Transcript chpt 19Force and newton`s Laws

Newton’s First Law
Newton’s Second law
Newton’s Third law
 Force
is a push or pull
 Force can be exerted in different ways
A
magnet can exert a force on a paper
clip, gravity pulls that paper clip to
center of earth, and there is a force
that acts on the paper clip when you
pick it up
 All
forces acting on an object is called the
net force
 Net force acting on a book might be gravity
pulling it toward center of earth and the
book pushing up on book. It doesn’t move
therefore it is balanced
 If two forces are in the same direction, they
are added together to form the net force
 If two forces are acting in opposite direction,
the net for is the difference between the
two forces and is in the direction of the
larger force
 When
two forces are exerted on an object,
and the net force is zero their effects cancel
each other out and they do not change
velocity it is a balanced force
 Unbalanced forces are a result when forces
on an object are not zero and the effects
don’t cancel each other and their velocity
changes
 Page
551 figure 2
 Sir
Issac Newton (1642-1727) developed laws
of motion
 First law describes how an object moves
when the net force acting on it is zero
 First law states that an object at rest will
remain at rest, or an object in motion will
continue in motion unless an outside force
acts on it. This occurs when a balanced
force is applied
 Inertia is an example of Newtons First Law
 Friction
is a force that brings all objects to
rest
 Before Newton people didn’t know about
friction or the force that acts in opposite
direction of movement
 When he realized objects in motion are
slowed down by an unbalanced force his first
law began to make sense
 An object in motion will continue forever
unless an unbalanced force acts on it(like
friction) or it will remain at rest unless an
outside force acts on it
 Static
Friction: resistance of objects to
begin to move… must overcome static
friction to move a book across a table
 Sliding friction: resistance of objects once
an object is moving. Moves in opposite
direction of motion
 Rolling friction: less force in opposite
direction than sliding friction
 Force:
a push or pull on an object
 Acceleration occurs any time there is a
change of velocity or direction of an object
 Newton’s Second law deals with the
acceleration of an object that will equal the
net force divided by the mass and is in the
direction of the net force
 A=Fnet
F= ma
m
 Unit of force is in Newtons kg*m/s2

 Gravity
is the attraction between any two
objects
 It is dependent on the size of the objects and
the distance between them
 Weight: is the size of the gravitational force
exerted on an object


It can change as you move farther from center
of the earth
W= m (9.8 M/S2) Where 9.8 m/sec2 is the
acceleration rate on earth
 Weight
and mass are different
 Weight is a force and is measured in newtons
 It is a record of the pull of earth’s gravity
and can change depending on distance to
center of earth
 Mass is the amount of matter in an object
and doesn’t depend on location
 Speeding
up: when an unbalanced force is
moving in the same direction as an object
…it will speed up
 Slowing down: if the unbalanced force is in
the opposite direction as objects velocity it
will slow down
 Can calculate acceleration a=fnet/m
 Page 559
 Curved
path is a result of forces that are
neither in same or opposite direction acting
on an object
C
 ircular motion: movement in a circle like a
merry-go-round
Your direction of motion is constantly
changing which also means you are
constantly accelerating
In order to cause an object to move in circular
motion at constant speed, the net force must be
at right angles to the velocity
Centripetal force is the net force on an object
moving in a circle and is toward center of path
 Satellites
circle the earth and are kept in
motion because of the speed they travel in
orbit
 They must travel at least 8 km/sec or
 29,000 km/h other wise it will fall back to
earth
 When
an objects moves, there is air
resistance
 The greater the velocity, the greater the air
resistance
 Terminal velocity is a result of balancing the
gravitational pull and air resistance and the
object’s velocity remain the same
 Center
of mass is the point in an object that
moves as if all the object’s mass were
concentrated at that point
 The center of mass on an oblong shape is the
point if followed in a straight line (Figure 12)
 3rd
law states that for every action there is
an equal and opposite reaction
 Forces exerted by two objects on each other
are often called an action-reaction force
 They can be action forces or reaction forces
because normally they aren’t equal
 Only time they are equal is if forces act on
the same object
 Bird flying utilizes 3rd law
 Depending
on mass of objects you may not
know that there is an equal but opposite
reaction ie: jumping off diving board
 Rocket launches uses 3rd law to push rocket
upward with gases burning in opposite
direction
 Weightlessness
is the condition that occurs in
free fall when the weight of an object seems
to be zero
 A falling object is in free fall when the only
force acting on the object is gravity and for
that split second weightlessness occurs
 In orbit all objects are in free fall and falling
in a curved path
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