Need some help on an angular speed problem

In summary: We'll see if we can get you pointed in the right direction.In summary, the conversation is discussing the use of angular momentum to solve a problem involving a potter's wheel and a lump of clay being dropped on it. The individuals are attempting to use the equation L = Iw and are discussing the need for conservation of momentum. They also mention converting to SI units and determining the moment of inertia for the potter's wheel. The correct approach is to set the initial angular momentum equal to the final angular momentum, as angular momentum is conserved in this situation.
  • #1
math34
11
0

Homework Statement



A potter's wheel, with rotational inertia 6.40 kg * m^2, is spinning freely at 19.0 rpm. The potter drops a 2.80 kg lump of clay onto the wheel, where it sticks a distance of 47.0 cm from the rotation axis.

Homework Equations



I know I need to use angular momentum here, but I am a little confused..

The Attempt at a Solution

conver to SI units:

19 rpm * (2 pi radians/1 revolution) * ( 1 minute / 60 seconds) = 1.99 radians
radius = 0.47 meters
mass = 2.8 kg

I know I need to use angular momentum: L = Iw

and i think this has to do with conservation of momentum but I need some direction.

so the total angular momentum needs to be the sum of the angular momentum before and after the lump is dropped on the wheel,

w(inital)mv^2 + Iw(final)= Iwfirst term being the angular momentum before the drop so we can replace the moment of inertia by mv^2 since it is a disk

second term being after the lump is dropped on the disk

the sum of these is the total angular momentum, is this on the right approach?
something doesn't seem quite right to me..
Thanks
 
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  • #2
math34 said:

The Attempt at a Solution




conver to SI units:

19 rpm * (2 pi radians/1 revolution) * ( 1 minute / 60 seconds) = 1.99 radians
radius = 0.47 meters
mass = 2.8 kg

I know I need to use angular momentum: L = Iw

and i think this has to do with conservation of momentum but I need some direction.

so the total angular momentum needs to be the sum of the angular momentum before and after the lump is dropped on the wheel,

w(inital)mv^2 + Iw(final)= Iw


first term being the angular momentum before the drop so we can replace the moment of inertia by mv^2 since it is a disk
There are 2 problems with that:
1. They tell you the moment of inertia for the potter's wheel. Just use that value for I, there is no need to calculate it.
2. For future reference: the moment of inertia of a disk is (½)mr2. v is irrelevant.

second term being after the lump is dropped on the disk

the sum of these is the total angular momentum, is this on the right approach?
something doesn't seem quite right to me..
Thanks
It sounds like you are adding the initial angular momentum to the final angular momentum, and calling that "the total angular momentum". It doesn't work that way.

Conservation of angular momentum means that it doesn't change between the initial and final situations. So the equation should look more like this:
Linitial = Lfinal
Either side of the equation can be called the total angular momentum.

As a start, go ahead and calculate what Linitial is.
 

1. What is angular speed?

Angular speed, also known as rotational speed, is the rate at which an object rotates around a fixed axis. It is measured in radians per second (rad/s) or degrees per second (deg/s).

2. How is angular speed calculated?

Angular speed can be calculated by dividing the change in angular displacement by the change in time. The formula is: angular speed = change in angular displacement / change in time.

3. What is the difference between angular speed and linear speed?

Angular speed measures how fast an object is rotating, while linear speed measures how fast an object is moving in a straight line. Angular speed is usually measured in rotations per second, while linear speed is usually measured in meters per second.

4. How is angular speed related to angular velocity?

Angular speed and angular velocity are closely related, but they are not the same. Angular velocity is a vector quantity that describes the direction and magnitude of the angular speed. In other words, angular velocity includes information about the direction of rotation, while angular speed does not.

5. How can I use angular speed in real life?

Angular speed is used in many different fields, such as physics, engineering, and astronomy. For example, it is used to calculate the speed of rotating machinery, the rotation of planets and galaxies, and the speed of objects in circular motion, such as a Ferris wheel or a spinning top.

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