AIM: What factors affect friction and gravity and why
acceleration during free fall?
OBJ: Given notes and activity sheet SWBAT explain
the factors affecting friction and gravity and why
acceleration during free fall with 70% accuracy.
DN: Distribute Friction and Gravity Packet
ACT: Introduction to Friction (Factors Affecting Friction,
Types) and Gravity (free fall, air resistance, terminal
velocity, projectile motion).
HW: Text, read p. 42-50, Complete Friction and
Gravity Packet; SFFG Exam, April 27
Friction Notes
Friction: the force that two surfaces exert
on each other when they rub against each
other.
Factors Affecting Friction:
1) surfaces pushing together
2) rough versus smooth surfaces
3) lubricants (oils, grease)
Example:
Press (rub) forcefully ~ high friction
Rough surface ~ high friction
Press (rub) lightly ~ low friction
Smooth surface ~ low friction
Friction: a force that acts in a direction opposite to
the direction of the object’s motion. Four (4) types:
Static Friction: friction that acts on objects that
are not moving. Static greater than sliding, rolling,
fluid friction.
Sliding Friction: friction when two surfaces slide
over each other.
Rolling Friction: friction when an object rolls
across a surface. To reduce friction, use wheels or
ball bearings.
Fluid Friction: Friction when an object moves
through a fluid. To reduce friction, use oils &
lubricants; streamline.
most friction
static
sliding
least friction
rolling, fluid
Types of Friction
Fluid
friction
Gravity Notes
Gravity: is an attractive force between
objects containing mass.
Factors Affecting Gravity:
Mass - amount of matter (atoms) in an object
Distance - length between two points
increase mass ~ increase gravity
increase distance ~ decrease gravity
Weight & Mass
Mass: the amount of matter (atoms) in an object.
Weight: the gravitational force exerted on an
object’s mass.
Weight = gravity x mass
So:
more weight(person) = more gravity(earth) x same mass(person)
less weight(person) = less gravity(moon) x same mass(person)
Why? Moon ~ less mass, less gravity
Earth ~ more mass, more gravity (10x Moon)
Gravity & Motion
Free Fall: only gravity acting on a falling object. Objects
accelerate as they free fall because gravity is an unbalanced
force. In a vacuum, all objects in free fall accelerate at the
same rate regardless of their masses. Gravitational
acceleration near Earth is 9.8 m/s2.
Bowling Ball Feather drop (start at 2:35 minutes)
https://www.youtube.com/watch?v=E43-CfukEgs
Air Resistance: is a type of fluid friction as an object
falls through the air. Greater surface area of object ~
increase in air resistance ~ slower the object falls. Air
resistance increases as velocity increases.
Gravity & Motion
Terminal Velocity: maximum velocity of a
free falling object occurs when the force of air
resistance equals the weight of the object.
Terminal velocity represents balanced forces
where upward air resistance equals downward
gravity.
Projectile Motion:
Projectile: an object that is thrown. A projectile
(with horizontal motion) will fall at the same rate as
any dropped object.