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Two spheres of the same mass are spinning in your garage. The first is 12cm in diameter and made of solid plastic. The second is 10 cm in diameter but is a thin sphere shell of iron filled with aire. Which is harder to slowdown? Why?

Sagot :

The solid plastic sphere is harder to slow down compared to the thin iron sphere filled with air. This is because of the distribution of mass within each sphere and how it affects their moments of inertia.

Inertia is the resistance of an object to changes in its state of motion. The moment of inertia depends not only on the mass of the object but also on how the mass is distributed relative to the axis of rotation. For a rotating sphere, the moment of inertia increases with the radius squared.

The solid plastic sphere has more mass concentrated towards its center due to its solid nature, resulting in a smaller moment of inertia compared to the thin iron shell filled with air, which has its mass spread out towards the surface. Since the moment of inertia is smaller for the solid plastic sphere, it requires more force to slow down its rotation compared to the thin iron sphere with the same mass.

Additionally, the air inside the thin iron shell adds to the moment of inertia of the second sphere, making it even easier to slow down its rotation compared to the solid plastic sphere.

In conclusion, the solid plastic sphere is harder to slow down due to its lower moment of inertia resulting from its mass being more concentrated towards the center, while the thin iron sphere filled with air has a larger moment of inertia due to the spread-out mass within the shell, making it easier to slow down its rotation.