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.
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.
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.
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.
Classroom Activity: Weight Weight Don’t Tell Me
Materials needed: Paper and pens/pencils
Students work in small groups to diagram the relative weight of an object as it moves away from Earth. Students compare an object’s (astronaut, for example) theoretical weight at the space station’s orbital altitude with observable weight on board the actual station – to derive and understand the conditions that create weightlessness on the space station.
Nicole Stott: Hi Everyone. My name is Nicole Stott and I’m a flight engineer with Expedition 20 on the International Space Station. And we’re here today with you on the International Space Station and the Japanese Laboratory Module.
Robert Thirsk: Hi I’m Bob Thirsk. I’m also a flight engineer aboard expedition 20. The topic today is weight vs. mass, a heavy duty concept.
Nicole: So what is weight? Weight is the vertical force exerted by a mass as a result of gravity. Weight also means the strength of the gravitational pull on the object. That is how heavy is it. Weight is dependent on gravity.
Narration: On earth, in a 1g environment, an astronaut in his space suit would weight 360 lbs. On the moon, in 1/6 of the Earth’s gravity, he would weight 60 lbs. In orbit, he would weight 0 lbs because weight is dependent on gravity and the effect of earth’s gravity is not observed.
Nicole: Using more physics and mathematical terms, weight is defined as the force with which a body is attracted to earth or another celestial body equal to the product of the object’s mass and the acceleration of gravity. So in equation form, gravity equals mass times acceleration due to gravity. On earth, that’s 9.8 meters per second squared.
Robert: But what is mass? Mass is the property of a body that causes it to have weight in a gravitational field. The mass of an object is not dependent on gravity. Mass is the amount of matter in an object. Bodies with greater mass are accelerated less by the same force.
Narration: If we use the same force on these two animals, the elephant, which has a greater mass, is effected less than the mouse, which has much less mass.
Nicole: And what is gravity? Gravity is a force that governs motion throughout the universe. It holds us to the ground; it keeps the moon and this station in orbit around the earth, and the earth in orbit around the sun. It is best described as the attraction between any two masses, most apparent when one mass is very large, like the earth. The acceleration of an object toward the ground caused by gravity alone near the surface of the earth is called normal gravity, or 1g. This acceleration is equal to 9.8 meters per second squared or 32.2 feet per second squared. If you drop an apple on earth, it falls at 1g. If an astronaut on a space station drops an apple, it falls too, it just doesn’t look like it’s falling. That’s because they’re all falling together – the apple or the onion, the astronaut and the space station - all falling together.
Robert: What is microgravity? The international space station and us astronauts aboard are traveling at approximately 28,000 kilometers per hour or 17,500 miles per hour. And we’re essentially falling around the earth, creating weightlessness. The weightlessness that is felt in free fall on a ride on an amusement park, or on the international space station when it circles around the earth is microgravity. Objects in a state of free fall in orbit are said to be weightless. The object’s mass is the same, but their weight would register zero on a scale. Weight varies depending on weather you’re on earth, the moon, or in orbit but your mass always stays the same.
Robert: The first think Koichi and I are going to do is demonstrate that we’re actually aboard the International Space Station and we’re in a weightless environment so let us show a few little tricks that will make that point. I’d like to have an arm wrestle with Koichi. Let’s see what happens.
Title: How is arm-wrestling affected by microgravity? (pause for discussion)
Koichi: This is great.
Robert: One of my favorite Japanese sports is sumo wrestling. We’re gonna try to sumo wrestle.
Title: Which astronaut has more mass?
Robert: The summersault. Ok Nicole, show us your stuff. Where’d she go? Come back! It’s superwoman!
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