SECTION 2 (PART 1) – Gravity
LEARNING GOALS
 Describe gravitational force.
 Distinguish between mass and weight.
GRAVITY
 Gravity: an attractive force between any
two objects
 Depends on the masses of the objects and
the distance between them
GRAVITY
Your book is close
enough to exert a
force you can feel
but it is too small.
You
The Earth is large
enough and
close enough to
exert a noticeable
force.
Jupiter is large
enough to exert a
noticeable force,
but its too far
away.
THE LAW OF UNIVERSAL
GRAVITATION
 Newton’s equation enables the force of gravity to be
calculated between any two objects if their masses
and the distance between them is known.
G = universal gravitation constant
m1 = mass of the 1st object
m2 = mass of the 2nd object
d = distance between the two masses
THE LAW OF UNIVERSAL
GRAVITATION
 As the distance between two objects
increases, the gravitational force
between them decreases.
 No matter how far apart two objects
are, the gravitational force never
completely goes to zero!
THE EARTH’S GRAVITATIONAL
ACCELERATION
 Close to the Earth’s surface, the acceleration
of an object in free fall is 9.8 m/s2.
 Free fall ignores all forces except gravity!
 This acceleration constant is sometimes
labeled g.
THE EARTH’S GRAVITATIONAL
ACCELERATION
 Using Newton’s
of Motion, the
force of Earth’s gravity on a falling
object is
F = force of gravity on a falling object (N)
nd
2 Law
F = mg
m = mass of the object (kg)
g = gravitational acceleration constant
(9.8 m/s2)
FORMULA SHEET
F = mg
m = F/g
g = F/m
EXAMPLE
 What is the gravitational force on a sky
diver with a mass of 600 kg?
WEIGHT
 Even if you aren’t falling, the force of Earth’s gravity is
still pulling you downward.
 Weight: the gravitational force exerted on an object
 Weight can be calculated by the following equation:
W = mg
FORMULA SHEET
W = mg
m = W/g
g = W/m
WEIGHT AND MASS
 Weight and mass ARE NOT THE SAME
THING!
 Weight is a measure of force, mass is the
measure of the amount of matter in an object.
 Weight changes as gravity changes; mass does
not.
EXAMPLE
 A person on Earth has a mass of 54 kg,
what is their weight?