Question

Consider four objects, each having the same mass and the same radius:

1. a solid sphere
2. a hollow sphere
3. a flat disk in the x,y plane
4. a hoop in the x,y plane
The order of increasing rotational inertia about an axis through the center of mass and parallel to the z axis is:

Answer 1

1. Solid Sphere: 0.4 kg m²

2. Flat Disk in x,y plane: 0.5 kg m²

3. A Hollow Sphere: 0.67 kg m²

4. Hoop in x,y plane: 1 kg m²

Step-by-step explanation:

The formulae for the rotation inertia of given shapes about z-axis are given as follows:

1. Solid Sphere: (2/5)(MR²)

2. A Hollow Sphere: (2/3)(MR²)

3. Flat Disk in x,y plane = (1/2)(MR²)

4. Hoop in the x,y plane = MR²

where,

M = mass

R = Radius

Since, it is stated that all shapes have same mass and radius. Therefore, we assume a mass of 1 kg and radius of 1 m for each shape. Hence values become:

1. Solid Sphere: (2/5)(1 kg)(1 m)²

2. A Hollow Sphere: (2/3)(1 kg)(1 m)²

3. Flat Disk in x,y plane = (1/2)(1 kg)(1 m)²

4. Hoop in the x,y plane = (1 kg)(1 m)²

Solving the values, we get:

1. Solid Sphere: 0.4 kg m²

2. A Hollow Sphere: 0.67 kg m²

3. Flat Disk in x,y plane = 0.5 kg m²

4. Hoop in the x,y plane = 1 kg m²

Therefore, it is clear from above calculations, that the order of increasing of rotational inertia of these shapes will be as follows:

1. Solid Sphere: 0.4 kg m²

2. Flat Disk in x,y plane: 0.5 kg m²

3. A Hollow Sphere: 0.67 kg m²

4. Hoop in x,y plane: 1 kg m²