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Centripetal Force. Today you are going to study an object that moves in a circle. An object that moves in circular motion must have a force acting on it that is directed toward the center of the circle. This could be something as simple as a string pulling a ball into circular motion.

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Presentation Transcript
slide2
Today you are going to study an object that moves in a circle.
  • An object that moves in circular motion must have a force acting on it that is directed toward the center of the circle.
  • This could be something as simple as a string pulling a ball into circular motion.
slide4
The string is pulling on the ball.
  • Strings cannot push.
  • The circular motion could be a road forcing a car to turn a curve.
slide6
Even in a vertical loop amusement park ride,

when a car is at the top of the loop,

the track is actually pushing it downward toward the center of the circle

in which it is travelling at that moment.

slide7
Forces that make objects move in circular motion are called

centripetal forces.

  • Centripetal means “center-seeking.”
  • This force should not be confused with the psuedo force commonly known as centrifugal.
  • Centrifugal means “center-fleeing,” and

centrifugal forces are not real.

slide8
Today you will measure the centripetal force in a particular circular motion and

show that it satisfies

Newton’s Second Law:

.

slide9

Radius

String

Pulley

Bob

Spring

Index

Slotted

Masses

The apparatus you will use is shown below.

slide11
The acceleration in this circular motion is one associated with a change in the direction of the velocity vector, not the length of the velocity vector.
  • It, just like the centripetal force, also points toward the center of the circle.
  • To calculate the acceleration you have to determine the angular velocity.
slide12
Angular velocity is an angle measurement divided by time.
  • For example if you make one full spin in 2 seconds of time, then your angular velocity would be 3600 divided by 2 seconds which reduces to 1800/s.
  • Many of you have heard about 33 and a 1/3 rpm phonographic records that your parents or grandparents had when they were young. The rpm stands for revolutions per minute, and it is an angular velocity measurement.
slide13
In lab today you will determine an angular velocity based on an angular measurement of radians instead of degrees.
slide14

6 segments gets

to here.

One radian is the angle subtended at the center of a circle by an arc equal in length to the radius of the circle.

2

3

1

57.30

2p segments gets completely around.

4

6

5

1 rev = 3600 = 2p radians (rad)

slide15
To get the angular velocity measured in terms of rad/s, you will make the following measurements.
  • Count the number (N) of cycles the apparatus makes, and measure the time (T) to make these N turns.
  • Repeat until you have three different time measurements involving N turns each time.
  • Take the average of the three T’s.
slide16
Divide this average T by N to get the average time (t) for one rotation. In other words

t= T/N

  • Then the angular velocity (w) is

w = 2p/t

.

.

slide17
Though it is not shown here, it is not difficult to show that the centripetal acceleration (a) is given by

a = w2R

where R is the radius of the circle.

slide18

Radius

String

Pulley

Bob

Spring

Index

Slotted

Masses

  • Once you have a, you will multiply it by the mass of the swinging object (the bob). (The value will be on the blackboard.)
  • You will then compare this force to the force necessary to position the bob at a distance R from its rotation axis when the apparatus is not spinning.

For your lab exam, you must know this method of

determining the centripetal force.