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Nicholas J. Giordano
Circular Motion and Gravitation
• Circular motion
• Acceleration is not constant
• Cannot be reduced to a one-dimensional problem
• Examples
• Car traveling around a turn
• Parts of the motion of a roller coaster
• Centrifuge
• The Earth orbiting the Sun
• Gravitation
• Explore gravitational force in more detail
• Look at Kepler’s Laws of Motion
• Further details about g
Uniform Circular Motion – Overview
• Circular motion is an example of accelerated motion
• It can be analyzed in terms of acceleration and forces
• A problem-solving strategy can be applied in a manner
similar to one- and two-dimensional motion approaches
Section 5.1
Uniform Circular Motion
• Assume constant speed
• The direction of the
velocity is continually
• The vector is always
tangent to the circle
• Uniform circular motion is
circular motion at constant
Section 5.1
Uniform Circular Motion, cont.
• Examine one trip around the track
• The distance traveled is the circumference of the track
• The period of the motion, T is
• r is the radius of the track
• v is the speed of the motion
• The period does not depend on the location on the object
• The speed depends on the radius of the circle of motion and so
depends on location
• Speeds are fastest at the outside edge
Section 5.1
Centripetal Acceleration
• Although the speed is
constant, the velocity is not
• Direction of acceleration
• Always directed toward the
center of the circle
• This is called the
centripetal acceleration
Centripetal means “centerseeking”
• Magnitude of acceleration
Section 5.1
Circular Motion and Forces
• Newton’s Second Law can be applied to circular motion:
• The force must be directed toward the center of the circle
• The centripetal force can be supplied by a variety of
physical objects or forces
• The “circle” does not need to be a complete circle
Section 5.1
Centripetal Force Example
• The centripetal
acceleration is produced
by the tension in the string
• If the string breaks, the
object would move in a
direction tangent to the
circle at a constant speed
Section 5.1
Problem Solving Strategy –
Circular Motion
• Recognize the principle
• If the object moves in a circle, then there is a centripetal
force acting on it
• Sketch the problem
• Show the path the object travels
• Identify the circular part of the path
• Include the radius of the circle
• Show the center of the circle
• Selecting a coordinate system that assigns the positive
direction toward the center of the circle is often convenient
Section 5.1
Problem Solving Strategy, cont.
• Identify the principles
• Find all the forces acting on the object
A free body diagram is generally useful
• Find the components of the forces that are directed toward
the center of the circle
• Find the components of the forces perpendicular to the
• Apply Newton’s Second Law for both directions
The acceleration directed toward the center of the circle is a
centripetal acceleration
Section 5.1
Problem Solving Strategy, final
• Solve for the quantities of interest
• Check your answer
• Consider what the answer means
• Does the answer make sense
Section 5.1
Centripetal Acceleration Example: Car
• A car rounding a curve
travels in an approximate
• The radius of this circle is
called the radius of
• Forces in the y-direction
• Gravity and the normal
• Forces in the x-direction
• Friction is directed toward
the center of the circle
Section 5.1
Car Example, cont.
• Since friction is the only force acting in the x-direction, it
supplies the centripetal force
• Solving for the maximum velocity at which the car can
safely round the curve gives
Section 5.1
Example: Car on Banked Curve
• The maximum speed can be
increased by banking the
• Assume no friction between
the tires and the road
• The car travels in a circle, so
the net force is a centripetal
• There are forces due to
gravity and the normal force
acting on the car
Section 5.1
Banked Curve, cont.
• There is a horizontal component of the normal force
• Letting the horizontal be the x-direction
• The speed at which the car will just be able to negotiate
the turn without sliding up or down the banked road is
• When θ = 0, v = 0 and you cannot turn on a very icy
unbanked road without slipping
Section 5.1
Examples of Circular Motion
• When the motion is uniform, the total acceleration is the
centripetal acceleration
• Remember, this means that the speed is constant
• The motion does not need to be uniform
• Then there will be a tangential acceleration included
• Many examples can be analyzed by looking at the two
Section 5.2
Non-Uniform Circular Motion
• If the speed is also
changing, there are two
components to the
• One component is tangent
to the circle, at
• The other component is
directed toward the center
of the circle, ac
Section 5.2
Circular Motion Example:
Vertical Circle
• The speed of the rock
varies with time
• At the bottom of the
• Tension and gravity are in
opposite directions
The tension supports the
rock (mg) and supplies the
centripetal force
Section 5.2
Vertical Circle Example, cont.
