Download Weight and Mass (or is it Mass and Weight?)

Weight and Mass
(or is it Mass and Weight?)
WHICH ONE DEPENDS ON THE OTHER?
True or False
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The force of gravity is the same for all objects on
earth.
Gravity is caused by the rotation of the earth.
There is no gravity on the moon.
A chicken in a vacuum experiences no gravity.
You exert as large a gravitational force on the earth
as it exerts on you.
Lab
What is the mathematical relationship between the
gravitational force on an object (aka: its weight) (N)
and its mass (kg) on earth?
Weight = mass x g
 The weight of an object is directly proportional to its
mass, as shown by the equation weight=9.8(mass).
 This means if an object’s mass is tripled, its weight
triples, and if its mass is halved, its weight is also halved.
 For example, a 4 kg watermelon would weigh 39.2 N,
which is four times the 9.8N that a 1kg mango would
weigh.
What is weight?
 The size of the gravitational force on a body, exerted
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by Earth (or the chunk or matter body is on).
NOT 9.8 for all objects
NOT the same for all objects
The magnitude of the gravitational force
Units: N (newton)
Fg=9.8m or Fg = mg (g = accel. due to gravity)
Measured with a spring scale
The size (magnitude) of the gravitational
force a planet exerts on an object on or near
the planet
Fg = mg
This force acts WITHOUT CONTACT so it
cannot be directly measured.
Fg: weight
A 1.0kg mass weighs 9.8N
SI unit: N (newton)
alternate units: pounds, stones, dynes
“Measured” with a scale
Weight is NOT 9.8 for all objects
(a 5.okg mass weighs 49N)
The force of gravity is NOT the same for all
objects!
What is mass?
 The measure of an object’s resistance to
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acceleration (inertia)
One of the two properties of all matter
A SCALAR
Measured in kg in the SI system
NOT the force of gravity
NOT weight
Measured by trying to accelerate object
Mass is a fundamental property of
matter, there is no formula to define it.
A measure of the amount of inertia
(resistance to acceleration) for an
object. Mass is one way to measure
how much of something you have.
m: mass
(scalar)
SI unit: kg (kilogram)
alternate units: gram, slug
1.0 liter of water has a mass of 1.0kg
Mass does NOT change for an object,
regardless of where it is in the universe,
unless the amount of matter changes
Ex
 What is the weight (in N) of a 30 kg suitcase?
 An astronaut weighs 700N on Earth. How much
does he weigh on the moon (where g=1.6m/s/s)?
 A 30 kg child sits on a swing that consists of a level
wooden platform suspended by two vertical chains.
Determine the tension in each chain if the child
positions herself in the exact middle (midway
between the chains).