Circular motion (planetary orbit) question

In summary, the problem involves a spinning turntable with a mass of 2kg and a radius of 0.1 meters, spinning at 100 rpms. Two blocks of mass 0.5kg each fall simultaneously on opposite ends of a diagonal and stick together. The question is asking for the new angular velocity of the turntable after the blocks stick. Using the equations for angular momentum and conservation of linear momentum, the final angular velocity can be calculated. This approach is correct and is based on the principle of conservation of angular momentum.
  • #1
theneedtoknow
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Homework Statement



A turntable is spinning at 100 rpms. It has a mass of 2kg and a radius of 0.1 meters. 2 blocks of mass 0.5kg fall simultaneously on 2 opposite ends of a diagonal and manage to stick. What is the new angular velocity of the turntable after the blocks stick?

Homework Equations



Moment of inertia of a spinning disk is 0.25 M R^2

The Attempt at a Solution



Well I know Linear momentum of the ntire system is supposed ot be conserved
Initially its just the table spinning... Linear momentum is L = Iw where I is the moment of inertia and w is the angular velocity.

So Linitial = 0.25 x (2kg) x (0.1m) ^ 2 x winitial
and since it does 100 rpm we can multiply it by 2Pi rads / revolution and again by 1 min / 60 sec and i get 10Pi/3 rad/s
so my L initial is Pi/6

Now since L is conserved, when the 2 blocks fall, the new L should be the same as L initial
the New L will be a combination of the angular mometum of the turntable and of the 2 blocks

so L final = Iwfinal + the linear momentum of the 2 blocks
treating them as point particles of mass 0.5 kg , 0.1m away from the axis of rotation, their individual momentum can be obtained ith the formula L = r x p = r x mv... and v is just wr so it can be simplified to L = wmr^2.. and the w is constrained to be equal to the final w of the turntable

so Lfinal = Ix wfinal + 2mr^2 x wfinal = Pi / 6
(since one is at distance r and the other at -r, but they get squared so it doesn't matter)

From here, I can solve for wfinal and that should give me the angular velocity of the turntable at the end.

I kind of had a question but i think I might have worked it out while typing this up to ask it... but just in case...is this a correct way of approaching these kinds of questions? I've never done anything on angular momentum before and was wondering if this is correct?

BTW don't ask what this has to do with planetary motion c ause it doesn't LOL
 
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  • #2
Your approach looks correct.

Since Angular Momentum is conserved then

I1ω1 = I2ω2

Where ω = 2πf
 
  • #3


Your approach to solving this problem is correct. You correctly used the conservation of angular momentum to determine the final angular velocity of the turntable after the blocks stick. The moment of inertia and the linear momentum of the blocks were also correctly calculated. This type of problem is a good way to practice using conservation laws in physics. Keep up the good work!
 

FAQ: Circular motion (planetary orbit) question

What is circular motion?

Circular motion is the motion of an object around a fixed point or axis, where the distance from the object to the fixed point remains constant. In the case of planetary orbit, it refers to the motion of a planet around the sun in a circular path.

Why do planets orbit the sun in a circular path?

The circular motion of planets around the sun is a result of the gravitational force between the two bodies. This force pulls the planet towards the sun, but the planet's tangential velocity (the speed at which it is moving perpendicular to the force) keeps it in a circular orbit.

How is the speed of a planet in circular orbit determined?

The speed of a planet in circular orbit is determined by its distance from the sun and the force of gravity between the two bodies. The farther a planet is from the sun, the slower its speed, and vice versa.

What is the difference between a circular orbit and an elliptical orbit?

A circular orbit is a perfectly round path around a fixed point, while an elliptical orbit is an oval-shaped path. In an elliptical orbit, the distance between the planet and the sun varies, resulting in different speeds at different points along the orbit.

Can a planet's orbit change from circular to elliptical?

Yes, a planet's orbit can change from circular to elliptical due to the influence of other planets or objects in the solar system. These external forces can cause the planet's orbit to become more elongated, resulting in an elliptical orbit instead of a circular one.

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