Solid surfaces exert a force, called a support force, on objects pressed against them.
Downward force (weight) balanced by upward force (support).
How much is the
net force on the brick?
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.
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:
Support of the floor (Upward)
Frictional force of the floor (Horizontal)
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.
Object is stable if CG is above the base.
Demo: Find your Center
Your CG is
roughly in the
center of your
Demo: Balanced Bird
Where is the bird’s center of gravity?
Center of Gravity
located above the
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.
Base of Support with a Partner
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?