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# Mass vs. Weight: Air Powered Mass

Resource for Grades 6-12

Media Type:
Video

Running Time: 1m 58s
Size: 14.2 MB

or

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

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In this video, astronauts on board the International Space Station conduct an experiment using an air gun to exert a consistent force on two difference masses -- an empty water bag and a full water bag. With different masses, the two bags respond differently to the force, in accordance with Newton's Second Law: F=ma.

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

• What if you tried to weigh yourself under water? What do you think the scale would read, compared to what it would read on land? Why?
• Imagine traveling between the Earth and the moon: How would your weight change? Is there some point where you would weight nothing?
• Is it easier to get something moving if it’s big and heavy or if it’s small and light? Why? What effect would this have on the acceleration?
• While Viewing

• How do you know the same force is being applied in both cases?
• Do the two bags appear to be different? If you had one bag with water and one bag without water on Earth, would they appear to be different?
• Which bag moves more easily in response to the force?
• Tip: Go back to the top-bottom, split-screen comparison and step through the sequence frame by frame, discussing the difference between velocity and acceleration.
• After Viewing

• One bag is filled with water and one bag has no water. Why is that difference important for the experiment?
• What are some other ways the astronauts might have applied a force to the bags?
• What would have happened if, instead of using a full drink bag, the astronauts had used several empty bags bound together to match the same mass as the drink bag? Would the results be the same, even if the combined bags took up more room than the single full drink bag?
• Bonus question: What if one astronaut used the air gun to accelerate the other? What do you think would happen? How would the astronauts’ motion compare to the motion of the bags?

Teaching Tips

Classroom Activity: "Air Power”

Materials needed: hair dryer(s) or other focused air source, toy race car on wheels, stack of quarters

Procedure
Students work in groups using a timed burst from the hair dryer to apply a force to a racecar. With each subsequent trial a quarter gets added to the car. Students observe and track the results – creating a graph of how long it takes the car for each trial to travel a given distance and discussing how this relates to F=ma and to the experiment in the video.

Discussion Questions

• Even though F=ma includes acceleration as a variable, this activity tracks the time it takes to travel a given distance. How exactly does that variable relate to acceleration – and why it is reasonable to use it to interpret the results?
• At some point, the car will be too heavy for the dryer to have any effect. Why does that happen? Would that happen on board the space station? Why or why not?
• Why is it important that the burst of air is timed – that it isn’t continuous?
• If the dryer has two power settings, try them both. How do the results differ?

Transcript

Koichi: Bob now has a water container which is an empty container. And also we have an air gun. And from this cylinder there is an air exit here and Bob can squeeze the air out of this. We will demonstrate how this water container will move with the force of the air. [Demonstration]

Koichi: Okay as you saw with the empty or the filled with the air, this drink bag moved pretty quickly. And now we have another bag which is filled with water and looks the same but inside is filled with water so the mass is different. Now we will demonstrate the same thing with this water bag. [Demonstration]

Koichi: And as you can see with the higher mass this bag did not move as quickly as the other bag with the air. We will do that again. [Demonstration]

Koichi: Okay so that was the demonstration and you saw the difference between the air filled bag and the water filled bag. Since the masses are different you saw the difference in acceleration as a result from the same forced applied to the bag.

Title: Which bag do you think is empty (less mass) and which one is full (more mass)?

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