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# Periodic Motion P2

## on Jan 14, 2008

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nice ppt of physics

nice ppt of physics

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## Periodic Motion P2Presentation Transcript

• Introduction to Periodic Motion CP Physics - Sly
• Periodic Motion: a comparison Projectile motion was an object going up and coming down, ONCE! -vs.- Periodic motion is a repeating motion, that can be described with Newtonian physics and our kinematics equations.
• So we will be looking at the following examples:
• Circular Motion
• Torque
• Simple Harmonic Motion
• (Springs and Pendulums)
The only thing is that these situations require us to describe things differently.
• The human body cannot sense constant velocity… But it has a great ability to sense small shifts in acceleration .
• I. Circular Motion Remember that acceleration is a change in speed or direction.. So an object spinning in a circle can have constant speed, but since its direction is changing, the velocity is changing and therefore acceleration. In circular motion its called Centripetal Acceleration . * Centripeta l: meaning center seeking or towards the center.
• Centripetal acceleration It it a difficult derivation, that I do not want to spend time on, but based on taking two points very close together on a circle (that can be connected by a tangent), we get… a c =  v = v 2  t r
• What is Uniform Circular Motion?
• Based on the car looping around the track, what do you gather?
• Understanding a circle to understand velocity
• Remember that circumference = 2  r
• Trig tells us one complete cycle is the period (T)
• So velocity is distance around circle divided by time
• V = 2  r
• T
• Why use this for velocity? This makes it easier to determine the velocity of the object, since distance and time can be measured without interfering in the motion.
• Put these two equations together. (derive as a class)
• We also have a force
• Since there is acceleration toward the center, there is also a force towards the center …
Centripetal Force F c = ma c = mv 2 = m(4  2 r) r T 2
• Circular Motion cont. Lets look at this in action…
• One last item, fictional forces
• To compensate, objects in an accelerating reference frame invent fictional forces to explain motions in Newtonian terms.
• In circular motion, this fictional force is called centrifugal force, or in outward direction opposite of centripetal.
• WHY? (based on momentum)
• So think about riding in a car
• Consider riding along in a car with your crazy friend, the one who likes to take turns 50 mph. What forces do you feel in a turn to the left in this scary song?
If you say a force to the right then you have identified centrifugal force due to your body wanting to move in a straight line.  NOT A REAL FORCE When you press against the door, the door applies a centripetal force (along with friction on the seat) to make you turn.