• At the top of the circle:
• Tension and gravity are in
the same direction
Pointing toward the center
of the circle
Section 5.2
Vertical Circle Example, Final
• There is a minimum value of v needed to keep the string
taut at the top
• Let Ttop = 0
If the speed is smaller than this, the string will become slack and
circular motion is no longer possible
Circular Motion Example: Roller Coaster
• The roller coaster’s path is
nearly circular at the
minimum or maximum
points on the track
• There is a maximum speed at
which the coaster will not
leave the top of the track:
• If the speed is greater than
this, N would have to be
• This is impossible, so the
coaster would leave the
Section 5.2
Circular Motion Example:
Artificial Gravity
• Circular motion can be used
to create “artificial gravity”
• The normal force acting on
the passengers due to the
floor would be
• If N = mg it would feel like
the passengers are
experiencing normal Earth
Section 5.2
Circular Motion Example: Centrifuge
• A centrifuge is a device used
in many laboratories
• It can be used to separate
particles or molecules
• Or remove them
• The effective force causes
the particle to move to the
bottom of the test tube
• Similar to artificial gravity
• To someone outside of the
test tube, the particle appears
to spiral
Section 5.2
Frames of Reference and the Centrifuge
• The stationary observer is in an inertial reference frame
• He can apply Newton’s Second Law
• His interpretation is correct
There is no actual force acting on the particle
The force FAG is a fictitious force
• The observer moving with the particle is in an accelerated
• He cannot apply Newton’s Second Law
• He thinks there is a force acting on the particle
• They agree on the particle’s motion
Section 5.2
Newton’s Law of Gravitation
• In many cases, the orbits of planets and moons are
approximately circular
• The Law of Gravitation plays a key role in physics
• It allows us to calculate and understand the motion of a wide
variety of objects
• Newton’s application of his law of gravitation to motion of
planets and moons was the first time physics was
successfully applied to describe the motion of the solar
Showed that the laws of physics apply to all objects
Had an effect on how people viewed the universe
Section 5.3
Newton’s Law of Gravitation, Equation
• Law states: There is a
gravitational attraction
between any two objects.
If the objects are point
masses m1 and m2,
separated by a distance r
the magnitude of the force
Section 5.3
Law of Gravitation, cont.
• Note that r is the distance between the objects
• G is the Universal Gravitational Constant
• G = 6.67 x 10-11 N . m2/ kg2
• The gravitational force is always attractive
• Every mass attracts every other mass
• The gravitational force is symmetric
• The magnitude of the gravitational force exerted by mass 1
on mass 2 is equal in magnitude to the force exerted by
mass 2 on mass 1
• The two forces form an action-reaction pair
Section 5.3
Gravitation and the Moon’s Orbit
• The Moon follows an
approximately circular
orbit around the Earth
• There is a force required
for this motion
• Gravity supplies the force
Section 5.3
Notes on the Moon’s Motion
• We assumed the Moon orbits a “fixed” Earth
• It is a good approximation
• It ignores the Earth’s motion around the Sun
• The Earth and Moon actually both orbit their center of
• We can think of the Earth as orbiting the Moon
• The circle of the Earth’s motion is very small compared to
the Moon’s orbit
Section 5.3
Gravitation and g
• Assuming a spherical
Earth, we can consider all
the mass of the Earth to be
concentrated at its center
• The value of r in the Law
of Gravitation is just the
radius of the Earth
Section 5.3
Gravitation and g, cont.
• Since the weight of a person is also the gravitational force
between the person and the Earth, we can find the value of
• The value of g is a function only of the Earth’s mass and
radius, and the value of G
Section 5.3
Gravitation Force From The Earth –
• We assumed
• A spherical Earth
• That the gravitational force could be calculated as if all the
mass of the Earth was located at its center
• Assumptions are true as long as the density of the object is
spherically symmetric
• The object has a constant density
• The object’s density varies with depth as long as the density
depends only on the distance from the center
Section 5.3
Measuring G
• Henry Cavendish
measured the force of
gravity between two large
lead spheres
• By an experimental set-up
similar to the picture, he
was able to determine the
value of G
Section 5.3
More About Gravity – Newton’s Apple
• G is a constant of nature
• Gravity is a weak force
• Much smaller than typical normal or tension forces
• Gravity is considered the weakest of the fundamental
forces of nature
• Newton showed the motion of celestial bodies and the
motion of terrestrial motion are caused by the same force
and governed by the same laws of motion
Section 5.3a
Johannes Kepler
• Kepler studied results of other astronomer’s measurements of
portions of the Moon, planets, etc.
