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Physics Unit 1
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
• To describe a force you need to
know direction and strength
(size) of the force
• ex. Push the box to the right with a
force greater than 20 N
Net force
• Is the total force acting on an
object
• To find net force:
• Add forces acting in the same
direction
• Subtract forces acting in opposite
directions
Weight
- Weight is a force
- Weight is a measure of the
amount of gravity acting on an
object
- To lift an object on Earth you
must over come the gravity
pulling on it, you must lift with a
force equal to or greater than
its weight
Newton
• metric unit of force
• named in honor of Sir Isaac
Newton
What is gravity?
• Sir Isaac Newton
• 1642 – 1727
• The Law of Gravity or Universal
Gravitation – states that all
objects are attracted to each other
by the force of gravity
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 there is no air and
no air resistance
• 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
• Rolling Friction
• friction between two surfaces that
are not in constant contact
• ex. wheels
Types of Friction
• Fluid friction – friction that
occurs when an objects move
through a fluid; through a gas or
liquid
Types of Friction
• Air Resistance is a Type of Fluid
Friction
• friction results from air pushing on
an object as it is moving
Types of Friction
• Static friction – friction of an
object at rest
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
Pressure
• pressure is the amount of force
acting on a surface
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
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