Task. (1) What is gravitational potential energy, E g ? (2) How do we calculate E g ? (3) Estimate your own gravitational potential energy relative to the lowest floor in the school when you are standing at the top of the stairs of the highest floor. POWER. SPH 4C – Energy Transformations
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(1) What is gravitational potential energy, Eg?
(2) How do we calculate Eg?
(3) Estimate your own gravitational potential energy relative to the lowest floor in the school when you are standing at the top of the stairs of the highest floor.
SPH 4C – Energy Transformations
Two monkeys of equal mass climb identical rope ladders 12 m in length, one in 4.0 s and the other in 6.0s. How does the work done by each monkey against gravity compare?
(Recall: the work done equals the gravitational potential energy at the top relative to the bottom, Eg=mgh.)
The work done by both monkeys is the same!
But if the times were different, there must be some other factor to explain the difference?
The monkey who does the work in a shorter time generates more power!
Definition: Power (P) is the rate of doing work or transforming energy.
power = work or power = energy transferred
time interval time interval
P = W or P = Δ E
Δ t Δ t
P = W work and energy are measured in joules (J)
time is measured in seconds (s)
P = Δ E
Δ tWhat unit do power we measure in?
Therefore power is measured in joules per second (J / s).
watt (W): 1 W = 1 (J / s)
Named after Scottish physicist and inventor, James Watt (1736 - 1819)
He designed and built the first practical steam engine
He compared the power of his steam engine to the rate at which a horse could pull coal out of a mine
The imperial unit “Horsepower” (hp) is the average power output of a working horse
1 hp = 746 WSI Unit
A speedboat transforms 2.25 x 105 J of energy in 5.00 s. What is the speedboat’s power? Express the answer in watts and kilowatts.
A 52- kg student climbs 3.0 m up a ladder in 4.7 s. Calculate
(a) the student’s gravitational potential energy at the top of the ladder relative to the bottom
(b) the student’s power for the climb
A 1.0 x 103 kg elevator lifts a 85 kg man and a 68 kg woman from the ground floor of a high-rise building upwards at a constant speed of 3.0 m/s for 6.4 s. Calculate
(a) the gravitational potential energy of the elevator
at 6.4 s.
(b) the power needed to lift the elevator
# 1, 2, 3, 4
(Nelson, Physics 12 College Preparation)