Friction •
The force that one surface exerts on another
when the two rub against each other is called friction.
•FRICTION
OPPOSES MOTION.•
The
strength of friction depends on two factors:
–1. the type
of surfaces involved–
2. how hard the surfaces push together.
The Usefulness of Friction
•Without
friction, we would not be able to walk!
•We would only slip
when we try to move.
•In fact, we often try to increase friction.
–Tire chains in winter for better traction on ice.
–Basketball shoes for quicker stops and starts.
Types of Friction
•Sliding
friction occurs when solid surfaces slide over each other.
•Rolling
friction occurs when an object rolls over a surface.
–The force needed to overcome rolling friction is much less than that of sliding friction.
•Fluid friction occurs when an object moves through a liquid or gas.
–The force needed to overcome fluid friction is usually less than that needed of sliding
friction.
Lubricants replace sliding friction with fluid friction.
Gravity
•Gravity
is the force that pulls objects toward each other.
•Free
fall occurs when the only force acting upon a falling object is gravity.
Air Resistance
•Objects falling
through air experience a type of fluid friction called air resistance.
•Air
resistance is not the same for all objects.
–The greater
the surface area of an object, the greater the air resistance.
•Air
resistance increases with velocity.
•Eventually, the air
resistance equals the force of gravity (balanced forces).
–When
these forces are balanced, there can be no acceleration.
–Even
though the object continues to fall, its velocity no longer increases.
–This
velocity, the greatest velocity the object reaches is called terminal velocity.
•In a vacuum, where there is no air, all objects fall with exactly the same rate of acceleration
Projectile Motion
•An object that is thrown or launched is called a projectile
•When a projectile is launched horizontally, it is in free fall.
Weight•
Since weight is a force,
we can rewrite Newton’s second law of motion.
Weight = Mass X Acceleration due to gravity
•Weight is usually measured in newtons, mass in kg, and
acceleration due to gravity in m/s2.
Universal Gravitation
•The law of universal gravitation states that the force of gravity acts between all objects
in the universe.
–Any two objects in the universe, without
exception, attract each other.
–Attraction between other
objects is not noticeable because the gravity between us and Earth is much greater than other objects.
–The farther apart objects become, the weaker the gravitational force becomes.
The force of
attraction between two objects varies with mass and distance.If mass is
doubled, force increases.If distance is doubled, force decreases.
If distance is halved, force increases.
