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# Mass vs. Weight: Stretching Mass

Resource for Grades 6-12

Media Type:
Video

Running Time: 0m 56s
Size: 6.7 MB

or

This media asset is from the Mass vs. Weight series produced by the Teaching From Space Office at NASA's Johnson Space Center.

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In this video, an astronaut on board the International Space Station demonstrates weightlessness by comparing two water bags tethered to a line by stretchable bands. Despite the fact that the two bags have very different masses (one is full; one is empty) neither experiences a gravitational force, as evidenced by the lack of "stretch" in the connecting tethers.

Background Essay

Mass
In normal conversation, when we use the word “massive” we’re usually referring to how big something is. Scientifically speaking, though, “mass” isn’t related to size (or volume). Mass is related to how much an object resists changes to its state of motion. Though it’s true that, for a given density, more volume will mean more mass, some objects can be “small” - in the sense that they don’t take up a lot of volume - yet still have a lot of mass. (One example would be the dense material that makes up a neutron star.) Conversely, large objects can have low mass. Think about a mountain of granite versus a mountain of cotton candy. Both might have the same volume, but they have different masses. (And one tastes better, too.) An object with more mass requires more effort – more force - to get moving from a state of rest, or to stop once it’s in motion. That quality of being easy or hard to set in motion or bring to a stop is “inertia.” An object’s “inertial mass” is its resistance to being accelerated (or decelerated) by a force.

Gravitational Force
All masses near Planet Earth feel a gravitational force proportional to their mass: the bigger the mass, the bigger the gravitational force. The equation for gravitational force is: FG = Mass x Gravity = mg. The value of “g” (the strength of the gravitational field) is unique to each planet. While g here on Earth may be ~10 m/s2, on Jupiter, g ~25 m/s2 and on the Moon, g is only ~ 1.6 m/s2. And since the value of g changes based on the gravitational field strength, the gravitational force also changes. Neither an object’s state of motion nor its specific location impacts gravitational force. What matters is the value of g.

Weight
Gravitational force is associated with acceleration in the direction of that force. Simply put, an object subject to a gravitation force will “fall” in the same direction in which the force is acting. The force it takes to counteract and balance out that falling is the object’s “weight.” Unlike mass and unlike gravitational force, weight will change based on whether there are forces acting that increase or reduce the “upward” force necessary to balance out the “downward” gravitational force; for instance the buoyant force that helps an object float in water, making it weightless.

Another example of an object’s weight changing is due to it’s acceleration relative to the gravitational field. If an object is pushed upwards so that it accelerates up, it’s weight on the surface pushing it up will increase. Alternatively, if the object is allowed to fall, it’s weight will be reduced. For example, when you’re in an elevator, going over the top of a steep bump in your car or riding a roller coaster, your weight changes because you are experiencing an upward or downward acceleration, so your weight does not completely offset the gravitational force. In each scenario, your weight is the net force required to counteract the downward force such that you experience a certain acceleration, and that value can change even though the gravitational force remains the same.

Weightlessness
When astronauts are in the space station, their mass is the same as it is on Earth. The gravitational force on the space station - contrary to what many people think – is only slightly less than the gravitational force on Earth. The space station, and everything in it, is subject to Earth’s gravity. Indeed, that’s what keeps it in orbit. However, since the station and everything on it moves together around Earth, the space station and its contents are constantly falling towards Earth; they are in free fall. They never fall to Earth, since the curvature of Earth exactly matches the shape of the orbit, but they are constantly falling, nonetheless. The station and its contents are weightless since no force is exerted to counterbalance the gravitational force. Based on what it means for something to have weight, this explains why – despite having mass and despite being subject to a gravitational force – the astronauts are weightless. The condition of weightlessness on board the space station allows astronauts to conduct experiments and demonstrations that would be impossible to do on Earth.

Discussion Questions

Before Viewing

• Can you tell how much something weighs just by looking at it? What are the effects of an object’s weight?
• Now consider the same question inside the space station, where objects have mass but appear to be weightless. Could you tell the difference between objects of different mass even if they had no weight? If so, how?
• What’s inside an “empty” bag?
• While Viewing

• Do the flexible rings holding the drink bags appear to respond differently when pulled? If so, how?
• The astronaut says one of the bags is “full” and one is “empty,” but what does she really mean?
• Which bag do you think is the one filled with liquid? What is your evidence?
• Tip: Replay the sequence when the astronaut pulls on each ring and let students observe and compare how each bag responds.
• After Viewing

• Why was it important for this demonstration to be done in space? How would the results be different if this were done on Earth?
• What are some techniques you might use to test to see which bag has liquid in it and which one doesn’t?
• Explain how an object can have mass but, at the same time, not have weight. In your own words, explain how that’s possible.
• Bonus question: Do a thought experiment in which the two bags (one filled with liquid and one not) move from the surface of the Earth to their spots on the space station. Along the way, what happens to each one’s mass? each one’s weight? Would the result be different if they were suspended in a single spot in space rather than orbiting the Earth inside the space station?

Teaching Tips

Classroom Activity: Bungee Scale

Materials needed: bungee cord; carabiners; various objects of different weights; bar, track, or other secure point from which to measure how much cord distends for each object.

Procedure
Students work in groups to measure and track how much each object hooked to the bungee cord causes it to deflect and distend. Groups create their own measurement systems using the cord with nothing attached to it as a “zero” point – and then marking and graphing each object’s effect.

Discussion Questions

• How would you describe the results of this experiment in qualitative terms? How would you describe the results in quantitative terms?
• What would happen if you exchanged the bungee cord for a material that’s tougher and less flexible? What about if you exchanged for a material that more flexible? Would your measurements change? Would the relative measurements change?
• How is this experiment similar to the demonstration in the video? How is it different? What would happen if you did the experiment in the video in the classroom? What would happen if you did this classroom experiment on board the space station?

Follow-up exercise
Students use a scale to actually weigh one or more objects in order to calibrate the measurement systems used. Students then overlay that result onto their graphs.

Transcript

Nicole: In a weightless environment, we don’t have the force of gravity pulling on us or anything around us. So what we’ve set up is two large rubber bands from here from the ceiling or what we’ll call overhead right now. We have two drink bags, one is empty one is full. I’m not going to tell you which. And if you look at the rubber bands that neither of these bags is experiencing the force of gravity. They don’t appear to have any load on them at all. It doesn’t look like the rubber band is stretching at all from one or the other. And what we can see here then is that both of these bags have mass but they don’t have weight. They don’t have the force of gravity acting on them. So the one that’s full and the one that’s empty – they both look the same.

Standards

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