Weight vs. Mass

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Mass and weight are very different.

 

Mass = the matter in an object. Mass is measured in kilograms.

 

Weight results from gravity pulling on a mass. Gravity gives masses weight. Weight is a force! Weight is measured in Newtons.

Weight is calculated by multiplying the mass of an object by the acceleration due to gravity. Weight = mg.

 

On Earth, the acceleration due to gravity is 9.8 m/s/s.  On the moon, the acceleration due to gravity is 1.622 m/s/s. Earth’s larger mass results in a larger acceleration due to gravity as compared to the moon. In other words, you weigh more on Earth; you would weigh less on the moon. For instance, if you take a 10 kg bowling ball to the moon, it would still have a mass of 10 kg. However, its weight would be much less because the moon’s gravity is relatively weak.

 

On Earth

bowling-ball-26066827[1] 

Mass on Earth = 10 kg

Acceleration due to gravity = 9.8 m/s/s

Weight on Earth = mg

Weight on Earth = 10 * 9.8

Weight on Earth = 98 Newtons

 

 

On the Moon

bowling-ball-26066827[1]

Mass on Moon = 10 kg

Acceleration due to gravity = 1.622 m/s/s

Weight on Moon = mg

Weight on Moon = 10 * 1.622

Weight on Moon = 16.22 Newtons

 

 

 

BD08124_100 kg

Mass = the amount of material in an object. Mass is typically measured in kilograms

 

 

 

BD08124_100 kg

 

Weight = mass x gravity

= 100 kg x 9.8 m/s/s

= 980 Newtons

 




 The folks at Newton's Apple wrote the following!

Your fuel gauge is below empty. Both engines of the cargo plane you're piloting have just sputtered and gone silent. The nose of the plane points down and you begin a terrifying dive toward Earth. In a panic, you make your way out of the cockpit and into the back of the plane where your parachute is stored. But a 2,000-kilogram crate is blocking your path. What do you do?

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No problem! Since the weight of the crate on the plane's floor is actually zero, you would not have to lift it in opposition to gravity or slide it in opposition to its friction with the floor. The force required to overcome the inertia of the crate would be small enough to allow you to move it by pushing hard with your feet braced against a wall. How is this so?

 

Let's look at the crate under normal flight conditions. The weight of the crate pushes down against the floor of the plane. What you might not realize is that the floor, which is supported by the airplane's wings and the forces that keep the airplane aloft, also pushes up against the crate. It pushes up with a force equal to the weight of the crate, so inside the plane, you're aware of how heavy the crate is.

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When your plane goes into free-fall, the crate is still pulled by gravity just as during a normal flight. But the floor is no longer pushing up on the crate, since it and the crate are now falling freely toward the earth. Gravity is still acting on both the crate and the plane, but inside the airplane, without the upward push from the floor, the crate now seems to be weightless. Both the crate and the pilot will float freely inside the airplane until something--like Earth--stops them.

 

Astronauts in orbit experience weightlessness just like objects in the falling aircraft. A space shuttle in orbit is actually in a state of free-fall as it travels around Earth.

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Hard to imagine? Picture yourself in a small spaceship a few meters above the ground. Now face the setting sun and go in a straight line for about 100 kilometers (62 miles). If you go in a perfectly straight line, you should notice that Earth is curving away from you.
A shuttle in orbit goes so fast that Earth curves "away" just as much as the shuttle falls. The shuttle falls, but never hits the ground!

 


 

* Falling appears to be different for different objects. For instance, which falls faster, a pen or a piece of paper? Why might one fall faster than the other?
* In real life, when do you experience something like free-fall? For how long?
* Which falls faster, a one-ton plane or a ten-ton plane?


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VOCABULARY

acceleration change in speed during a certain period of time
ascent going up
descent going down
force that which, when acting alone on an object, causes a change in the motion of the object
gravity force on Earth which pulls all objects toward its center
orbit falling around and around Earth
resistance a force opposing the motion of an object or opposing the forces trying to set an object in motion
weightlessness feeling or being observed as having no weight



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