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Forces
Using Newton’s Laws
Objectives
 Describe
how weight and mass are
related.
 Differentiate between gravitational force
weight and apparent weight.
 Define friction force, static and kinetic
friction.
 Describe Simple Harmonic Motion and
explain how the acceleration due to
gravity influences such motion.
Mass and Weight
 Mass
and Weight are NOT the same thing!
 Weight is a force. It incorporates gravity.
 On a falling object, both Force and
acceleration are downward.
Mass and Weight
 On
Earth, the
magnitude of an
object’s weight is its
mass x acceleration
(9.8 m/s2)
 On other planets the
acceleration due to
gravity varies, so the
object’s weight will
vary, but its mass will
not.
Scales
 The
scale exerts an upward force on the
bottom of your feet.
 It reads Fn, the number that is necessary in
order to put your weight in equilibrium.
Fnet = 0
Fspg – Fg = 0
Fspg = Fg
Apparent Weight
F
= ma Fg = mg
 Fg

changes when g changes.
Either from planet to planet or…
Apparent Weight
 If
there are other contact forces,
then the weight will change.


Contact forces may include a push
or a pull (e.g. someone pushing on
your shoulders while you’re standing
on a scale in an elevator).
Or, no contact forces pushing up
on you (e.g. the elevator cable
breaks while you’re standing on a
scale inside the elevator).
 Your
zero!
apparent weight would be
Friction Force
 Static


friction force
Exerted on one surface by the other when
there is no relative motion between the two
surfaces.
µs = Static coefficient of friction
 Kinetic


friction force:
Exerted on one surface by the other when
the surfaces are in relative motion.
µk = Kinetic coefficient of friction
Terminal Velocity




As an object falls, the
downward force of gravity
causes the object to
accelerate.
As the speed of a falling
object increases, the
upward force of air
resistance also increases.
This causes the net force on
a falling object to decrease.
Finally the upward air
resistance force because
large enough to balance
the downward force of
gravity
Terminal Velocity
 The
net force on the object is zero and
the velocity of the object doesn’t change
 The object falls with a constant velocity
called the terminal velocity.
 The terminal velocity is the highest velocity
the falling object will each.
 The terminal velocity depends on the size,
shape, and mass of the falling object.
Simple Harmonic Motion (SHM)
 When
the force that restores
the object to its equilibrium
position is directly proportional
to the displacement of the
object
Period (T)and Amplitude
 Period


The time needed to repeat one complete
cycle of motion.
The period depends on the length of the
pendulum and acceleration due to gravity.
 Amplitude

The maximum distance the object moves
from equilibrium.
Period (T)and Amplitude
T = 2π√l/g
T


= period
l = length of pendulum
g = gravitational acceleration