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

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, 19.62 m/s, 29.43 m/s] Actual falling objects experience air resistance that eventually equals the force of gravity – and the object reaches its terminal velocity

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…

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

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