2. (a) A disk with a rotational inertia of 5.4 kg.m 2 is rotating at 4.5 radians

Question

2. (a) A disk with a rotational inertia of 5.4 kg.m2 is rotating at 4.5 radians/sec. Calculate the rotational KE of the disk in Joules, to 2 sf.

(b) Two point masses are rotating about a common axis, with an angular speed of 2.9 radians/s. One mass is 2.6 kg and is 0.18 metres from the axis, the other is 2.8 kg and is 0.14 metres from the axis. Calculate the rotational kinetic energy of the system in Joules, to 2 sf.

(c) A moving object A of mass 5.1 kg and speed 1.7 m/s collides with a stationary object B of mass 2.2 kg. The collision is totally inelastic. Calculate the speed of the combined object after collision.

Explanation / Answer

2 (a) Rotational KE = ½ * Moment of Inertia * angular velocity^2

Rotational KE = 0.5 x 5.4 x 4.5^2 = 54.675 Joule

(b)  I for each mass can be described as I=mr^2. for the system, sum all

so I of the system = 2.6 x 0.18^2 + 2.8 x 0.14^2 = 0.13912

So Rotational KE of the system = ½ * Moment of Inertia * angular velocity^2 = 0.5 x 0.13912 x 2.9^2 = 0.585 Joule

Part c is a different question, please type it seperately. Thanks.

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