# Upcoming ClassesPage 3

#### WATCH ALL SLIDES

Solid surfaces exert a force, called a support force, on objects pressed against them.

100 Newton

Gold Brick

100 Newton

Support force

Downward force (weight) balanced by upward force (support).

How much is the

net force on the brick?

Slide 19

## Friction Force

Origin of friction is molecular interaction between solid surfaces.

Friction is complicated.

Friction depends on support force and on properties of the surface.

Basic properties of friction first established by Leonardo da Vinci.

Slide 20

## Forces on a Dancer

Newton’s First Law of Motion states that a stationary object (not moving) remains stationary if the net force on it is zero.

For a dancer, the three main forces are:

Gravity (Downward)

Support of the floor (Upward)

Frictional force of the floor (Horizontal)

Slide 21

## Center of Gravity

Average position of an object’s weight distribution is called the center of gravity (CG).

Force of gravity acts on an object as if pulling straight down at the CG.

Center

of

Gravity

Slide 22

## Stability & Balance

Object is stable if CG is above the base.

CG

CG

Weight

Weight

BASE

Axis

Axis

BASE

STABLE

UNSTABLE

Support

Support

Slide 23

roughly in the

center of your

body.

Varies with

orientation.

CG

CG

STABLE

UNSTABLE

Slide 24

Demo: Balanced Bird

Where is the bird’s center of gravity?

Slide 25

## Balance & Dance

Center of Gravity

located above the

dancer’s toe

The force of gravity and the support force of the floor can balance only when a dancer’s center of gravity is located above the base of support.

Slide 26

Base of Support with a Partner

x

x

Center of

Gravity

Two dancers, together, can form a larger base of support.

The center of gravity is roughly located in between the two dancers, as shown.

What happens if he shifts his front foot back?

What if he shifts his back foot forward?

Slide 27

## Ballet Barre

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