10.1 – ROTATION & REVOLUTION
Rotation – object that turns
on its own axis
 The whole object is
connected
 The earth spinning
Revolution – does not turn
on its own axis
 Objects are not connected
 The earth in its orbit
10.2 – ROTATIONAL SPEED
 Linear Speed (tangential speed) – the distance covered in a
specific amount of time
 Varies as distance from axis changes
 Rotational Speed – the number of rotations (revolutions) per
time
 The same for all distances from axis
 Expressed as RPM
10.2 – Rotational Speed – Cont.
 Tangential speed α distance
from axis & rotational speed
 v = 2πr / T
 Tapering causes linear speed
to vary depending on radius
 Therefore the cup changes
direction
10.2 – Rotational Speed – Cont.
 Railroad trains have tapered wheels to cause the train to
change direction
 The smaller diameter wheel travels slower and causes the
train to move in that direction
10.3 – CENTRIPETAL FORCE
 Accelerations are caused by:
speeding up, slowing down or
changing direction –
centripetal acceleration, ac
 A net force causes an object to
follow a circular path
 It is always directed towards the
center
 Centripetal Force
 “center seeking”
 Fc = m v2 / r
 Not a “real” force
10.3 – Centripetal Force – Cont.
 Centripetal Force is always caused by other “real” forces –
Friction, normal, weight, tension
 Our task is to find out what the “real” force is and make
it equal to centripetal force
10.3 – EXAMPLES OF CENTRIPETAL
FORCE
 Tension is a string connected to a
can spun in a circle
 Friction between two surfaces
provide the necessary centripetal
force
 In a centrifuge, normal force of the
test tube causes it
 If “real” force is not sufficient, the
object does not follow a circular
path (meeting the requirements)
10.4 – CENTRIPETAL &
CENTRIFUGAL FORCES
 Centrifugal force – the
apparent “center
fleeing” force an object
experiences
 All objects have inertia
and want to go in a
straight line
 Something forces them
to change direction
 Always acts radially
10.5 – centrifugal force in a
rotation reference frame
 What if you are a bug
inside a spinning can
 To it, there is an force
pushing it outwards
(centrifugal)
 It is not real, as the
spinning slows and
stops, so does this
imaginary force