Circular Motion Rotation vs Revolution Rotation when an
- Slides: 19
Circular Motion
Rotation vs. Revolution • Rotation – when an object turns about an internal axis. – Think of this as spinning • Revolution – when an object turns about an external axis. – Think of this as going around something. • Example: Think of a bug on a record player.
Rotation vs. Revolution • The Earth rotates about its axis once every 24 hours – This is what creates a day – The axis of rotation is inside of the Earth (internal axis) • The Earth revolves around the Sun once every 365. 25 days – This creates a year – The axis of revolution is the Sun (external axis)
Activity • We are going to go outside for a moment and try an activity. • We are going to hold hands and spin in a circle, trying to keep a straight line of students.
Tangential speed • Which student had to move the fastest in order to keep the line straight? • Who did not have to move tangent to the circle at all? • Tangential speed = (radial distance)*(rotational speed) • An object that is twice as far from the center of rotation will have twice the tangential speed.
Which moves faster? • A merry-go-round has two rows of horses. One near the axis of rotation, and one nearer the outside of the ride. • Which horse moves faster?
Linear speed vs. rotational speed • Linear speed is the distance an object moves per unit of time. • Therefore, the horse on the outside has greater linear speed since it moved a greater distance within the same amount of time as the inner horse. • In general, linear speed is greater for objects on the outer edge of a rotating object.
Linear speed vs. Rotational speed • Rotational speed is the number of rotations (or revolutions) per unit of time. • Therefore, both horses have the same rotational speed since they both complete one revolution in the same amount of time. • Units for rotational speed - RPM
Centripetal force • Spinning an object on a string over your head. • How do you keep the object rotating? • In order for an object to be in revolution, there must be a force pulling it towards the center of the circular path.
Centripetal force • Definition: any force that is at a right angle to the path of a moving object that produces a circular motion. • Example: The Sun exerts a centripetal force on the Earth
Cars going around Bunker Hill Circle • What force keeps a car from leaving the traffic circle in front of our school? • Friction is what keeps the car from leaving the circle, therefore friction is the centripetal force.
Washing machines • During the spin cycle, the tub in the washing machine rotates at a high speed. • The inner wall exerts a centripetal force (like a normal force) on the clothes keeping them in a circular path. • The holes in the tub allow for the water to escape. • The water escapes not due to a force, but due to inertia.
Centripetal vs. Centrifugal • Centripetal means “center-seeking” – This is a force that keeps the object in a circular motion • Centrifugal means “center-fleeing” – This is the tendency of the objects to want to break away from the circular motion due to inertia.
Misconception Alert! • Consider a ladybug in a can that you are spinning around your head. • There is only one force acting on the ladybug. – Centripetal force holding the bug in circular motion. • The lady bug pushes against the edge of the can due to inertia or centrifugal tendency. • The outward push is NOT due to a force, it is due to the lady-bug’s MASS (inertia)
Roller coaster • Describe what happens to a person as they are riding a roller coaster that goes upside-down. • Why don’t the fall out of the coaster when they are upside down? – Inertia “pushes” the person to their seat. – The inertia is caused by the speed of the roller coaster. • What keeps the person in the circular motion and not leaving the circle? – The normal force exerted by the track.
Centripetal force Mathematically • Acceleration =
Cars around a turn • A 1000 kg car rounds a curve on a flat road with a radius of 50 m at a speed of 14 m/s. Will the car make the turn or will it skid? The pavement is dry and has a coefficient of static friction of 0. 6.
FLYING PIG!!!
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