Transcript Motion and Forces

Motion Chapter 8.1
 Speed
 Velocity
 Momentum
Speed
 Distance
traveled divided
by the time during which
motion occurred
Speed
 Constant
speed means that
an object travels the same
distance in the same
amount of time
 Objects at rest have a
speed of 0 m/s
Speed Equation
Distance
Speed =
Time
d
v= t
Speed Example
 Suppose
a wheelchair racer
finishes a 132 m race in
18 s. What was the racer’s
average speed?
Speed Example
 speed
= d/t
 Speed = 132 meters/18 s
 Speed = 7.3 m/s
Are speed and velocity the
same?
 A car travels at a constant
speed of 30 miles/hour.
The car makes a left at a
speed of 30 miles /hour.
Are speed and velocity the
same?
 Speed
did not change
 Velocity did change
Velocity
 Quantity describing both
speed and direction
 Same calculation as
speed except that it also
designates a direction.
Velocity Example
 Find
the velocity of a
swimmer who swims
exactly 110 m toward
the shore in 72 s.
Velocity Example
V
= d/t
 V = 110 meters/72 seconds
 Speed = 1.5 m/s
 Velocity = 1.5 m/s towards the
shore
Momentum
A
quantity defined as the
product of an object’s mass
and its velocity
 The greater the mass or the
faster the velocity, the
greater the momentum
Momentum Equation
 Momentum
= mass x
velocity
 Equation is p=mv
 p is used to represent
momentum
Momentum Example
 Find
the momentum of a
75 kg speed skater
moving forward at 16
m/s.
Momentum Example
 momentum
= mv
 Momentum = (75 kg)(16m/s)
 Momentum= 1200 kg • m/s
forward
Conservation of
Momentum
 The
total amount of
momentum in a system is
conserved
Conservation of Momentum
 If
two vehicles with different
masses are traveling with
different velocities and they hit
head on, the momentum of the
two cars before the collision is
the same after the collision.
Acceleration and Force Chapter 8.2
 Acceleration
 Force
 Friction
 Gravity
and Air-resistance
Acceleration
 change
in velocity divided
by the time in which the
change occurred
 The greater the acceleration
the faster the object is
speeding up.
Acceleration
 If
the speed remains
constant, acceleration is
zero.
 SI unit is meter/second per
2
second (m/s/s or m/s )
Acceleration Equation
Acceleration =
∆ velocity
Vf-vi
a = Δt
Time
Acceleration Example
 Find
the acceleration of a
northbound subway train
that slows down from
12 m/s to 9.6 m/s in 0.8 s.
Acceleration Example
a
= change in v/t
 Acceleration = (9.6 – 12m/s)/0.8s
2
 Acceleration = -3 m/s or m/s/s
For constant velocity
Zero Acceleration
Accelerating has
positive slope
decelerating has a
negative slope
Force
 The
cause of acceleration
or the change in an
object’s velocity
 SI unit of force is a
newton. (N)
Force
 Two
Types:
1. Balanced forces do not
change motion
Force
2. Unbalanced
forces result in
a net movement in one
direction.
 This occurs because the net
force on one side is less
than the net force on the
other.(ex. Tug of War.)
Friction
 the
force between two
objects in contact that
opposes the motion of either
object
 friction maintains balanced
forces
Gravity



Gravitational force is determined by the
distance between the two masses.
Everything falls at an acceleration of
9.8 m/s2 in the absence of air resistance
Gravity is opposed by air resistance
Gravity Experiments
The Physics of Falling
Horizontal motion has no affect on gravity
Gravity has no affect on horizontal motion.
Gravity only works if you look down. True or False
FALSE
Air Resistance
 Air
resistance is a form of
friction
 Air resistance opposes gravity.
 Objects with bigger volumes
or smaller densities can
experience more air resistance.
Laws of Motion Chapter 8.3
 Newton’s
st
1
 Newton’s
nd
2
Law
 Newton’s 3rd
Law
Law
Newton’s
 An
st
1
Law
object at rest remains at
rest and an object in motion
maintains its velocity
unless it experiences an
unbalanced force.
 Inertia
How does this work?
Inertia and a roller coaster
Newton’s
 The
nd
2
Law
unbalanced force
acting on an object equals
the object’s mass times its
acceleration.
 F=ma
Measuring Force
 Force
is measured in
newtons.
2
 1 N=1 kg*m/s
Force Example
 What
is the force necessary
for a 16000 kg automobile
to accelerate forward at 2.0
2
m/s ?
Force Example
 Force=
ma
2
 Force = (16000 kg)(2.0m/s )
 Force = 32000 N
Free Fall
 the
motion of a body when
only the force of gravity is
acting on it.
 Acceleration of gravity is
2
9.8m/s
 Everything will fall at this
acceleration
Mass vs. Weight
 Mass
represents the
measured amount of matter
in an object.
 Weight is the gravitational
force an objects experiences
based on its mass.
Mass vs. Weight
 Weight
= mass x gravity
 w=mg , weight is a force.
Terminal Velocity
 The
maximum velocity
reached by a falling
object.
Terminal Velocity
 This
occurs when air
resistance is equal to the
force due to gravity.
 Terminal Velocity of a
skydiver is ~200 miles/hr
Newton’s third Law
 For
every action there is an
equal and opposite reaction
 Rockets move as a result of
action and reaction.