Transcript Newton`s Laws of motion

NEWTON’S LAWS OF
MOTION
Newton’s first law
Every object in motion
tends to remain in motion
unless an external force is
applied to it. The same
applies to an object at
rest.
What does this mean?
• An object at rest will remain at rest and an object in
motion will remain in motion unless something happens to
them.
• On earth the something that happens is often friction or
air resistance.
• Examples:
• Chairs and tables don’t move on their own.
• A ball rolled across the floor will eventually stop because of friction
caused by its contact with the floor.
• Satellites in space keep going without propulsion because of the
lack of particles to cause friction in space
• When playing pool you hit balls in particular places to cause them
to move in specific directions.
Newton’s second law
F = ma: This is the relationship
between an object's mass m, its
acceleration a, and the force
applied to it, F .
What does this mean?
• Force is directly proportional to acceleration, so if I
increase the force on an object then I will increase its
acceleration, and if I decrease the force on an object then
I will decrease its acceleration.
• Force is also directly proportional to the mass of the
object, so if I have a larger mass then I must apply a
larger force to get the same acceleration.
• Mass is indirectly proportional to acceleration, so if I have
a larger mass its acceleration will be less than that of
smaller mass object using the same force to cause the
acceleration.
• Examples:
• You would rather have a small Honda run into your car
at 35 mph than a large truck at the same velocity
because the larger mass means a larger force and thus
more damage to your car.
• The harder you hit a baseball, the farther it will
go because a larger force is acting on the same
mass.
• A bowling ball and a softball dropped from the
same height at the same time will hit the
ground at the same time, but the bowling ball
will make a deeper hole in the ground
because of its mass.
Newton’s third law
For every action there is an
equal and opposite reaction.
What does this mean?
• Every time you have a movement or a push in one
direction there is the same amount of movement or push
in the opposite direction.
• Examples:
• When a rocket takes off the combustion of fuel
pushes downward, forcing the rocket to move
upward.
• When you shoot a rifle the bullet moves forward, forcing
the stock backward into your shoulder.
• The amount of stretch you give to a rubber band (force)
will determine how much recoil (opposite force) it has.
The more you stretch it the further you can shoot it.
Reminder about levers
• Levers consist of an force or effort and force arm, a
resistance or load and resistance arm, and a pivot point or
fulcrum.
• Force X length of force arm = resistance X length of
resistance arm.
• The mechanical advantage (MA) = length of the force arm
divided by length of the resistance arm. This means that
you have a greater mechanical advantage if the force arm
is longer than the resistance arm.
• Mechanical advantage indicates how much easier a job is
to complete while using a simple machine. It is the ratio of
resistance force to effort force.
Types of levers
• First class = force – pivot – resistance
• The force has MA if it is farther from the pivot
than the resistance.
• Examples: seesaw, crowbar
• Second class = pivot – resistance – force
• The force always has MA because by definition it has to be further
from the pivot than the resistance.
• Examples: opening a door, using a stapler
• Third class = pivot – force – resistance
• The force never has MA because by definition it is closer to the
pivot than the resistance.
• The lack of MA is compensated for by the fact that you get a
greater range of motion for the resistance with little motion by the
force.
• Examples: opening a door if you push on the wrong side, a catapult
Atlatls are a third class lever system
• The extension of the forearm (the forward throwing
motion) uses the elbow as the pivot point; the triceps
muscle is the force and it is attached to the forearm a
short distance further on the forearm; the resistance is the
object held in the hand furthest out from the pivot.
• The flick of the wrist at the end of the throwing motion is
also a third class lever system.
• The triceps produces the force with very little motion and
the end of the atlatl moves over a great distance thrusting
the spear forward with great momentum.