Transcript Force motion and machines powerpoint

Force, motion, and machines
Force of gravity
• Gravity is a force that
pulls objects toward
each other.
• All matter has gravity
Force of gravity
• 2 main things affect how
gravity pulls on an object:
– The objects mass (more
mass = more pull from
gravity)
• Example: Why do the
planets orbit the sun.
– The distance between the
two objects (The further
the distance = less pull
from gravity.)
• Example: Why does the
moon orbit the earth?
Weight and mass
• Mass is a measurement
of how much matter is
in something.
• Weight is a
measurement of how
gravity is pulling down
on something.
• Weight is affected by
gravity, mass is not!
Laws of motion
• Isaac Newton came up
with 3 laws of motion in
the late 1600s.
Laws of motion
• The first law of motion,
(sometimes called the law
of inertia), says that an
object at rest will stay at
rest, and an object in
motion will stay in
motion unless it is acted
upon by a force.
• Inertia is when an object
resists change in motion.
• The more mass
something has, the more
inertia it has.
Balanced and unbalanced forces
• Newton’s second law of motion
can be summarized by the
equation F=ma.
• More mass takes more force to
move. (Kick a wall or a ball?)
• Newtons second law of motion
explains why an unbalanced
forces cause an object to
accelerate in the direction of the
greatest force.
• Balanced forced lead to NO
acceleration – or constant speed
Which way will it accelerate?
Newton’s third law of motion
• Newton’s third law of
motion says that for
every force there is an
equal and opposite
force.
• (When you push on the
wall, the wall pushes
back on you!)
• Newtons laws of
motion quick quiz
Friction
• Friction is the force
created when two
surfaces rub against
each other.
Friction
• There are 4 main types
of friction
–
–
–
–
Static friction
Sliding friction
Rolling friction
Fluid friction
Friction
• Some devices use
friction to control the
motion of an object.
Simple Machines
• A machine makes work
easier by changing one
of 3 things:
– The amount of force you
have to put out.
– The distance you use the
force over.
– The direction of the
force.
Simple Machines
• A machine’s mechanical
advantage is the
number of times that a
machine increases a
force.
• (Example: if it makes
your job 3 times easier,
it would have a
mechanical advantage
of 3)
Simple Machines
• There are 6 types of
simple machines:
–
–
–
–
–
–
Inclined plane
Wedge
Screw
Lever
Wheel and axel
Pulley
Simple Machines – inclined plane
• An inclined plane is a
flat sloped surface. (Like
a ramp).
• An inclined plane lets
you exert your force
over a longer distance.
• Where in the school
would you find inclined
planes?
Simple Machines – inclined plane
• You can find the
mechanical advantage
of an inclined plane by
using the formula:
– Length of incline/height
of incline.
Simple Machines – wedge and screw
• A wedge and a screw
are other simple
machines that are made
from an inclined plane.
• Where are some places
in the school that you
might find wedges and
screws?
Simple machines - Levers
• A lever is a rigid bar that
is free to rotate or pivot
on a fixed point.
• The point that the lever
rotates on or around is
called the fulcrum.
• Levers change the amount
of force you have to put
out, and they can change
the direction of the force.
Simple machines - Levers
• First class lever:
– A first class lever has the
fulcrum in the middle.
– Examples of first class
levers include:
•
•
•
•
Prying open a paint can
Scissors
Rowing a boat
Pliers
Simple machines - Levers
• A second class lever has
the resistance (load) in
the middle.
• Examples of second
class levers include:
– Wheelbarrows
– Nutcrackers
Simple machines - Levers
• A third class lever has
the effort in the middle
• Examples of third class
levers include:
–
–
–
–
–
Brooms
Tweezers
Tongs
Fishing poles
Hockey sticks
What type of lever is this?
Simple machines - Levers
• You can find the mechanical
advantage of a lever using
the formula:
– Distance from the fulcrum to
the input force / Distance
from the fulcrum to the
output force.
– Input force is sometimes
called “Effort”
– Output force is sometimes
called “load” or “resistance”
Calculate the mechanical
advantage for the levers
?
Simple machines – Wheel and axle
• A wheel and axle is made
of 2 cylinders of different
sizes that rotate together.
• A wheel and axle changes
the distance over which a
force is exerted.
• Examples include:
– Screwdriver
– Doorknob
– A faucet
Simple machines – pulleys
• A pulley is made of a
grooved wheel with a
rope wrapped around
it.
• Pulleys change the
amount of force needed
and the direction of the
force.
• Where might you find
pulleys at the school?
Simple machines – pulleys
• The mechanical
advantage for a pulley is
found by counting the
number of rope
segments (not counting
the one that is
attached to the effort.)