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Questions to think about
• Why do balls fall
• What pushes a flying ball in an
upwards direction
• Do objects fall sideways
• Is the moon falling
What pulls things down?
Every object in the Universe attracts every
other object due to g r a v i t y .
•
•
•
Gravity is a type of interaction.
The effect is proportional to the mass of each object.
The effect depends on the distance between the
objects.
Near the earth’s surface, the gravitational force (Fg),
exerted on an object is proportional to the mass (m) of the
object only
Fg = m ⋅ g
Geeee !
Fg = m ⋅ g
g
• Vector g, describes the local strength of gravity (the
gravitational field).
• At the surface of the earth, vector g is directed toward
the center of the earth and its magnitude has a value
of 9.8 m/s2.
g
• If the object does not interact with other objects
besides the earth, its acceleration (with respect to the
ground) has a value equal to g. Consistently, g is
often referred to as the acceleration due to gravity.
g
Warning! Do not confuse acceleration due to gravity
with the acceleration of the object.
g
Weight
The same symbol g, is traditionally used for the actual acceleration
due to gravity in any reference frame (not necessary inertial).
The weight of an object is defined as the product of the
mass of an object and the acceleration due to gravity
W=m⋅g
Puzzle 1.
Compare your weight on earth with
the gravitational force exerted on you.
W = 0 ≠ Fg
Puzzle 2.
Compare your weight in space
(earth’s orbit) with the gravitational
force exerted on you.
W = Fg
free fall (from 60 m)
vg
t [s] a [m/s2] v [m/s] x [m]
0.00
0.0 -9.8
0.0
60.0
1.0
1.00 -9.8
-9.8
55.1
acceleration
a=g
velocity
v = v0+a⋅t
2.00 -9.8 -19.6 40.0
2.0
3.00
3.0 -9.8 -29.4 15.9
position
x = x0+v0⋅t+½⋅a⋅t
upward throw
vg
t [s] a [m/s2] v [m/s] x [m]
3.00
3.0 -9.8
0.0
60.0
2.0
2.00 -9.8
-9.8
55.1
acceleration
a=g
velocity
v = v0+a⋅t
1.00 -9.8 -19.6 40.0
1.0
0.00
0.0 -9.8
29.4 15.9
position
x = x0+v0⋅t+½⋅a⋅t
projectile motion
The initial velocity has both a non-zero horizontal component
and a non-zero vertical component.
Vertical acceleration affects the vertical component of velocity.
The horizontal component of velocity is constant.
vertical
component
y
v
v
g
g
v
v
g
g
horizontal
component
x
The object moves along a parabolic trajectory (path), dependent
on the initial position, initial velocity, and acceleration.
Why doesn’t the moon fall?
Orbit: 384,403 km
Speed: 1.03 km/s
With zero initial velocity, an object
falls straight down.
7.9 km/s
With a small horizontal velocity (so
that the surface of the earth might
be considered flat), objects fall
down along a parabolic trajectory.
With a greater horizontal component,
the trajectory becomes elliptical with
the acceleration always directed toward
the earth’s center.
With a precise horizontal velocity, the
object will move along a circular orbit.