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Unit 4 –Circular Motion
Objectives
1. Recognize the period of an object in circular motion is the time needed for the object to make
one complete circle or orbit. Period, T, is equal to the orbit circumference divided by the
tangential velocity. T = 2r/v
2. Recognize that as an object travels in a curved path, the direction of its velocity changes.
Centripetal acceleration is the name given to a change in velocity due to curved motion.
Centripetal acceleration is directed toward the center of the circle. ac = v2/r. Centripetal
acceleration is also referred to as radial acceleration.
3. Recognize the net force needed to curve an object away from straight line motion is directly
proportional to the object's mass, directly proportional to the square of the object's velocity and
inversely proportional to the radius of the object's path. Fnet = m v2/r
4. Recognize a net force causing circular motion is also called centripetal force = Fc = net force
toward the center of the circle.
5. Draw force diagrams for circular motion and show a net force toward the center of the circle.
6. Recognize centripetal and centrifugal force are different concepts.
Centripetal force is the name for the "net force toward the center of the circle" that causes
circular motion to occur.
Centrifugal force is the name for the apparent force that "pushes" objects away from the center
of a circle from the viewpoint of the object that is undergoing circular motion. Of course, there is
no force pushing an object away from the center of the circle, it is simply the object traveling in a
straight line, tangent to the circle as described by the law of inertia.
7. Recognize that if the speed of the object changes while moving in a circle there is also a
tangential acceleration. This is the rate of change of speed of the object and can be determined
using kinematics.
8. Determine the resultant acceleration of an object in circular motion.
9. Write down the equation that results from applying Newton's Second Law to the body, and
take components of this equation along appropriate axes to determine the magnitude and
direction of the net force, or of one of the forces that makes up the net force, the speed, or some
other unknown in situations such as the following:
(a) Motion in a horizontal circle (e.g., mass on a rotating merry go round, or car rounding a
banked curve).(b) Motion in a vertical circle (e.g., mass swinging on the end of a string, cart
rolling down a curved track, rider on a Ferris wheel).
Reference: Chapter 5 – Section 2 to 5