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Gravity
Gravity is a force of attraction that acts between all matter.
Gravity is measured by the rate at which it can accelerate mass.
SI acceleration is measured in meters per second per second (m/s
2
). Imperial acceleration is
measured in feet per second per second (ft/s
2
).
x Gravity is a fundamental property of all mass. All mass has gravity all
mass exerts gravitational forces that attract all other masses.
x The force of gravity is a vector quantity. It has both magnitude and
direction.
Gravity
Is the mutual
attraction of mass
x The force of gravitational attraction is proportional to the masses
attracting one another. The more mass an object contains, the greater
the gravitational force it exerts. Gravitation is additive.
x The earth is the most massive object that we interact with.
x The earth’s mass and our mass exert mutually attractive forces that
hold us in contact, or bring us back in contact when we attempt to
leave the earth’s surface.
x A bathroom scale provides a numerical measure of the force of
gravity acting on our mass.
x If we increase our mass (Gain weight) the reading on the scale goes up. Conversely if we reduce
our mass (Lose weight) the reading on the scale goes down.
x Since the scale is positioned between the earth and us, it records the change in the force of gravity
acting between the earth and us.
x The gravitational attraction between two objects is proportional to their masses and inversely
proportional to the square of their distance
:
2
21
D
mm
GF
Where: F = Mutual gravitational force
G = Gravitational constant
D = Distance
m = mass
Weight
Weight is a derived measure of force. Weight is the result of the
interaction between mass and gravity.
x Weight is not a fundamental quantity or force like mass or
gravity.
x Weight is a force we can “feel” when we pick up an object.
x In the SI system, force is measured in units called Newtons.
x The force exerted by a mass under the influence of gravity is
measured in Newtons.
x Weight is a measure of the effect
of the force of gravity
acting on the mass of an object.
x The measure of the weight of a given object can change
with
respect to other factors or frames of reference.
Weight
is a measure
of
the attractive
force of gravity
acting on mass
An 8-inch (203mm) pencil on earth, might feel like it weighs 8g (0.035oz.), but if you take that
same pencil to the moon it might feel like it weighs only 1/6 of what it felt like on earth. In fact if
you weighed it on both earth and the moon using a spring scale, you could prove this to be true.
A spring scale measures force not mass.
Sample Science Curriculum