rod ( I = m l 2 12 ) ( I = \frac{ml^2}{12})

Now substituting the value of v y v_y we get ω = 6 \omega = \boxed{6}

Problem is nice but shouldn't it be specified to take g as 10 because i took 9.8 initially and got wrong answer

Prakhar Bindal - 5 years, 4 months ago

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Sorry for trouble, i have mentioned it

neelesh vij - 5 years, 4 months ago

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Thanks ! also i had a strong feeling from inside to take g as 9.8 because in answer box it was written that decimals ok :)

Prakhar Bindal - 5 years, 4 months ago

What is the equation of e

Austin Joseph - 5 years, 1 month ago

Can u explain the physical meaning of the e equation and how u got there

Austin Joseph - 5 years, 1 month ago

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The equation of e the the equation for coefficient of restitution (e) = velocity of separation of objects velocity of approach of objects \dfrac{\text{velocity of separation of objects}}{\text{velocity of approach of objects}}

neelesh vij - 5 years, 1 month ago

how did you get initial angular momentum = m 4 v 5 m * \frac{4v}{5}

space sizzlers - 4 years, 8 months ago

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sorry its just a typo it should be m × 4 v 5 × l 4 m \times \dfrac{4v}{5} \times \dfrac l4 .

It is because we need to take component of velocity perpendicular to rod to calculate initial angular momentum.

neelesh vij - 4 years, 8 months ago

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Great solution, are you in fiitjee south delhi

space sizzlers - 4 years, 8 months ago

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@Space Sizzlers no. fiitjee kanpur

neelesh vij - 4 years, 8 months ago

i first did by a different method but ended with a wrong answer ,

i'll be glad if u tell my mistake ,first i found the velocity of particle in direction perpendicular to the rod let it be v' ,

v be the initial perpendicular velocity of the particle which is ( 2 g h ) \sqrt (2gh) cos 37 \cos 37 the net impulse on rod is m(v+v')

Now using angular impulse - momentum theorem M l 2 12 \frac{Ml^{2}}{12} angular velocity = m(v+v') l/4

after solving i get 5.3333

can u tell what is wrong in my approach ?

i firstly didn't feel like conserving momentum as impulse is acting at the point of contact and hinge will also apply an impulse.

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Look at my comment...

Mark Hennings - 4 years, 1 month ago

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which comment sir ?

A Former Brilliant Member - 4 years, 1 month ago

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@A Former Brilliant Member Sorry, report.

Mark Hennings - 4 years, 1 month ago

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@Mark Hennings When you are using impulses, you are conserving angular momentum, presuming that the particle has perpendicular speed v v' after the collision. This is right. However, you never say how you calculate v v' , so I don't know how the second half of your argument goes...

Mark Hennings - 4 years, 1 month ago

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@Mark Hennings v' is calculated by usual conservative of momentum and elasticity condition .

A Former Brilliant Member - 4 years, 1 month ago
Aryan Goyat
Feb 13, 2016

Just conserve angular momentum and put the condition of elasticity.

Can you please post the full solution.Thanks

neelesh vij - 5 years, 4 months ago

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By conservation of energy and angular momentum I am getting ans as 3. Please explain how to use condition of elasticity. Also I am not able to see the correct ans even after clicking on veiw solution. So pl tell the correct ams

Avadhoot Sinkar - 5 years, 4 months ago

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