HELP - ROTATIONAL INERTIA (no numbers given)

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SUMMARY

The discussion centers on the comparison of mass and rotational inertia between a child's bowling ball and an adult's bowling ball, given that the child's ball has half the radius. The mass of the child's ball is reduced by a factor of 8, while its rotational inertia is reduced by a factor of 32. The relevant equations used include the moment of inertia formula I = MR² and the specific case for a sphere I = (2/5)MR². The relationship between density, mass, and volume is crucial for understanding these reductions.

PREREQUISITES
  • Understanding of rotational inertia and its calculation using I = MR²
  • Knowledge of the properties of spheres, including volume and density
  • Familiarity with basic algebraic manipulation of equations
  • Concept of mass density and its implications in physical problems
NEXT STEPS
  • Study the derivation of the moment of inertia for different shapes, particularly spheres
  • Learn how to calculate the volume of a sphere and its impact on mass
  • Explore the relationship between density, mass, and volume in physics
  • Investigate the implications of scaling objects in physics, particularly in rotational dynamics
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Students studying physics, particularly those focusing on mechanics and rotational dynamics, as well as educators looking for examples of mass and inertia calculations.

Quarkn
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Homework Statement


A bowling ball made for a child has half the radius of an adult bowling ball. They are made of the same material (and therefore have the same mass per unit volume). By what factor is a) mass and b) rotational inertia if the child's ball reduced compared with the adult ball?


Homework Equations


I=MR²
(sphere) I=(2/5)MR²


The Attempt at a Solution


I only got to: R(adult) = (1/2)R (child).

PS. the answers to a) reduced by a factor of 8 and b)reduced by a factor of 32
 
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Set the two densities equal to each other, and then examine how the masses vary (since the radius of the child's bowling ball differs from the adult's). Based on this, you should have enough information to determine its moment of inertia.
 
physicsvalk said:
Set the two densities equal to each other, and then examine how the masses vary (since the radius of the child's bowling ball differs from the adult's). Based on this, you should have enough information to determine its moment of inertia.

So would you find the volume of each spheres first?

And if i set the two densities equal, wouldn't the masses just cancel?

ex. D=M/V, M/v=M/V ?
 
Yes, you would need to find the volumes.

The densities are said to be equal and the volumes differ, therefore, the masses can't be the same.
 

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