Transcript 6.1 Power Notes

Chapter 6.1 Notes
Power
• The time it takes to complete
an activity is as important as
the work required.
• Compare running up stairs to
walking up stairs. When you
run, you do the work faster.
• The rate of doing work or
expending energy is power.
•Work requires that an
object moves.
•Power is a measurement
of how fast something
moves.
•Units for power is watts.
•Power equation if
work is known=
Power = Work /
time
•P = W / t
• An engine does 5000
Newtons of work in 10
seconds. How much
power is the engine doing?
• P = W/t
• P = 5000 J / 10 s
• P = 500 J/s or Watts
• Power equation if
force and distance are
known = (Force x
distance) / time
• P = (F x d) / t
•To increase your power,
you must increase Force
or distance.
•To increase your power,
you can also decrease the
time.
• A force of 20 Newtons moves a
car a distance of 50 meters in 5
seconds. How much power was
required?
• P = (F x d) / t
• P = (20 N x 50 m) / 5s
• P = (1000 Nm)/5s
• P = 200 Watts
•Power when gravity is
only force = (mass x
gravity x distance) /
time
•P = (m x g x d) / t
• Mass of an elevator is 1200 kg. A
motor raises the elevator 15
meters at a constant speed for 10
seconds. What is the power of
the motor?
• P = (m x g x d) / t
• P = (1200 x 10 x 15) / 10
• P = (180,000) / 10
• P = 18,000 Watts
•Power if a constant
force other than
gravity is at work =
force x velocity
•P = f x v
• A car has a drag force of 300 lbs
and is capable of a constant
velocity of 60 mph. How much
power does the engine have?
• P=fxv
• P = 300 x 60
• P = 18,000 Watt or HP
• Not all energy or power is
converted. Some is lost due to
friction, air resistance, and
gravity
• To find the efficiency of an
engine’s power = Output
Power / Input Power
• Eff = Po/Pi
• An elevator motor has an output
power of 2 kW and an input
power of 15 kW. What is the
efficiency and percent efficiency?
• Eff = Po / Pi
• Eff = 2/15
• Eff = .13 or 13%
•Power in a rotational
system = Torque x
angular speed
•P = t x w
• The angular speed of a record
is 750 rad/s. The Torque is
.01 J. How much is the
power of the record?
• P=txw
• P = .01 x 750
• P = 7.5 Watts