Unformatted text preview: How do we describe motion? Speed = distance traveled in a
given time Velocity = speed and direction Acceleration = change in velocity
(speed or direction) in a given time
Car accelerates when
its speed increases Car accelerates when it
turns(direction changes) Car accelerates when
its speed decreases Acceleration owing to
gravity is same for all
objects Near Earthʼ s surface
gravity increases the
downward velocity by
about 10 meters per
second each second (g = 9.8 m/s/s) Momentum = (mass) x (velocity) The only way to change an objectʼs
momentum is to apply a force to it How is mass different from
weight? 1 Mass = total amount of matter in
an object Weight = the force that gravity
exerts on an object The weight reading on a scale changes as an elevator
accelerates, even though mass stays constant
If the elevator falls freely, you experience weightlessness Theory of gravity united
heavens and Earth in one
universe Laws of motion laid the
foundation of physics How did Newton change our
view of the universe? Also:
Invented calculus
Built 1st reﬂecting telescope
Explored nature of light
Isaac Newton Newton’s First Law What are Newton’s three laws
of motion? An object moves at
constant velocity
unless a force acts to
change its speed or
direction 2 Newton’s Second Law
Force = mass x acceleration Newton’s Third Law
For any force, there is
an equal and opposite
reaction force (F = ma or a = F/m) Conservation of Momentum
In any interaction, the
total amount of
momentum does not
change Forces merely transfer
momentum from one
object to another Newton’s 3rd law of motion
equivalently,
conservation of momentum
is consistently violated in
Hollywood movies. (Send
examples to me!) What keeps a planet rotating
and orbiting the Sun?
Gravity constantly exerts forces (=transfers
momentum) between planets and Sun. 3 Objects orbit around
their common center
of mass because the
total amount of
momentum must
stay ﬁxed Angular momentum = (mass) x (velocity) x (radius) Conservation of Angular Momentum
In any interaction the
total amount of
angular momentum
does not change Keplerʼs second law is an example of conservation of
angular momentum The orbital velocity increases as the radius of the
orbit decreases (see animation kepler_2_area..) As skaterʼs radius
decreases, her spin
velocity must increase IMPORTANT IDEA! Conservation of Energy Where do objects get their
energy? Energy can be converted from one form into
another, but the total amount of energy never
changes. IMPORTANT IDEA! 4 Forms of Energy
• Kinetic energy: energy of motion
• Potential energy: stored energy
– Gravitational (can fall from a height)
– Chemical (food, gasoline)
– Massenergy (E = mc2) • Radiative energy: energy of light Definition: Temperature
Kelvins: temperature unit
T (K) is directly proportional to the random
Kinetic Energy of the gas particles
“Absolute zero” means zero motion
0 K = 460 deg F = 273 deg C
A Kelvin is the same size as a Celsius degree. Keplerʼs second law is also an example of conservation
of energy As the orbitʼs radius decreases, gravitational potential
energy is converted into kinetic energy What determines the strength
of gravity? The nature of an orbit depends on the total amount of energy Objects with lots of kinetic energy follow trajectories that do
not return Escape velocity speciﬁes the speed needed to escape an
objectʼs surface
The force of gravity between two objects depends on
the product of their masses and the square of the
distance between them (KE = 1/2 mv2) (Gravity is an inverse square law.)
Orbit trajectory cannonball 5 Astronauts in orbit
are essentially
falling around the
Earth They feel
weightless because
they feel no force
opposing gravity Kepler’s First Law Revisited How did Newton’s gravity
extend Kepler’s laws? Kepler’s Second Law Revisited The shape of an orbit
depends on the total
amount of energy Orbits with lots of
kinetic energy never
return
The speed of an object in its orbit is governed by
conservation of energy and angular momentum Kepler’s Third Law Revisited:
Newton’s version of Kepler’s 3rd Law
p2 = 1
(M1 + M2) a3 How does gravity cause tides? Measuring the orbital period (p) in years and average
separation (a) in AU of an orbiting system tell us its total
mass (M1 + M2) in solar masses (1 solar mass = 2 x 1030
kg) IMPORTANT IDEA! 6 Gravity of Moon pulls harder on the near side of the
Earth than on the far side, stretching the Earth Tides occur because water deforms more than land Tides generally occur twice a day as Earth rotates
under Moon Tides are strongest
when Sun, Earth,
and Moon are
aligned
Tides are weakest
when Sun and
Moon form a right
angle with Earth
When is the
LOWEST low
tide?
A. new/full moon
B. 1st/3rd quarter
moon 7 ...
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 Fall '08
 Donahue

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