# Conservation of Angular Momentum of Bug Problem

• tylertwh
In summary: The angular momentum of the system is conserved.HencemvL = ML2/3*ωSolve for ωω= 3v/(2L)In summary, the bug has a mass of 13.0g and stands at one end of a thin uniform bar with a mass of 65.0g and a length of 120cm, which is initially at rest on a smooth horizontal table. The bar pivots about a nail driven into the table and can rotate freely, without friction. The bug jumps off in the horizontal direction, perpendicular to the bar, with a speed of 25.0cm/s relative to the table. Using conservation of angular momentum, the angular velocity of the
tylertwh

## Homework Statement

A small 13.0g bug stands at one end of a thin uniform bar that is initially at rest on a smooth horizontal table. The other end of the bar pivots about a nail driven into the table and can rotate freely, without friction. The bar has mass 65.0g and is 120cm in length. The bug jumps off in the horizontal direction, perpendicular to the bar, with a speed of 25.0cm/s relative to the table.

mass bug = 0.013 kg
mass bar = 0.065 kg
distance from axis of rotation = 0.12 m
velocity bug = 0.025 m/s

Find
What is the angular speed of the bar just after the frisky insect leaps?

## Homework Equations

L = Iω
I(bug) = Mr^2
ω(bug) = v/r
L(bug) = Iω = Mr^2ω = Mrv
I(bar) = Mr^2
ω(bar) = v/r
L(bar) = Iω = Mr^2ω

## The Attempt at a Solution

Conservation of Angular Momentum

L(bug) = L(bar)
M*r*v = M*r^2*ω
0.013*0.12*0.025 = 0.065*0.12^2*ω
3.9*10^5 = 9.36*10^4ω
ω = 0.204

This is the incorrect answer. I am not quite sure where I made my mistake?
All help would be appreciated!

Three problems that I can see.

The first is that the length of the bar needs to be in terms of meters. You've changed 120cm into 0.120 meters which is not right. 120cm=1.20m

The second is that the change is the fly's linear momentum transfers to the bars angular momentum. Find the fly's change in linear momentum (easy) and set it equal to the bar's angular momentum (slightly more work) and then solve for the angular speed of the bar.

The third is that the moment of inertia for a bar which rotates about its end is I=(1/3)ML^2. You forgot the 1/3

See if that helps

Last edited:
ΔLinear Momentum (fly) = ΔAngular Momentum (bar)

mv(final) - mv(initial) = Iω(final) - Iω(initial)
= mr^2ω(final) - mr^2ω(initial)

0.013*0.025 - 0 = 0.065*1.2^2ω(final) - 0
3.25E-4 = 0.0936ω(final)
ω(final) = 0.00347

Does this look better?

I apologize, i just made an edit on my last post. Look at what I wrote about the moment of inertia for the bar.

Also, it's not as straight forward as I thought. You need to figure out the torque that results as the fly jumps off of the rod. It's related to the force that the fly applies to the bar as it jumps off as well at the distance down the bar in which the fly jumped.

I will keep looking at this and get back to you although I'm sure somewhat will read this and correct me before I get another chance.

Last edited:
So when I calculate the torque that the fly applies to the bar how would I integrate it into finding the angular velocity? I am just confused how the torque would be added to the angular velocity

$\tau$ = rFsin(theta)

since here we know that theta = 90...

$\tau$ = rF

would F here be = ma(centripetal) = (mv^2)/r

**All for the bug**

Try conservation of energy. KE of fly = KE of bar

If this doesn't work out I promise to stop trying to help

Alright. No problem.

Useful Equations

KE = 1/2Iω^2

KE(bug) = 1/2Iω^2
KE(bug) = 1/2(mr^2)ω^2
KE(bug) = 1/2(mr^2)(v/r)^2
KE(bug) = 1/2mv^2

KE(bar) = 1/2Iω^2
KE(bar) = 1/2(1/3mr^2)ω^2
KE(bar) = 1/6mr^2ω^2

KE(bug) = KE(bar)
1/2mv^2 = 1/6mr^2ω^2
4.0625E-6 = 0.0156ω^2
ω^2 = 2.6042E-4
ω = 0.01614

how confident are you in this answer...
I am running out of "tries" in my online homework.
(no offense meant; I wouldn't have had any idea of other strategies w/o your help)

I'm pretty darn confident this time. Conservation of energy never fails. The reason we can't use cons of momentum is because you'll need to know the torque applied by the fly which requires us to know the force the fly applied on the bar. And in order to figure out the force we would need to know the amount of time the fly applied the force which we don't have.

The alternative is to wait a little while and see if anyone corrects me or has a better idea.

Well... unfortunately that answer was incorrect also... I am not sure where to go with this, but I did repost this question as a new thread.

I apologize, I'm obviously missing something that's leading me down the wrong path. Let me know how to properly do it when you figure it out.

I think you were correct for the the angular momentum of the bug about the axis of rotation after jump as mvL

The angular angular momentum of the rod after jump will be Iω = ML2/3*ω

Now apply law of conservation of angular momentum.

## 1. What is the "Conservation of Angular Momentum of Bug Problem"?

The "Conservation of Angular Momentum of Bug Problem" is a thought experiment that presents a scenario where a bug crawls along a rotating platform. As the platform rotates, the bug moves towards the edge, but due to conservation of angular momentum, the bug's speed decreases as it gets closer to the edge.

## 2. Why is this problem important in the field of physics?

This problem is important because it demonstrates the principle of conservation of angular momentum, which is a fundamental law of physics. It helps explain the behavior of rotating objects and has implications in various fields such as astrophysics, mechanics, and engineering.

## 3. What factors affect the conservation of angular momentum in this problem?

The main factors that affect the conservation of angular momentum in this problem are the mass and velocity of the bug, the distance from the center of rotation, and the rotational speed of the platform. Additionally, the friction between the bug and the platform can also play a role.

## 4. How does the conservation of angular momentum apply in real-life situations?

The conservation of angular momentum applies in numerous real-life situations, such as the rotation of planets and stars, the movement of satellites, and the behavior of spinning objects like tops and gyroscopes. It also plays a crucial role in the stability of objects in motion and the transfer of energy.

## 5. Are there any limitations to the conservation of angular momentum?

The conservation of angular momentum is a fundamental law of physics and has been proven to hold true in most situations. However, it does not apply in extreme cases such as the collapse of stars or the behavior of subatomic particles. Additionally, external forces or other factors such as friction can cause deviations from the conservation of angular momentum in certain situations.

• Introductory Physics Homework Help
Replies
11
Views
5K
• Introductory Physics Homework Help
Replies
335
Views
9K
• Introductory Physics Homework Help
Replies
8
Views
1K
• Introductory Physics Homework Help
Replies
3
Views
312
• Introductory Physics Homework Help
Replies
4
Views
1K
• Introductory Physics Homework Help
Replies
18
Views
5K
• Introductory Physics Homework Help
Replies
10
Views
1K
• Introductory Physics Homework Help
Replies
18
Views
2K
• Introductory Physics Homework Help
Replies
7
Views
1K
• Introductory Physics Homework Help
Replies
5
Views
2K