Transcript Forces - Needham.K12.ma.us

Forces
Use the force
FORCE
• Any push or pull where one body
exerts another
– Pushing, pulling, stretching, squeezing,
etc…
• Usually measured in Newtons (N)
• Measured with a Scale
Two Effects of Forces
1. Balanced Forces
2. Unbalanced Forces
Balanced Forces
• there is no change in
movement.
–Example: Two football players
push with the same force
against each other.
–also called equilibrium-
Unbalanced forces
-Unbalanced forces result in a
change in movement
• also called net force
• Example: There are 2 football
players pushing against each other.
One pushes with 50 N and the
other pushes with 75 N. What is
the net force?
75 – 50 = 25 N
What if they pushed together?
Friction
• Force that opposes motion between two
surfaces
• The amount of force depends on:
– Kind of surface
– Force pressing surfaces together
Friction
• Some examples of friction?
• Friction can be both harmful and
beneficial
GRAVITY
Gravity
• A force that every object in the
universe exerts on every other object.
• It is often too slight to notice
• Example: The force between your
hand and your book. There is one
there, but you can’t feel it.
• When CAN you feel gravity?
Gravitational Force
• The amount of a gravitational
force between two objects
depends on:
–Mass of the objects
–Distance between them
Earth’s Gravity
• The amount of Earth’s gravity
acting on an orbiting satellite
depends on its distance from Earth
– The closer to Earth, the stronger
the pull of gravity
How is Mass different than
weight?
• Weight
– Measure of the force of gravity on an
object
• Not the same as mass
• Mass is measured in grams or
kilograms
Gravity is A Force
• Gravity is a force and weight is a measure
of the force of gravity on an object;
therefore, weight is measured in
….Newtons
• Weight is a measurement of the pull of
gravity on a mass
Mass and Weight
• Mass- The measurement of the
amount of matter.
• Matter- Is anything that takes
up space and has mass. “stuff”
Mass and Weight
• Mass is measured with a balance.
• Weight is measured with a scale
– Weight is a FORCE due to gravity. So if
gravity is lower, your weight is lower
Practice….
• Describe the forces acting on a box that is
being pushed to the right along the floor…
•
•
•
•
Gravity
Friction
Man pushing
Normal force
Normal Force?!
• Think about it…. Gravity is a downward
force on the object- if there wasn’t some
upward force to cancel it out, the box would
be falling through the floor (net force means
object moves)
• We call this the NORMAL FORCE – the
floor exerts the NORMAL FORCE on the
box
Force due to Gravity
Applied
force
Force due to friction
Normal force
Isaac Newton used three laws to explain the
way objects
are often call
Sirmove.
IsaacThey
Newton
Newton’s Laws. The First Law states
that an object that is not being pushed or
pulled by some force will stay still, or
will keep moving in a straight line at a
steady speed. It is easy to understand that
a bike will not move unless something
pushes or pulls it.
Newton’s First Law of Motion
• – also called the Law of Inertia
“An object at rest stays at rest
unless another force acts upon
it.
An object in motion stays in
motion unless another force
acts on it.”
Inertia
• The tendency of an object to
resist a change in motion
Demonstrate with paper
Inertia and Mass
Rule of Inertia- The greater
the mass, the greater the
inertia
It is harder to understand that an object will
continue to move without help. Think of
the bike again. If someone is riding a bike
and jumps off before the bike is stopped
what happens? The bike continues on
until it falls over. The tendency of an
object to remain still, or keep moving in a
straight line at a steady speed is called
inertia.
Another way to think about it
• The behavior of all objects can be described
by saying that
Objects tend to "keep on doing what they're
doing" (unless acted upon by an unbalanced
force).
Can you think of any
everyday examples that
shows inertia?
Accelerated Motion
Newton’s Second Law
-is about force and acceleration
My Second
Law
Newton’s 2nd Law of Motion
“A net force acting on an object causes
an object to accelerate in the
direction of the force applied.”
nd Law Rules
2
• The acceleration of the object is affected
by two factors:
–Size of force – the larger the force acting
on the object, the greater the acceleration.
–Mass of object- the greater the mass, the
greater the force needed to give it the same
acceleration.
The Equation
• Newton’s 2nd Law of motion can be
expressed with the following equation:
Force = Mass X Acceleration
F = ma
So, why then does…..
David Ortiz hit a baseball
farther than a 5 year old tball player?
A soccer ball go further
when I kick it than a bowling
ball?
Newton’s
rd
3
Law of Motion
“For every action there is
an equal and opposite
reaction.”
When one object exerts a force on a
second object, the second object
exerts a force on the first that is
equal in size and opposite in
direction.
Action Forces and
Reaction Forces
• Forces are always
equal in size and
are acting on each
other.
Example
Action
Force
Reaction
Force
See Saw
You push
down
They go up
Rocket
Propulsion
Example?