Monday, Jan 11th
Physics I
• Agenda:
• Power
• Assignment:
• HW #1 due tonight!
I can…
RECOGNIZE THE UNITS FOR
WORK, ENERGY & POWER.
EXPLAIN THE RELATIONSHIP
BETWEEN WORK AND POWER.
CALCULATE POWER IN TWO
DIFFERENT WAYS.
https://www.youtube.com/watch?v=4mf6G0hIMUM
Power
• Power (P) is the amount of work (W) done per unit of time  it is the rate at
which work is accomplished or the rate of energy transfer
• When we see the word “rate” we know time is a factor
• Powerful machines are both strong (large F) and fast (large v)
• Units
• Watt (W):
𝐽
𝑠
𝑚2
𝑠3
• 𝑜𝑟 𝑘𝑔 ∙
= 𝑊 “watt”
• A watt of power is expended when one Joule of work is done in one
second
• 1 kW is 1000 watts
• Horsepower (hp)
• 1 hp = 746 watts
• What is velocity?
What do we
already know
to derive the
equation?
𝑑
𝑣=
𝑡
• What is Work?
W=Fxd
𝑊 𝐹 ×𝑑
𝑑
𝑃=
=
= 𝐹 ∙ = 𝐹𝑣
𝑡
𝑡
𝑡
Unit Conversion Example
• Convert an engine with a power rating of 134 hp to kilowatts
• Set up your conversion factors
• 1 hp = 746 W
• 1000 W = 1 kW
• You are comparing leaf blowers on Amazon. Two blowers are
exactly the same price. One has 150hp and another has 150kW.
If your only consideration is power rating, which one would you
buy?
𝑊
𝑃=
𝑡
Power
Relationships
• Power and Time  inverse
• To increase power, time must
decrease (work is constant)
• Power and Work  direct
• If we increase work, power must
increase as well (time is constant)
• Work and Time  direct
• If work increases, the time must
increase to maintain constant
power
Power Example #1
• Two physics students, Will N. Andable and Ben Pumpiniron, are in the weightlifting room. Will lifts the 100pound barbell over his head 10 times in one minute; Ben lifts the 100-pound barbell over his head 10 times in
10 seconds.
• Which student does the most work?
• Which student delivers the most power?
Power Example #2
An escalator is used to move 20 passengers every minute from the first floor of a department
store to the second. The second floor is located 5.20 meters above the first floor. The average
passenger's mass is 54.9 kg. Determine the power requirement of the escalator in order to
move this number of passengers in this amount of time.
K
U
E
SS
Complete HW #1
on GoFormative