Lesson 1 of 6 · Work and power
Work transfers energy; power sets the pace
8 minute lesson · 3 checks along the way
Your goal for this lessonDistinguish work, energy, and power, and compare a motor’s energy demand with its required operating rate.
Follow a linked name to explore its archive card. “Back to your lesson” brings you to the same part of the reading.
Words with a dotted underline open a short definition right here in the lesson.
A force needs a displacement
A ground crew can pull hard on a locked mechanism without moving it. Their effort is real, but mechanical on the stationary mechanism is zero in the ideal rigid model. Work describes an energy transfer by a force acting through displacement. A constant force parallel to the displacement does work W = F × d, measured in . One joule equals one newton meter.
Direction matters. A force in the direction of motion does positive work; an opposing force does negative work. A force perpendicular to the motion does zero work at that instant. For changing forces or directions, add the contributions along the path. Holding equipment motionless can still consume electrical or bodily energy internally, so zero mechanical work on the equipment does not mean zero energy use everywhere.
An amount and a rate answer different questions
Energy measures an amount that can be transferred or transformed. measures the rate of energy transfer, in : one watt equals one joule per second. Average power is transferred energy divided by elapsed time. A motor rated for high power can perform the same ideal lifting work in less time than a lower-power motor, provided the rest of the mechanism can support that operation.
A battery’s usable energy and its maximum output power are separate limits. A supply might contain enough energy for a maneuver but be unable to deliver it quickly enough. Conversely, it might briefly deliver high power and then run out of usable energy. Engineers therefore examine both the whole operation’s energy budget and its changing power demand, including conversion losses and the spacecraft’s other loads.
Check 1 of 3
10 pointsUse the reading above. You can try again without losing points.
Work through a decision
Compare two fictional equipment moves
Suppose a motor pulls a test carriage with a constant 60-newton force through 5 meters along the force direction. The motor does 300 joules of mechanical work on the carriage. Completing that pull in 10 seconds requires 30 watts of average mechanical output. Completing the same pull in 3 seconds requires 100 watts. The work is unchanged because force and displacement are unchanged.
If the motor converts half its electrical input energy into this mechanical output, it needs 600 joules of electrical input in either case. The average electrical demands are then 60 watts and 200 watts. A fictional supply limited to 150 watts cannot support the faster case at that , even if its stored energy exceeds 600 joules. These figures describe a teaching mechanism, not an actual spacecraft motor.
Check 2 of 3
10 pointsUse the reading above. You can try again without losing points.
Read an array photograph as a clue
The International Space Station Solar Array Wings photograph shows a large illuminated collecting surface above Earth. Solar cells convert some incoming radiation into electrical energy; batteries can store energy for periods without sunlight. This connects a visible structure to an operational question: can the station meet its loads both during sunlight and while passing through Earth’s shadow? Array area is useful evidence, but it is not itself a wattmeter.
To estimate delivered power, you would need illumination, orientation, cell performance, temperature, and electrical measurements. To estimate delivered energy, you also need the duration and any changes during it. The bright appearance of the panels does not establish their electrical efficiency. Keeping amounts, rates, and measurement conditions separate lets you evaluate an energy claim before choosing a larger battery or a more powerful device.
International Space Station Solar Array Wings above Earth in NASA image ISS040-E-087258. The preserved frame shows illuminated panels and their supporting mast. It is a conventional photograph, not a map of electrical output; its golden appearance does not measure conversion efficiency.
Look closely
Find the broad collecting panels. What else would you need to estimate their delivered electrical energy over an orbit?
See what to notice
You would need power through time, or illumination, orientation, cell and electrical performance, and duration sufficient to estimate it. The photograph shows surface geometry and lighting at one moment, not the power history or battery state.
Check 3 of 3
10 pointsUse the reading above. You can try again without losing points.
Put it together
Your lesson progress
Each check connects to a part of the story. Complete all 3 to earn your 25-point lesson bonus.
Go to the source
Explore the evidence behind this lesson. These optional readings and videos open in a new tab.