Download 13.1 Mass versus Weight

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13.1 Mass versus Weight
What is the difference between mass and weight?
mass
Mass is a measure of the amount of matter in
an object. Mass is not related to gravity.
The mass of an object does not change when it
is moved from one place to another.
weight
Weight is a measure of the gravitational force
between two objects.
The weight of an object does change when the
amount of gravitational force changes, as when
an object is moved from Earth to the moon.
Mass is commonly measured in grams or
kilograms.
Weight is commonly measured in newtons or
pounds.
Weightlessness: When a diver dives off of a 10-meter diving board, she is in free-fall. If she
jumped off the board with a scale attached to her feet, the scale would read zero even though she
is under the influence of gravity. She is “weightless” because her feet have nothing to push
against. Similarly, astronauts and everything inside a space shuttle seem to be weightless because
they are in constant free fall. The space shuttle moves at high speed, therefore, its constant fall
toward Earth results in an orbit around the planet.
Weight = mass x
gravity
(newtons = kilograms x 9.8 N/kg)
W = mg
•
On Earth’s surface, the force of gravity acting on one kilogram of mass is 9.8 newtons (N)
(9.8 N is equal to about 2.2 pounds). So, if an object has a mass of 2.0 kilograms, the force of
gravity acting on that mass on Earth will be:
2.0 kg x (9.8 N/kg) = 19.6 N (
•
pounds)
On the moon’s surface, the force of gravity is about six times weaker than on Earth or 1.6
newtons per kilogram (0.35 pounds). The same object, if it were carried to the moon, would
have a mass of 2.0 kilograms, but its weight would be just 3.1 newtons.
2.0 kg x (1.6 N/kg) = 3.2 N (
pounds)
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13.1 Mass versus Weight
Problems to solve:
1. What is the weight (in newtons) of a 7.0-kilogram bowling ball on Earth’s surface?
2. What is the weight (in newtons) of a 7.0-kilogram bowling ball on the surface of the moon?
3. What is the mass of a 7.0-kilogram bowling ball on the surface of the moon?
4. Would a triple beam balance function correctly on the moon? Why or why not?
5. The weight of an object is 600 N, what is the mass of the object in kg?
6. A weight lifter can lift 300N, what mass in kilograms can she lift?
7. Calculate the weight of an object with a mass of 150 kg?
8. Anna weighs 98 N, if her mass is 10 kg, how much force due to gravity is pulling on Anna?
9. A wooden toy has a mass equal to .43 grams, determine its weight in newtons.
Challenge Question
10. Take a bathroom scale into an elevator. Step on the scale.
a. What happens to the reading on the scale as the elevator begins to move upward? to move
downward?
b. What happens to the reading on the scale when the elevator stops moving?
c. Why does your weight appear to change, even though you never left Earth’s gravity?