Unit 2
Force
Force – push or pull
• a force always acts in a certain
direction
• ex. if you push something, the
force is in the direction of the push
Forces in Nature
• Electric force
• ex. lightning
• Magnetic force
• ex. magnetic poles
• Gravity
Weight is a force
Newton
• metric unit of force
What is gravity?
• Sir Isaac Newton
• 1642 – 1727
• The Principia
Gravity
• force of attraction between all
objects in the universe
Gravity
• the larger an object’s mass, the
greater the gravitational force
Gravity
• the greater the distance
between two objects, the
smaller the gravitational force
• the force of gravity decreases by
the amount equal to one divided by
the distance (d) squared
• 1/d2
What is air resistance?
• air resistance
• force that opposes the movement
of an object in air
Air Resistance
• a larger surface area usually
results in greater air resistance
• lighter objects feel more air
resistance than heavier objects
Terminal Velocity
• speed at which air resistance
and gravity acting on an object
are equal
• as an object falls, it’s speed
increases at a steady rate until it
reaches it’s terminal velocity
Vacuum – empty space
• in a vacuum, all objects fall at
the same speed
What is friction?
• Friction
• force that opposes the motion of
an object
• the force of friction works in the
opposite direction of the force of
motion
Types of Friction
• Sliding Friction
• the source of friction is the
contact between two surfaces, at
least one of which is in motion
Types of Friction
• Air Resistance
• friction results from air pushing on
an object as it is moving
Types of Friction
• Rolling Friction
• friction between two surfaces that
are not in constant contact
• ex. wheels
Friction
• friction makes motion possible
• friction also makes it hard to
move objects
• reducing friction makes it
easier to move objects
How can friction be
reduced?
• by changing sliding friction into
rolling friction
• by using lubricants
• Lubricants – substances that
reduce friction
What is pressure?
• Pressure
• force per unit area
Pressure
• pressure is the amount of force
acting on a surface
• pressure equals force divided by
area
• p = f/a
Pressure
• pressure can be changed by
changing the amount of force
acting on an area
• pressure can be changed by
changing the area on which a
force acts
Fluid Pressure
• pressure in gases and liquids
Air Pressure
• air pressure is caused by the
motion of particles in the air
• you do not feel the weight of the
air because the pressure inside
your body is equal to the air
pressure around you
Air Pressure
• air pressure changes with
altitude
• the higher you are, the lower the
air pressure
Air Pressure
• Barometer
• instrument used to measure air
pressure
Water Pressure
• the force of water pressure on
an object is the same in all
directions
• water pressure changes with
depth
• the deeper you are, the greater the
water pressure
Water Pressure
• Manometer
• instrument used to measure
pressure in a liquid
What is Bernoulli’s
principle?
• Bernoulli’s Principle
• principle that states that as the
speed of a fluid increases, its
pressure decreases
Bernoulli’s Principle
• airplane wings are designed to
use Bernoulli’s principle
• air traveling over the wings moves
faster than air underneath
• this reduces the pressure on top of
the wing, allowing it to be pushed
upward
Bernoulli’s Principle
• three forces combine to help an
airplane fly
• the upward force on a plane’s wing
is called lift
• a forward force, or thrust, helps
the plane take off and maintain air
speed
• the air resistance on a plane is
called drag