Transcript CPO Chapter 3 Notes

Chapter 3
Forces
&
Motion
Newton’s 1st Law of Motion
• An object at rest will remain at rest unless
acted upon by an unbalanced force. An
object in motion will continue with constant
speed and direction, unless acted upon by
an unbalanced force.
• Unless you apply a force, things tend to
keep doing what they were already doing.
Newton’s 2nd Law of Motion
• The acceleration of an object is directly
proportional to the net force acting on it
and inversely proportional to its mass.
• Forces cause an object to accelerate,
while the object’s mass resists
acceleration.
Newton’s 2nd Law Equation
a=
F
m
• Unbalanced forces cause a change in
motion. They will speed things up, slow
them down, or change their direction.
• Forces that don’t add up to be equal in size
AND opposite in direction are unbalanced.
Newton’s 2nd Law Equation
a=
F
m
F=m*a
F=m*a
F=m*a
Newton’s 3rd Law of Motion
• Whenever one object exerts a force on
another, the second object exerts an equal
and opposite force on the first.
• For every action, there is an equal and
opposite reaction.
Inertia
• The property of an object to resist
changing its state of motion.
• Inertia is measured by the mass of an
object. More mass means more inertia.
Force
• A push or a pull
• Any action that has the ability to change
motion.
• Units
– Newtons in the metric system
– Pounds in the English standard
– One pound equals 4.48 Newtons
Mass vs. Weight
• Mass: the amount of matter in an object.
• Weight: the response of an object (mass)
to gravity.
• Mass is constant; weight varies as gravity
changes.
– The mass of your text book will be the same
on the moon as on earth. It will weigh more
on earth than the moon because earth’s
gravitational pull is more than the moon’s.
Mass, Weight & Volume
• Why do farmer’s sell milk by the pound
and not in gallons?
• Why do aircraft measure their fuel in
pounds instead of gallons?
Gravity
• An attractive force that pulls every mass toward
every other mass.
• Applying Newton’s 2nd Law to weight:
Fw = m*g
Fw = weight of an object
m = mass of the object
g = acceleration of gravity
(9.8 m/s/s on earth)
Law of Universal Gravitation
• The force of attraction between two
objects is directly related to the masses of
the objects and inversely related to the
distance between them.
• You will not have to do any calculations
using the formula given in the book.
Friction
• The resistance to motion.
• Always work against the motion of an
object.
• Two types of friction
– Static friction: the friction involved before
something begins to move
– Kinetic friction: the friction involved when an
object is in motion (air, sliding, viscous,
rolling)
Examples of Friction
• Air Friction (air resistance): the air is slow to
move out of the way of an object moving through
it.
• Sliding Friction: two surfaces rub against each
other caused by irregularities in the surfaces in
contact. (i.e. skiing)
• Viscous Friction: objects moving through a fluid.
(i.e. oil, grease, water)
• Rolling Friction: one object rolls over another.
(i.e. ball bearings, wheel)
Net Force & Equilibrium
• Net force: the sum of all the forces acting
on an object.
• Equilibrium: When the net force is zero.
There is no change in motion. These
forces are said to be balanced.
Friction & Motion
• Friction will cause an object to come to
rest if no other force balances it out.
• All machines have friction. A continual
force must be applied to keep an object in
motion.
Action & Reaction
• Forces always come in pairs.
– Action & Reaction
• Action and reaction forces act on opposing
objects.
– If you push on the ground, the ground pushes back
on you.
– The reaction force (in this case) makes you move
because it is an unbalanced force on you.
– These forces don’t balance out because they are on
two different objects.
Momentum
• Mass of an object multiplied by its velocity.
• Law of Conservation of Momentum: As
long as a system is not influenced by
outside forces [like friction], the total
amount of momentum of the system
cannot change.