Download Chapter 2 - Gordon State College

Chapter 2
MOTION
MOTION
• - one of the more common intangible
concepts in science
• - is the act or process of changing position
• TWO IMPORTANT ASPECTS OF MOTION
• A) change in position
• B) Change in time
3 PROPERTIES OF MOTION
• A. SPEED – how fast an object moves
• - distance per unit time
• Speed = change in distance
change in time
2nd PROPERTY of MOTION
• B. VELOCITY - describes the SPEED and
DIRECTION of a moving object
• Vectors are important because of the concept
of direction.
3rd PROPERTY of MOTION
• C. ACCELERATION – “change in velocity” and
time; rate at which motion was changed.
• Acceleration = change in velocity
change in time
Motion can be changed by:
• 1. changing speed
• 2. changing direction of travel
• 3. changing both the speed and direction
(this is the same as changing the velocity of
an object = ACCELERATION)
HOW TO CHANGE MOTION
• FORCE can be used to change the motion of
an object.
• FORCE – is the push or pull that can change
motion
• Net Force – is the sum of all the forces acting
on an object. (Again, vectors are important)
Important Concept
• Moving objects that are not
pushed or pulled come to a rest
in a short period of time.
• Objects keep moving only if a
force keeps acting on it.
• Aristotle thought that a heavier
rock would fall faster than an
lighter rock. He thought that the
speed of a falling object is
proportional to its weight. (He
was wrong!!!)
• Galileo corrected Aristotle.
• The velocity of a falling object in FREE FALL
(without air resistance; only gravity involved)
is NOT proportional to its weight.
• However, Galileo observed that the velocity
of a falling object increases as time
increased.
• Galileo observed that the velocity of a falling
object depended on the time and the
distance that it fell.
• “Distance is proportional to time2.”
• An object should fall 4x as far in 2 seconds
(22), 9x as far in 3 seconds (32) and 16x as far
in 4 seconds (42).
IMPORTANT CONCLUSIONS!!!
• The velocity of a falling object increases at a
constant rate. “Distance is proportional to time2.”
• Acceleration due to gravity is constant for all objects
on Earth (9.8 m/s2). [no air resistance]
• All objects in free fall experience constant
acceleration.
• Objects falling from the same distance will hit the
ground at the same time. [Time is not dependent on
the mass of the object.]
2 Type of Motion Discussed so far:
• 1. Horizontal Motion
• 2. Vertical Motion
• 3. Motion due to an object being thrown
vertically or horizontally
INERTIA
• Is the behavior of matter to remain
in unchanging motion, whether at
rest or in motion
VERTICAL PROJECTILES
• Ex. Bullet goes up, slows down and
stops, then accelerates back to earth.
• Bullet slows down and stops because
gravity is decreasing the velocity of the
bullet.
HORIZONTAL PROJECTILES
• If air resistance is ignored, then there are no
forces involved in horizontal projectiles.
• The reason an arrow shot horizontally falls to
the ground is because the vertical forces
(gravity) come into play.
Newtons’ First Law of Motion
• “Every object retains its state of rest or
straight-line motion unless acted upon by an
unbalanced force.”
• An object resists any change in its state of
motion unless acted upon by an unbalanced
external force.
Newton’s Second Law of Motion
• “The acceleration of an object is directly
proportional to the net force acting on it and
inversely proportional to the mass of an
object.”
• The acceleration of an object depends on the
net force and the mass of an object.
• Equation: Force = mass x acceleration
Unit of Force
• Newton (N)
• 1 Newton =1 N = 1 kg-m/s2
Mass vs. Weight
• Mass = is the amount of matter in an object
• = is the property that determines how much
an object resists a change in motion (inertia)
• The greater the mass of an object, the
greater the inertia.  this is the reason that
big rigs takes a longer distance and time to
come to a standstill when the brakes are
applied.
Weight
• = is the force of gravity acting on a mass.
• Because weight is a FORCE, its unit is Newtons.
• Equation:
w = mg
or
weight = mass x acceleration due to gravity
or Downward force = mass x acceleration due to gravity
Acceleration due to Gravity
• Near the Earth’s surface, g
(acceleration due to gravity) has an
approximate value of :
g = 9.8 m/s2
• Pounds is also a unit of force, aside from
Newtons. So pounds is also a unit of weight.
• Kilogram is a unit of mass.
• Conversion: 1 lb = 4.5 Newtons
Newton’s Third Law of Motion
• “ Whenever two objects interact, the force
exerted on one object is equal in strength
and opposite in direction to the force exerted
on the other object.”
• “For every action, there is an equal and
opposite reaction.”
• Example: 2 Sumo wrestlers doing belly bump
MOMENTUM
• Momentum (p) = mass x velocity
=mxv
• It takes a longer time to stops an object’s
movement when it has momentum.
• Momentum involves both inertia and velocity.
Law of Conservation of Momentum
• “The total momentum of a group of
interacting objects remains the same in
the absence of external forces.”
IMPULSE
• Is the product of the force and the time of application
of the force.
Impulse = Force x time = FT
• Brings about a change in momentum (A greater
impulse means greater change in momentum.)
• Equations:
Impulse = Force x time = FT
• Change in momentum =(applied force)(time of contact)
D p = FT
• The longer the time of contact, the weaker the force.
FORCES and CIRCULAR MOTION
• Centripetal force – is the force that pulls an
object out of its straight-line path and into a
circular path
• Centrifugal force – is the outward tug or the
outward force
• The acceleration of an object moving in
uniform circular motion is equal to the
square of its speed around the circle (v2) and
inversely proportional to the radius of the
circle (1/r)
• Equation: ac = v2/r
GRAVITY
• Gravity is the force, a pull that exists
between ALL objects in the universe.
• Example: The Earth attracts you and
you attract the Earth.
Newton’s Law of Gravitation
• “ Every object in the universe is attracted to
every other object with a force that is directly
proportional to the product of their masses and
inversely proportional to the square of the
distances between them.”
• Equation: Force = G x mass1 x mass2
distance2
- where G is a universal constant with the
value 6.67 x 10-11 N.m2/kg2 and not the
acceleration due to gravity.
• Equation: Force = G x mass1 x mass2
distance2
• This equation gives the magnitude of the
attractive or gravitational force that each
object exerts on the other.