Chapter 8 - Rotational Motion
Circular Motion
Trip to the Carnival…
You and your two friends decide to go ride the merrygo-round. Your one friend says she wants to sit on
an outside horse because it moves faster. Your
other friend wants to sit on the inside because he
says it doesn’t matter where you sit – all the horses
are connected so they are all going the same speed.
Who do you
agree with???
So many speeds…
• Linear Speed
– What we’ve been calling speed up to this point;
distance in meters per unit of time
• Tangential Speed
– The speed of something moving in a circular path
– Called tangential because the direction of motion is
always tangent to the circle
*For circular motion, the terms linear speed and tangential
speed can be used interchangeably*
An object will always fly off tangent
to it’s circular path…
So many speeds…
• Are tangential speeds the same for each point of a
rotating object?
• Ex: Bike Wheel
– Inner point and outer points
have the same amount of
time to travel to very different
distances
• Conclusion…
– DIFFERENT!!!
Which horse has the greatest tangential speed?
OUTTER MOST HORSE
So many speeds…
• Rotational Speed
– Refers to the number of revolutions or rotations per unit of
time
– Also called Angular Speed
– All parts of a rotating object share the same rate of
rotation
– Commonly expressed in revolutions per minute or RPM
• Relationship
– Tangential speed is directly proportional to both rotational
speed and radial distance.
Which horse has the greatest rotational speed?
ALL THE SAME
Example #1
On a rotating platform, if you sit halfway
between the rotating axis and the outer edge
and have a rotational speed of 20 RPM and a
tangential speed of 2 m/s, what will be the
rotational and tangential speeds of your friend
who sits on the outer edge?
Example #2
A lady bug sits halfway between the axis and
the edge of a phonograph record. What will
happen to its tangential speed if the RPM rate
is doubled? At this doubled rate, what will
happen to its tangential speed if it crawls out
to the edge?
…so
dizzy
…
Review Questions
1.
Why is the linear speed greater for a horse on the outside
of a merry-go-round than for a horse closer to the center?
2.
How are tangential speed and rotational speed related?
If the rotational speed of a platform is doubled, how does
tangential speed anywhere on the platform change?
3.
Consider a bicycle that has wheels with a circumference
of 2 m. What is the linear speed of the bicycle when the
wheels rotate at 1 revolution per second?