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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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Task

Task

(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.


Power

POWER

SPH 4C – Energy Transformations

Miss Miner


Problem

Problem

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.)


Problem continued

Problem Continued…

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?

POWER!

The monkey who does the work in a shorter time generates more power!


Power1

Power

Definition: Power (P) is the rate of doing work or transforming energy.

power = work or power = energy transferred

time interval time interval

Symbolically,

P = W or P = Δ E

Δ t Δ t


What unit do power we measure in

P = W work and energy are measured in joules (J)

Δ t

time is measured in seconds (s)

P = Δ E

Δ t

What unit do power we measure in?

Therefore power is measured in joules per second (J / s).


Si unit

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 W

SI Unit


Example 1

Example 1

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.


Example 2

Example 2

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


Example 3

Example 3

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


Lab activity

You will calculate the work and power output of a machine… you!

Lab Activity


Homework

Homework

  • Complete Pre-Lab for tomorrow’s Work and Power Lab:

    • Calculate your mass in kg (2.2 lb = 1 kg)

    • Find all necessary constants and formulas you will need for calculations

    • Bring running shoes to tomorrow’s class

  • Section 4.1, page 177

    # 1, 2, 3, 4

    (Nelson, Physics 12 College Preparation)


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