# Power

## Problems

### practice

- A typical adult in the United States consumes something like 2000 calories of food per day. Determine the average power generated by such an adult (assuming he or she is not gaining or losing weight).
- Determine the cost of operating a 7000 Btu, room-sized air conditioner in New York City for the duration of the summer. Assume that electricity costs 14¢ per kilowatt hour and that the air conditioner will run about 10 hours a day for 80 days.
- A supertanker doesn't come with brakes. Using engines alone, it takes a loaded supertanker 13 km (8 miles) to stop. A typical vessel of this class has a gross mass of about 150 million kilograms and a cruising speed of 50 kph (30 mph). Determine…
- the average stopping force applied to the ship and
- the average power dissipated while stopping it.

- The athlete in this video clip is performing a weightlifting maneuver known as the snatch. In this maneuver, the barbell must be lifted from the platform to a point above the head, with the arms and legs fully extended, in a single movement. The barbell must then be held motionless until the referees give the signal and then returned to the platform. In this particular video…
- the mass of the barbell is 77.5 kg (for comparison, the mass of athlete is 58 kg)
- the disks on the barbell have a diameter of 450 mm
- the video advances at 25 frames per second (with 71 frames total).

- height
- velocity
- acceleration
- applied force
- work
- power

### numerical

- A 64 kg student travels from the first floor to the fourth floor of a school (a height of 15 m).
- What total work did she do climbing the stairs?
- How long would this trip last if the student produced 240 W of power?

- A motorized winch is rated at 10.0 kW. At what maximum constant velocity can this winch raise a mass of 27,500 kg?
- A pedaling cyclist turns a 17.5 cm crank arm at 200 rpm. (The crank arm distance is measured from one pedal to the axle.) Calculate the average force exerted on the pedals if the cyclist does work at the rate of 600 W.
- How fast must a cyclist climb a 12° hill to maintain a power output of 190 W? Ignore friction and assume the mass of the cyclist plus bicycle is 85 kg?
- The graph below shows the power output vs. time for an elevator motor in operation.
- What does the area under this curve represent?
- Calculate its cumulative value at 2 s intervals. Compile your results in a table like the one below.

interval ending at 0 s 2 s 4 s 6 s 8 s 10 s interval area cumulative area - Sketch a graph of this quantity with respect to time.

- The world's most powerful laser in 1996 was the Petawatt — a special prototype laser built at the Lawrence Livermore National Laboratory (LLNL) in California. This laser produced a peak power of 1.25 petawatts, ten times more power than the previous record holding laser (which was also built at LLNL) and 1200 times more powerful than the entire electrical generating capacity of the United States. Although it is incredibly powerful, the Petawatt is not particularly energetic. Pulses from the Petawatt typically last less than half a picosecond. How long could an ordinary 60 W light bulb run on the energy delivered in one pulse of the Petawatt?
- A problem for Americans and other children of the former British Empire. James Watt defined the horsepower as being sufficient to raise 33,000 pounds 1 foot every 1 minute (often stated in reduced form as 550 foot pounds per second). Show that one horsepower is approximately equal to…
- 476.75 watts
- one pound of thrust at 375 mph

- A document based question. Read the following excerpt from the
*New York Times*.Lance Armstrong's strength and endurance sometimes seem too extraordinary to be believed.

Armstrong, a six-time winner of the Tour de France bicycle race who next month will try for his seventh straight victory, can cover 32 miles [51.5 km] in one hour of riding. In contrast, the average cyclist covers 16 miles [25.7 km]; a top marathon runner can cover 21 miles [33.8 km] on a bike.

Armstrong can ride up the mountains in France generating about 500 watts of power for 20 minutes, something a typical 25 year old could do for only 30 seconds. A professional hockey player might last three minutes — and then throw up….

- In 2008, a team of engineers at Rensselaer Polytechnic Institute in New York
developed a technique for measuring the forces generated while swimming. They first tested olympic athletes and then moved on to dolphins. The table below combines measurements taken by the researchers with the Olympic results for some of the swimmers tested. Complete the table and determine the power output of humans and dolphins.
dolphins humans Primo & Puka Ariana Kukors Megan Jendrick Beth Botsford venue Santa Cruz 2008 Seattle 2008 Sydney 2000 Atlanta 1996 style dolphin kick freestyle breaststroke backstroke distance n/a 100 m 100 m 100 m time n/a 49.62 s 67.05 s 61.19 speed 9.0 m/s typical force 1600 N 290 N 290 N 290 N power - The English scientist Thomas Young (1773–1829) was the first person to use the word energy in its modern sense. In the passage below he almost defined a new unit for power — something along the lines of the horsepower, but using people.
The daily work of a labouring man, of middle age, and in good health, will serve as a convenient unit for the comparison of moving powers of all kinds. It may be most easily remembered in this form: a man can raise a weight of 10 pounds to the height of 10 feet in a second, and can continue this labour for 10 hours a day.

- What is the power of a "labouring man, of middle age, and in good health" in watts? In horsepower?
- What is the "daily work of a labouring man" in joules? In kilocalories? In Btu?

- A 940 W motor is used to lift 200 kg of supplies 11 m above street level to the roof of a building.
- If the motor ran for 24 s how much work did it do?
- What is the final potential energy of the supplies relative to street level?
- How much work was done against friction?
- What was the average force of friction on the cable?

