Transcript Forces

Chapter 12
Forces &
Motion
Forces
 “a push or a pull”
 A force can start an
object in motion or
change the motion
of an object.
 A force gives
energy to an object
Representing forces
 We often use arrows.
 Larger the arrow, the
greater the force
 Combined balanced
forces equal zero
 Normal force = the
force acting
perpendicular on an
object in contact with
another object.
Balanced Forces
 Forces opposite in
direction and equal in
size.
 Net result: no change
in motion.
No change in motion!
 Balanced forces will keep an object moving
at a constant velocity
FRICTION
ENGINE
Friction:
One of the most common forces.
Always acts in a direction opposite
the direction of motion.
There are four main types of
friction.
Static Friction
 The force that must one must overcome to
get an object to move.
 Examples:
 Trying to move a heavy object
 Tires on pavement when a car is moving
 Walking
Sliding Friction
 Two surfaces
sliding over each
other.
 Caused by two
factors:
1. Weight of moving
object
2. Types of surfaces
in contact
Rolling Friction
 An object rolling
over a surface
 Not as strong as
sliding
 Can be reduced
by using wheels
and ball bearings.
Fluid Friction
 When an object
is moving
through a fluid
(either gases or
liquids)
 ex. a fish
swimming under
water, air
resistance
Is friction ever helpful?
 YES. Examples:
1. Stopping a car
2. Driving on ice
3. Walking
GRAViTY
The pull of
one body of
mass on
another.
How fast do objects fall on earth?
 Galileo 16th century
 Dropped two metal
spheres of different
sizes off the Leaning
Tower of Pisa
 Which hit first?
 They hit at the same
time.
Acceleration due to gravity
 All objects fall at the
same rate, regardless
of their mass or weight
 The acceleration due
to gravity on earth is
about 9.8 m/sec2.
 This value will change
with elevation and
location on earth.
Terminal Velocity
 After a period of
free fall, the force
of gravity will be
canceled by the
force of air friction
and objects will
travel at a constant
velocity
Projectile motion
 Any object thrown in
the air becomes a
projectile.
 The object will move
forward due to inertia,
and downward due to
gravity.
 Always a curved path.
Aristotle
 384 B.C. – 322 B.C.
 Greek scientist and
philosopher
 First to work with
force.
 Incorrectly stated that
a force is required to
keep an object moving
at constant speed
Sir Isaac Newton
1642-1727
British
Physicist
Considered the
“Founder of
modern
physics”
Newton’s First Law
of motion
 An object at rest will remain at rest and
an object in motion will remain in
motion unless acted upon by an outside
force.
 Often referred to as the Law of Inertia.
(the property of matter that resists any
change in motion)
Newton’s Second
Law of Motion
 The force of a moving object is directly
proportional to the object’s mass and
acceleration.
 The most important of Newton’s laws.
How do we label force???
 F = ma
 Mass is measured in kg
 Acceleration is measured in m/sec2
 If we multiply these two units :
1 kg m/sec2 = 1 newton (N)
Weight and Mass
MASS
WEIGHT
 The measure of inertia
 A measure of the force
of an object.
 The amount of matter
in an object
 Stays constant with
changes in location
 A scalar
of gravity acting on an
object.
 Measured in newtons.
 Changes with changes
in location.
 A vector
Newton’s Third Law
of Motion
 For every action there is an equal and
opposite reaction, or
 All forces occur in pairs
 examples:
Momentum
 All moving objects have momentum.
 It is the product of an object’s mass and
velocity.
 Which has more momentum? A moving car
or a moving train?
 Not enough information.
 A 1500 kg car traveling 50 m/sec? Or a
48,000 kg train traveling 2 m/sec?
Universal Forces
Electromagnetic force:
 Forces that attract and repel.
Nuclear forces
 STRONG
 Responsible for
holding the nucleus
together
 Strongest known force
 WEAK
 Very small
attractive force in
the nucleus
Newton’s Law of Universal
Gravitation
 Every object in the universe attracts
every other object.
 Depends on two factors:
1. Distance between the objects
2. Mass of the objects
The Earth, Moon and Tides
 Earth’s gravitational
pull on the moon
combined with the
inertia or momentum
of the moon keeps the
moon in orbit around
the earth.
 The pull of the moon
and sun causes the
tides
Satellites
 Satellites must be
placed at a specific
height above the
earth’s surface in order
to stay in orbit.
 If it is too close, or
moving too slow, it
can spiral into the
earth’s atmosphere and
burn.
Assignment:
Page 385-386
1-11, 16, 17, 18, 20, 23,27,
31,32,33
1-6 on page 387