SECOND POST

ANGULAR  MOMENTUM

Question 1 :

Why angular velocity is increases  when the skier do their skates?

Answer :

During ice skating, the skier opens her arm to start a rotation. When she folds her arms, she will rotate faster. Her mass does not change but the moment of inertia is reduced due to her arms closing and thus, the total moment of inertia becomes smaller. By conservation of angular momentum, her angular velocity will increase.

Question 2 :

A uniform rod of length 40 cm is freely pivoted at its centre. A wad of gum of mass 30g moving at a speed of 6 m s^-1 strikes and sticks to one of the end rod. The rod rotates in a horizontal circle. The moment of inertia of the rod and the gum about the pivot is 0.12 kg m².

a)      Define the SI unit of the angular momentum and calculate the angular momentum of the gum.

b)      What is the initial angular velocity of the rod ?

Answer :

Explanation :

The angular momentum , L = (mv)r = mvr

a)      The SI unit of the angular momentum is kg m²s^-1

L  =  mvr

   = (30 x 10^-3) (16) (40 x 10^-2)

  = 0.03 (16) (0.4)

   = 0.192  kg m²s^-1

b)              L = Iω

0.192 = 0.12 ω

ω                 = 0.192 / 0.12

                   = 1.6 rad s^-1