Moment of inertia equation

3 Problems and solutions about Moment of inertia equation

1. A solid cylinder has a radius of 8 cm and a mass of 2 kg. Meanwhile, a solid ball has a radius of 5 cm and a mass of 4 kg. If the two objects rotate with an axis through their center, determine the ratio of the moment of inertia of the cylinder and the ball.

Known:

Solid cylinder radius (r) = 8 cm = 0.08 m

Solid cylinder mass (m) = 2 kg

Solid ball radius (r) = 5 cm = 0.05 m

The mass of the solid ball (m) = 4 kg

Wanted: Comparison of the moment of inertia of a cylinder and a ball

Solution:

The formula for the moment of inertia of a solid cylinder:

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I = ½ M R2 = ½ (2)(0,08)2 = 0,0064

The formula for the moment of inertia of a solid ball:

I = 2/5 M R2 = 1/5 (4)(0,05)2 = (0,8)(0,0025) = 0,002

Comparison of the moment of inertia of the cylinder and the ball:

0,0064 : 0,002

3,2 : 1

2. The tin in which the biscuits were placed was used as a toy. The mass of the can is 200 grams and the radius is 15 cm. The can is rolled on a horizontal floor. If the lid and bottom of the can are neglected, determine the moment of inertia of the can.

Known:

The can resembles a hollow cylinder.

Mass (m) = 200 grams = 0.2 kg

Radius (r) = 15 cm = 0.15 m

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Wanted:

Solution:

The formula for the moment of inertia of a hollow cylinder through the axis:

I = M R2 = (0,2)(0,15)2 = (0,2)(0,0225) = 0,0045 kg m2

3. A disc which is free to rotate about a vertical axis is capable of rotating at a speed of 80 revolutions per minute. If a small object with a mass of 4 x 10-2 kg is attached to a disc 5 cm from the axis, it turns out that the rotation is 60 revolutions per minute, then determine the moment of inertia of the disc.

Known:

Initial angular velocity (ωo) = 80 put / 60 s = 4/3 put/s

Initial moment of inertia (Io) = I

Final angular velocity (ωt) = 60 put / 60 s = 1 put/s

Final moment of inertia (It) = I + m r2 = I + (0.04)(0.05)2 = I + (0.04)(0.0025) = I + 0.0425

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Wanted: moment of inertia of the Disc

Solution:

The law of conservation of angular momentum

I ωo = I ωt

I (4/3) = I + (0,0425 (1))

4 I / 3 = I + 0,0425

4 I = 3 I + 0,1275

4 I – 3 I = 0,1275

I = 0,1275 kg m2

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