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Circular Motion and Equilibrium
Exam 3
31
Physics
Undergraduate 4
11/10/2011

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Term
Define force and torque
Definition
Force- act necessary to produce linear motion

Torque- the act necessary to produce angular motion (or to make an object twist, spin, or rotate)
Term
State the two conditons of equilibrium
Definition
Fnet=0

τnet=0
Term
Discuss the physics of circular motion
Definition
To produce circular motion, an object must be moving at a constant speed on a horizontal surface. It is accelerating because of the change in direction. Also, there must be a net force acting on the object for it to accelerate, and any force can produce circular motion. The acceleration and Fnet are always towards the center of the circle, and there must always be a force to keep the object in circular motion.
Term
Discuss centripetal and centrifugal forces, including frames of reference
Definition
Centripetal is a force acting towards the center of the circle

Centrifugal is a false force, which acts away from the center of the circle

Frames of reference are a set of axes which the position or motion of an object can be described

Equilibrium exists in the rotating frame if all forces balance, including an inertial "centrifugal" force
Term
What must occur for an object to tip over?
Definition
The objects center of mass must stay inside the axis of rotation
Term
Why won't a sphere tip over?
Definition
The center of mass of a sphere is located at its center, and it is always supported at its center when balancing. Its axis of rotation is also at its center, so its center of mass cannot go outside of the axis of rotation
Term
What will a net torque do to an object? Net force?
Definition
An object will stay in rotation unless acted upon by an external net torque.

If there is a net force acting on a mass, then it must be accelerating.
Term
Define center of mass.
Definition
the point at which the entire mass of a body may be considered concentrated
Term
Torque depends upon...
Definition
Lever arm (distance from the axis of rotation to the applied force) and Force
Term
If torque is constant, how are the other two quantities related?
Definition
F α1/r; inversely proportional
Term
Why does a mechanic put a pipe extension on a stubborn nut?
Definition
To increase the lever arm; τ α r, so increasing the lever arm increases the torque and the nut will come off.
Term
At maximum torque, how are the two quantities related?
Definition
F is perpendicular to r (torque is at a maximum at 90°
Term
Why is it difficult to open a jar with wet, soapy hands?
Definition
If friction is one the force acting to produce the circular motion to open a jar, then wet, soapy hands decreases the coefficient of friction, making it more difficult to open.
Term
Discuss the physics of a rock being whirled at the end of a string at constant speed.
Definition
UCM, the acceleration and force act towards the center of the circle. There is always a force keeping the rock in circular motion, once this force stops acting on the rock, it will accelerate linerally.
Term
State and discuss Kepler's laws of planetary motion.
Definition
I. Orbit of the planets are elleptical with the sun.

II. Equal areas in equal times (the closer a planet is to the sun, the faster its moving)

III. The square of a planet's orbital period (T^2) is proportional to its avg distance to the sun cubed (r^3) [T^2 α r^3]

Used experiments to discover the laws of nature
Term
State and discuss Hooke's Law.
Definition
F=kΔx

Force is proportional to displacement
Term
Who is Tycho Brahe? What did he contribute to astronomy?
Definition
well known astronomer who was assisted by Kepler. When he died, Kepler used his astronomical results to develop his own theories of astronomy.

He observed the planetary motion particularly of mars, and obtained crucial data that Kepler later used.
Term
What forces can produce circular motion?
Definition
Any force:
-normal
-tension
-friction
-gravitational
-centripetal v centrifugal
Term
Why does jupiter have a period longer than 1 year?
Definition
Earth is 1 AU from the sun, and a period is one year. Jupiter is almost 5 AU from the sun, so the period is much longer.
Term
Where is the earth moving faster around the sun, January or July?
Definition
January, because it is closer to the sun. Kepler's second law, equal areas equal times, says that when a planet is closer to the sun it is moving faster.
Term
How does a tight rope walker stay on the wire?
Definition
Balancing center of gravity, keeping their center of mass withing their axis of rotation
Term
Define linear velocity
Definition
V=Vo-at; how fast an object accelerates linearly
Term
Define angular velocity
Definition
ω=ωo-αt; how fast an object is rotating (accelerating angularly)
Term
convert 10rev/min to rad/sec
Definition
10 rev/min (2
Term
How are v and ω related mathematically?
Definition
v=Rω; v is proportional to ω
Term
Discuss τnet=Iα
Definition
I is the moment of intertia (resistance to an objects change in angular velocity)

This equation means that an object will stay in rotation unless acted upon by an external net torque.
Term
Define I, moment of inertia
Definition
Resistance to an objects change in angular velocity
Term
Write Newton's laws using τnet
Definition
Term
Define geosynchronous orbit. How far from earth's surface is a satellite in geosynchronous orbit?
Definition
Term
Can a yo-yo be whirled fast enough to be completely horizontal?
Definition
Term
Use the gravitational force to determine g as a function of distance from the center of earth. Is g constant?
Definition
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