Circular motion

1
Circular motion

• All circular motion is accelerated motion.
• To accelerate means a change in speed or
  DIRECTION.
• If either changes then there has to be some
  unbalanced force acting on object to CAUSE the
  change in behavior.
• Circular motion is curved motion around some
  central point.
• Cars going around a racetrack, planets orbiting
  the sun, galaxies spinning on their axis of
  rotation, swinging a bucket of water on a rope.

2
Centripetal force

• If the object is accelerating there must be an
  unbalanced force acting. That force is called a
  centripetal force. It acts toward the center of
  the circle of travel. It pulls the object from
  its translational path. It deflects the object
  from straight line motion.

3
Remember when?

• When you were a child, do you recall tying a
  bucket of water onto a rope and swinging it
  around in the air?
• The water did not fall out even when you swung
  the bucket upside downremember?
• That is due to centripetal force which keeps the
  bottom of the bucket in contact with the water!.

4
Which way?

• So as you swing the bucket around and around in
  the air, do you remember which way you exerted
  the force? Didnt you have to constantly tug the
  rope toward you..toward the center of the
  circleyou.
• That is the rule here. Centripetal force always
  acts toward the center of the circle.

5
And what if.?

• And what if you let go??
• Would the bucket continue to orbit your head or
  would in fly off in a straight line?
• Well, it would fly off of course since inertia
  keeps it moving in a straight line in the
  direction it was aimed when you let go!
• Let go then there is no longer an unbalanced
  force deflecting the objects path from a
  straight line into a curved path

6
Rotate or Revolve?

• Two terms
• Rotate means to spin on ones own axis. The Earth
  spins on its axis and thats what causes our day
  and night as we spin toward or away from the sun.
  
• Our Milky Way galaxy rotates around the central
  axis. We think that center point might be a
  black hole.
• Revolve means to follow a circular path around
  some external central point. It is like the Earth
  revolving around the sun. That is cause for our
  year. One year is once around the sun.
• The gravitational force of the sun is like the
  rope tied to the bucket. The suns gravity tugs
  us off a straight line path and deflects us into
  a curved path around the sun. The pull is always
  toward the sun, toward the center of the circle.
  If the sun should let go we would fly off in a
  straight line.. off into outer space, just like
  the bucket of water would fly off in a straight
  line.

7
Torque

• Torque is a force that causes objects to
  rotate..to turn around some central point.
• In this case, consider a seesaw, a teeter-totter,
  a wrench loosening a bolt.
• The external force that pushes or pulls so that
  the seesaw starts to turn is torque.
• The amount of torques applied is equal to the
  force you apply times the distance the force is
  applied from the axis of rotation.

8
Quiz

• No specific quiz on this lecture but DO expect
  these principles to show up on worksheets,
  problems etc.