• He found the motion of the Moon and planets could be
described by a series of laws
• Now called Kepler’s Laws of Planetary Motion
• Kepler’s Laws are mathematical rules inferred from the
available information about the motion in the solar system
• Kepler could not give a scientific explanation or derivation of
his laws
• Newton’s Laws of Motion and Gravitation give the explanation
Section 5.4
Kepler’s First Law of Planetary Motion
• Planets move in elliptical
• The Sun is at one focus
• This was very different
from the previous idea
that the planets moved in
perfect circles with the
Sun at the center
Section 5.4
Kepler’s First Law of Planetary
Motion, Planetary Orbits
Section 5.4
Kepler’s Second Law of Planetary Motion
• A line connecting a planet to
its sun sweeps out equal
areas in equal times as the
planet moves around its orbit
• If the time required for the
planet to sweep out area A1
is equal to the time to sweep
out A2, the areas will be
• The planet’s speed will be
slowest when it is farthest
from its sun and fastest
when it is closest
Section 5.4
Kepler’s Third Law of Planetary Motion
• The square of the period of an orbit is proportional to the
cube of the orbital radius
• For simplicity, apply to a circular orbit
• Period is T and the gravitational force supplies a centripetal
• Also applies to satellites with MSun replaced by Mplanet
Section 5.4
Orbit Examples
• Low Earth orbit
• International Space Station, for example
• T ~ 90 minutes
• r ~ 6.66 x 106 m
• Geosynchronous Orbit
• T = 1 day
Always above the same position above the Earth
• r = 4.2 x 107 m
Section 5.4
Kepler’s Laws and Orbits, Summary
• Kepler’s Three Laws of planetary motion apply to all
types of gravitationally produced orbital motion
• Different motions result from different ways of initially
setting objects into motion
• For example, launching a satellite to the east into an
equatorial orbit takes advantage of the speed of the Earth’s
Section 5.4
• Tides are the fluctuations of the level of the Earth’s
• Tides are due to the Moon’s gravitational force on the
• Also exerts a force on the solid Earth
• The bulge in the ocean is due to the slightly greater force
acting on the side of the Earth facing the Moon
• Due to the dependence of gravity on distance
• The bulge is a high tide
Section 5.5
Tides, cont.
• One high tide occurs when
the Moon is directly
• The acceleration of the water
closest to the Moon is more
than that of the rest of the
• Twelve hours later the
acceleration of the solid
Earth is more than the ocean
water on the farther side
• There are generally two high
tides per day, 12 hours apart
Section 5.5
Tides, final
• The Sun also affects tides
• Its effects are smaller than the Moon’s
• When the Sun and the Moon are aligned and on the same
of the Earth, the tide is higher than when produced by the
Moon alone
Section 5.5
Inverse Square Law
• Newton’s Law of Gravitation is an example of what is
called in mathematics an inverse square law
• A number of other forces in nature are also inverse square
• Force between two electric charges, for example
• How can you explain why so many forces follow this
Section 5.6
Field Lines
• Imagine that an object
possesses gravitational field
lines that emanate from it
• Also imagine that the
number of lines is
proportional to the mass
• When the lines intersect
another object, there is a
force on that object directed
parallel to the lines
Section 5.6
Field Lines, cont.
• Since the force lines emanate in three-dimensional space,
the number of lines that intercept the second object falls
off with distance as 1/r2
• The result implies that gravity follows an inverse square
law because we live in three-dimensional space
• It also means we should expect other forces described by
the field line picture to have the same inverse square
Section 5.6
Field Lines, final
• No one has devised a way to see the line
• You can observe the resulting force that the lines are
presumed to cause
• Gravitational force is also an example of “action at a
• Newton’s Law of Gravitation tells us that action at a
distance does occur
• The Law doesn’t tell us how it occurs
• The field line picture was invented to answer this “how”
Section 5.6
• The mass in the gravitational force is sometimes called
gravitational mass
• The mass in Newton’s Second Law of Motion is called
inertial mass
• The gravitational mass of an object is exactly equal to its
inertial mass
• Needed the theories of relativity to explain why they were
• Also tells us that a photon of light will be accelerated by
Even though it is a massless particle
Section 5.6