- Read the following passage from Wired describing an extreme cycling competition called the UCI Hour Record.
The hour is widely considered to be cycling's purest record, albeit an unusual one: Instead of requiring them to traverse a set distance, this event allots cyclists a set time of 60 minutes to pedal as many laps as they can around a velodrome. And whereas other competitive pursuits typically pit multiple athletes against one another, the hour is a solo affair. The race, if you can call it that, is against the clock….

Despite her preparation, [Olympic cyclist Evelyn] Stevens' hour attempt nearly broke her. During minutes 50 through 55, "I was physically in the most painful place I had ever been," she says. She remembers sounds fading away, her vision going dark, and her thoughts turning to all the wrong things. "You want oxygen, you want water, your body is screaming: Stop, stop, stop." Around the 55th minute, the idea of letting down her coach brought her back. "I just thought, oh gosh, he'd be so disappointed. His family, who sacrificed so much of their time with him so he could coach me, would be so disappointed!" The guilt returned her attention to her breath, to her mantra, to the state of mind she needed to traverse a then-unprecedented 47.980 km — 29.81 miles — in 60 minutes flat….

To stand a shot at the hour record, an athlete must maintain for 60 minutes a power output that most people would struggle to hold for 60 seconds. Stevens averaged just over 300 watts for the duration of her attempt. British cyclist Bradley Wiggins, who, in 2015, pedaled 54.526 kilometers (33.881 miles) to set the current men's record, is estimated to have averaged 440 watts. If you've ever paid attention to your numbers during spin class, those figures will no doubt astound you. If spinning's not your thing, imagine this: 440 watts is the energy it takes a 150-pound person to climb a flight of stairs in about 5.5 seconds. Now imagine climbing 655 flights at that pace. That's an hour.

Using the quantities stated in this passage, complete a table like the one below.

The hour record * The hypothetical 150 pound person is climbing a flight of stairs in 5.5 seconds. The real-life athletes are cycling for one hour. quantity Evelyn

StevensBradley

Wiggins150 pound

person*time

(s)distance

(m)power

(W)speed

(m/s)applied force

(N)total work

(J) - The Petawatt at Lawrence Livermore National Laboratory in California was the world's most powerful laser from 1996 to 1999, when it was decommissioned. Since 2015, when it was last upgraded, the world's most powerful has been the Laser for Fast Ignition Experiment (LFEX) at Osaka University in Japan. Both have beams of about 10
^{15}watts — a petawatt. Read the following passages and compute a more exact value for the power of the Petawatt and the LFEX. (The passage on the left is from a publication meant for a more general audience. The passage on the right is from a scientific journal.) How do the two lasers compare?Livermore's Petawatt laser operated for three years, until its last shot was fired on May 27, 1999. At full energy of about 680 joules, the shots delivered more than a quadrillion watts (or petawatt, which equals 10

^{15}watts) of power, exceeding the entire electrical generating capacity of the U.S. by more than 1,200 times. But the Petawatt's shots lasted for just a fleeting moment — less than a trillionth of a second, or 440 femtoseconds to be precise.In this work we show a method to reduce the laser pulse pedestal level, by implementing a plasma mirror (PM) device on an LFEX laser at the Institute of Laser Engineering, Osaka University. The LFEX laser is a kJ, PetaWatt class laser capable of delivering 1.6 kJ of laser light on target in 1.5 ps, with an intensity of ~2 × 10

^{19}W cm^{−2}. The PM is constituted by a transparent dielectric (usually BK7 glass) with an antireflective (AR) coating….

### investigative

- Ringworld (paid link) is the title of a classic science fiction novel written by Larry Niven in 1970. Set in the year 2850, it is the story of four adventurers (two human and two alien) who are chosen to explore an engineered world encircling a sun-like star. The Ringworld is an enormous cylindrical band with a radius roughly equal to that of the Earth's orbit and a width about the same as the diameter of the sun. It was constructed by some unspecified form of matter transmutation using the planets and minor bodies that once orbited the Ringworld's sun as raw material. The flat, inner surface is covered with a natural-looking, earth-like terrain and it spins at a speed fast enough to provide its inhabitants with the sensation of earth gravity. Thousand mile high walls along the edges keep the Ringworld's atmosphere from spilling out into space. The Ringworld is the home of hundreds of hominid species, but they are mostly non-technological. The sufficiently advanced civilization that engineered the Ringworld collapsed centuries ago and the adventurers find only its remains.
- Fill the following table with the data you will need to solve the remaining problems.
ringworld

parameterterrestrial

equivalentcalculated

value

unitradius earth-sun distance m width diameter of sun m ⋮ mass of jupiter kg ⋮ mass of saturn kg ⋮ mass of uranus kg ⋮ mass of neptune kg mass total mass kg available power luminosity of sun W n/a radius of earth m - Determine the surface area of…
- Ringworld
- the earth
- a sphere with radius equal to the Earth-sun distance

- How fast does Ringworld spin to provide its inhabitants with the sensation of normal earth gravity? State your answer in…
- meters per second
- earth days per rotation
- rotations per earth year

- Determine the…
- kinetic energy of Ringworld due to its rotation
- number of days it would take to accelerate a newly constructed Ringworld from rest up to its final rotational speed if all the solar energy that landed on its surface was converted to kinetic energy.

- Fill the following table with the data you will need to solve the remaining problems.