Physics 101: Lecture 13 - Rotational Kinetic Energy and Inertia, Study notes of Physics

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Physics 101: Lecture 13, Pg 1

Physics 101: Lecture 13

Rotational Kinetic Energy

and Inertia

z

Today’s lecture will cover Textbook Chapter 8.1-8.2with a review of circular motion from chapter 5

Exam II

Physics 101: Lecture 13, Pg 2

Overview of Semester

z

Newton’s Laws

Î

Σ

F = m a

z

Work-Energy

Î

Σ

F = m a

multiply both sides by d

Î

Σ

W =

∆

KE

Energy is “conserved”

Î

Useful when know Work done by forces

z

Impulse-Momentum

Î

Σ

F = m a

multiply both sides by

∆

t

Î

Σ

I =

∆

p

Momentum is “conserved”

Î

Useful when know about EXTERNAL forces Î

Works in each direction independently

Physics 101: Lecture 13, Pg 4

Energy ACT

z

When the bucket reaches thebottom, it’s potential energyhas decreased by an amountmgh. Where has this energygone?

1. Kinetic Energy of bucket2) Kinetic Energy of flywheel3) Both 1 and 2.

Physics 101: Lecture 13, Pg 5

Rotational Inertia I

z

Tells how much “work” is required to getobject spinning. Just like mass tells you howmuch “work” is required to get object moving.

Î

K

tran

= ½ m v

2

Linear Motion

Î

K

rot

= ½ I

ω

2

Rotational Motion

z

I =

m

ri^

(^2) i

(units kg m

z

Note! Rotational Inertia depends on what youare spinning about (basically the r

i^

in the

equation).

Physics 101: Lecture 13, Pg 7

Inertia Rods

Two batons have equal mass and length.

Physics 101: Lecture 13, Pg 8

Rolling Race

(Hoop vs Cylinder)

A hoop and a cylinder of equal mass roll down a

ramp with height h. Which has greatest KE atbottom? A) Hoop

B) Same

C) Cylinder

Physics 101: Lecture 13, Pg 10

Main Ideas

z

Rotating objects have kinetic energy

Î

KE = ½ I

ω

2

z

Moment of Inertia I =

mr

2

Î

Depends on Mass Î

Depends on axis of rotation

z

Energy is conserved

Physics 101: Lecture 13, Pg 11

Massless Pulley Example

Consider the two masses connected by

a pulley as shown. Use conservationof energy to calculate the speed ofthe blocks after m

2

has dropped a

distance h. Assume the pulley ismassless.

Physics 101: Lecture 13, Pg 13

Massive Pulley Act

Consider the two masses connected by a

pulley as shown. If the pulley ismassive, after m2 drops a distance h,the blocks will be moving

1. faster than2) the same speed as3) slower thanif it was a massless pulley

• Physics 101: Lecture 13, Pg
• Physics 101: Lecture 13, Pg