1. The Effects of Gravity
Gravity exerts a force on all objects that is proportional to each object’s mass
We call the force that gravity exerts on an object the “weight” of the object [weight is a force!]
The proportionality constant between the force of gravity (weight) and mass is “the acceleration of gravity” [remember F = m x a]
We use “g” to represent the acceleration of gravity g = 9.81 m/s2
We use the equation: Fg = m x g

2. Falling Objects
Because all objects are subject to a downward force due to gravity, objects that are not supported in some way accelerate downward – that is, they fall
Objects that fall in the absence of air resistance are said to be in “free-fall” and they accelerate at 9.81 m/s2 [9.81 m/s, m/s, m/s]
Actual falling objects experience air resistance that eventually equals the force of gravity – and the object reaches its terminal velocity

3. Friction in Fluids
Objects moving through fluids (gases & liquids), experience a resisting force called “fluid friction”
Fluid friction results from the work that must be done to move the fluid out of the path of motion
The amount of fluid friction depends on the viscosity of the fluid
extremely low viscosity – gases
low viscosity – water
medium viscosity – honey, motor oil
high viscosity – molasses in January…

4. Friction on Surfaces
Objects on surfaces experience varying amounts of frictional force depending on several factors
rougher surfaces exert greater friction
more massive objects experience greater friction (friction is proportional to weight)
the condition of motion
static friction – when the object is not moving but is experiencing an applied force
sliding friction – when the object is sliding across the surface

5. Friction on Surfaces – cont’d
rolling friction – when the object is rolling across the surface
For any particular object moving on a surface of any particular roughness,
the static friction (the force required to initiate movement) has the greatest magnitude
the sliding friction (the force required to maintain a sliding motion) is next greatest
the rolling friction (the force required to maintain a rolling motion) is the smallest