Homework Help: Angular momentum of a rod

1. Feb 2, 2010

Shahid0072

A rod AB having mass M ,pivoted at mid point at 0 is in horizontal..An insect having same mass M Fall vertically on rod at mid point C between 0 and B with speed V..It starts moving towards end B such that angular velocity of rod remains constant ..If the insect reached at B when rod has rotated to an angle of 90degree..Calculate V...How can we calculate time insect take to cover from C to B?? How moving of insect towards end B made angular velocity of rod constant?

2. Feb 2, 2010

tiny-tim

Welcome to PF!

Hi Shahid0072! Welcome to PF!

Once the insect lands on the rod, the total angular momentum will be constant (because then there is no external torque).

if the insect moves towards B, that will increase the moment of inertia (which will tend to slow the rod, exactly as a skater slows when he extends his arms).

So calculate the moment of inertia when the insect is at distance r from O.

3. Feb 3, 2010

Shahid0072

Thank you tim..Yeah..Great..When insect will move from C to B,moment of inertia of system will increase which will tend to descrease their angular velocity..But what abt the torque due to weight of insect Of mass m..Won't it be increasing the angular speed of rod..But torque will also be changing as insect will move to B...Will both effects cancel each other,thus maintaining angular velocity of rod constant

4. Feb 3, 2010

tiny-tim

Yup! That's why we have to have specially trained insects!

You need to teach yours the following formulas:

τ = d/dt(Iω) = Idω/dt + ωdI/dt

dω/dt = 0

5. Feb 3, 2010

Shahid0072

Sir..In this question we have to calcute V..Velocity with wh4ch insect falls on rod...For that we ha ve to calculate angular velocity using the fact the insect reached the mid way between 0(pivot point)to end B..During the time rod rotated by pie/2 radians...So if we calculate the time insect takes to reach from C to end B..That time is equal to time rod takes to rotate by constant angular velocity...So dividing the time by pie/2 will give angular velocity and eventually speed by which insect falls on rod..But how to calculate time??

6. Feb 3, 2010

tiny-tim

(don't call me sir! … i'm only a little goldfish! )

No, you calculate the intial angular velocity by conservation of angular momentum …

exactly the same way as you'd use conservation of ordinary momentum for an ordinary collision.

7. Feb 3, 2010

Shahid0072

Alryt goldfish..I know how to apply conservation of angular momentum in this case..Problem is that we are given velocity of insect with which it falls as V..We have to calculate V IN Numerals..We are given length of rod(L) =1.8 m...And mass M SAME as of insect..Read the question again...After applying coam..We get omega in terms of V and L..But we just know that time taken by insect to reach end b is equal to time taken by rod to rotate by 90 degree..Can u calculate the time,tim?

8. Feb 3, 2010

tiny-tim

start writing out all the equations, and see where you get.

9. Feb 3, 2010

Ketman

I arrived on this forum as a questioner, not a helper, but even I know that there is an external torque acting on this system tending to increase the velocity while the action of the insect tend to decrease it. It isn't a simple problem by any means.

10. Feb 3, 2010

tiny-tim

Welcome to PF!

Hi Ketman! Welcome to PF!
And that's exactly why it's best to write out all the equations first, and not try to work out some energy-saving short-cut.

11. Feb 3, 2010

Ketman

Thanks for the welcome.

Returning to the problem, is there an external torque or not? I don't think the questioner can begin writing out his equations until he knows the answer to that.

12. Feb 3, 2010

tiny-tim

Hi Ketman!

Yes, that's the "τ" in "τ = d/dt(Iω) = Idω/dt + ωdI/dt"

13. Feb 3, 2010

Ketman

Yep. It's just that it conflicts with your first reply to him, which says no external torque.

I hope my posts don't seem too much like butting in, but I'm trying to get back into practice with theoretical mechanics after many years away from the subject. And one way is to look at other people's problems and have a go at solving them, which is why I'm interested in this one. In the meantime I hope someone will have a go at the medieval catapult problem I posted, which is certainly foxing me.

Shahid - why not start with an equation for how much angular momentum there is after the insect lands, even if it's only variables to begin with? At least post one equation. You know, one step at a time...

14. Feb 3, 2010

Shahid0072

Thanks for helping you two.. Tim,don't get me wrong ..When i asked you to calculate time,i meant the approach to calculate it.Ketman,u r ryt..It ain't easy..Before attempting ny question, i try to solve it in mind first..First we apply angular momentum conservation to get angular velocity in terms of v and l..Now we need time taken by insect to reach end B..how can we calculate that..Tim,your chain rule formula is ryt but torque is variable in this case ,its increasing as insect is going towards end B,simultneously its decreasing as it rod is rotating..First the rod is horizontal it makes angle0..When its vertical it makes pie/2..Torque varies as Mgx cos§ where x is distance from rod's centre 0..And § is angle rod makes with horizontal..Tell me please...Angular velocity of rod is constant but angular momentum is changing..As insect is moving away,MOI changes...

15. Feb 3, 2010

Shahid0072

Alryt first we as insect land at C( mid point between 0(centre ) and end B) with velocity V..We apply conservation of angular momentum we get omega=12 V /7L where L=1.8 metres given..Now we have to find V in nos...First lets denote omega (angular velocity) by¥..We have V=7L¥/12 ..We need to calculate ¥ using the fact time taken by rod to rotate 90(from horizontal position to vertical) is equal to time taken by insect to reach from C TO B...If we could calculate time,we can divide it by pie/2 to get omega ..

16. Feb 3, 2010

Staff: Mentor

Perhaps I'm missing something, but I don't see how the angular velocity of the rod can remain constant. There's a torque on the system due to the insect's weight, thus the system will have an angular acceleration.

17. Feb 3, 2010

Ketman

It'll have angular acceleration due to gravity, but angular deceleration due to the increased moment of inertia as the insect travels away from the axis. If they cancel each other out, the angular velocity will stay constant.

18. Feb 3, 2010

Ketman

12?.... 7?.... Where did those numbers come from?

Edit: okay I get the 12, but still don't know about 7.

Last edited: Feb 3, 2010
19. Feb 3, 2010

Staff: Mentor

Right.
I don't know what you mean. The rotational inertia will increase as the insect moves along, which (by itself) acts to reduce the angular acceleration.

20. Feb 3, 2010

Shahid0072

Ketman..Applying conservation of angular momentum about rod centre..We have(i am using LL INSTEAD OF L SQUARE
MVL/4=(MLL/12 +MLL/16)¥