Rotational Kinetic energy
Rotational Kinetic Energy
Rotational kinetic energy is the kinetic energy due to the rotation of an object about a fixed axis and
it is part of the the object’s total kinetic energy.
Requirement: ω must be in rad/s
Unit: Joules
It can be useful to use
v
v = ωr or ω = r
Rotational Kinetic Energy
• When to use rotational kinetic energy
• You are told an object is rolling
• You are given the moment of inertia
• You are asked about angular speed
Example
• Starting from rest, a basketball rolls from the top of a hill to the bottom, reaching a translational speed
of 7.34 m/s. Ignore frictional losses. I = 2/3 MR2
• (a) What is the height of the hill?
• (b) Released from rest at the same height, a can of frozen juice rolls to the bottom of the same hill.
What is the translational speed of the frozen juice can when it reaches the bottom? I = ½ MR2
Example
A projectile launcher uses a spring with a spring constant of 500 N/m, to shoot a small solid, round ball of mass 100 grams and radius 2 cm across
a room. The spring in the launcher is compressed 10 cm from its equilibrium position. The launcher is located on a level table. The table is 1.3 m
in height. Rotational inertia for a solid sphere I = 2/5mr2
(a)With what speed does the ball leave the end of the table?
(b) How long does it take the ball to reach the floor after leaving the table?
(c) Where on the floor does the projectile land?
(d) If the mass of the ball is increased, how would the landing distance be effected? Increase, decrease, stay the same. Justify your answer.
Rotational Kinetic Energy of a skater
• When a figure skater moves her arms in closer to her body while
spinning, what happens to the skater’s speed?
• Increases, Decreases, stays the same
• How does this affect her rotational kinetic energy?
• Increases, decreases, stays the same
• Where does the extra kinetic energy come from?
Rotational Kinetic Energy
• Find the velocity of the following system (m1>m2) after it has
moved through some height h. The pulley has mass M, radius R and
I = MR2