Lesson 5 of 6 · Mass and weight
Keep mass when apparent weight disappears
8 minute lesson · 3 checks along the way
Your goal for this lessonDistinguish mass, gravitational weight, and support-force measurements in different gravity and free-fall conditions.
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Give kilograms and newtons different jobs
A tool does not lose its when it leaves Earth’s surface. Mass, measured in kilograms, describes how strongly an object resists for a given net force. Gravitational weight is a force: near a location with gravitational acceleration g, its magnitude is W = mg. Because weight is a force, its SI unit is the newton, not the kilogram.
The same sealed tool kit can have different gravitational weights on Earth and the Moon while retaining essentially the same mass. The local value of g changes. Everyday language often calls a scale’s kilogram display “weight,” but the instrument has converted a measured force into a mass estimate using an assumed gravity. That calibration is not automatically valid at another world or during acceleration.
Ask what a scale actually senses
A floor scale senses the contact force between its platform and the object pressing on it. For an object stationary on a horizontal scale near Earth’s surface, the upward support balances , so the reading can represent mg. If the scale accelerates vertically, support and gravitational weight need not match. A changing reading does not necessarily mean that material appeared or disappeared.
In ideal , the object and scale accelerate together under gravity without the platform needing to support the object. The support-force reading becomes zero even though gravity remains. Astronauts in orbit experience approximately this situation. Calling it distinguishes the tiny support forces from the substantial gravitational attraction that keeps an orbiting spacecraft’s path curved.
Check 1 of 3
10 pointsUse the reading above. You can try again without losing points.
Work through a decision
Check a kit’s transport label
An invented 12 kg kit is tested at rest on level scales at two sites. Use rounded values g = 10 m/s² on Earth and g = 1.6 m/s² on the Moon. Its gravitational weights are 120 N and 19.2 N respectively. Its mass remains 12 kg. A shipping label changing the kit’s mass to 1.92 kg on the Moon has confused reduced force with reduced .
In a separate horizontal test with other horizontal forces negligible, a net 6 N push gives that same kit acceleration 6/12 = 0.5 m/s². Lower lunar gravitational weight does not make the kit respond like an almost massless object. In a freely falling cabin, a crew member must still exert force to start or stop its motion relative to the cabin.
Check 2 of 3
10 pointsUse the reading above. You can try again without losing points.
Use the archive to ask about handling
The STS-73 archive photograph shows one crew member holding a metal tool and an open tool tray inside the laboratory module. It offers a useful handling problem: a tool can be easy to support in an orbiting cabin yet still require a controlled push to move or stop. The photograph does not supply the tool’s mass, force history, or acceleration.
For a practical description, record three separate quantities: mass in kilograms, local gravitational acceleration, and any measured . State whether the object is resting, accelerating with a floor, or freely falling. These distinctions explain why “weightless cargo” still needs careful restraint. They also prevent a zero scale reading from being misused as evidence that either gravity or the cargo’s mass has vanished.
The archived STS-73 color photograph STS073-351-024 shows one crew member holding a metal tool and an open tool tray inside the laboratory module. Supporting a tool in an orbiting cabin differs from accelerating or stopping its mass; neither its mass nor a scale reading is measured by this frame.
Look closely
Would easy support of this tool in a freely falling cabin establish that it takes no force to stop the tool once moving?
See what to notice
No. The tool retains mass and inertia. Changing its velocity requires a net force even when the supporting contact force is small. The person’s pose does not supply the tool’s mass or a numerical force measurement.
Check 3 of 3
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Put it together
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