How does gravity
affect falling
objects?
Gravity and Falling
Aristotle vs Galileo
Aristotle believed that increased mass would
equal increased acceleration, but Galileo
proved that all objects accelerate to Earth at
the same rate due to GRAVITY.
So objects dropped from the same height will
travel at the same speed and land at the
same time.
Free Fall and Acceleration
due to Gravity
Free fall- only occurs when gravity is the only force
acting on an object (free fall acceleration is
directed toward the center of earth)(NO Air
Friction or applied force)curs in space and a
vacuum
The acceleration of gravity (g) for objects in free
fall at the earth's surface is 9.8 m/s2. (This
would be different on another planetary body)
Galileo found that all things fall at the same rate
in the absence of air resistance, regardless of
their mass.
Falling Objects
The rate of falling increases by 9.8
m/s every second.(constant
acceleration leads to an increase in
velocity)
So, every second the object is
falling it gains velocity.
Objects dropped from a higher
position will hit at a faster speed
and with more force. F=MA
Force= Mass x Acceleration
The longer an object is falling the
faster it is moving.
Weight = the force of gravity
The weight of an object is, by definition,
the strength of the force of gravity pulling
the object downward.
W=mg
force of gravity
newtons
kg
So objects with more mass will be pulled with more force from gravity, therefore
the object weighs more.
The object will land with more force,
but not more speed!!!
How can we calculate the speed at which
an object hits a surface?
Formula: velocity= g x t
Velocity will be 0m/s at rest.
Velocity= the speed at which it hits or stops
g= 9.8m/s/s ( this is the acceleration due to gravity
t= time it took the object to fall
Let’s practice a problem.
Alice drops a penny from the top of her apartment
building. What is the penny’s velocity after it falls
for 2 seconds.
Velocity = g(acceleration) x t
Velocity= 9.8m/s/s x 2 =
Velocity of the penny = 19.6m/s down
If it takes the penny 10 seconds to hit the ground,
what is its final velocity?
Velocity= g x t
Velocity = 9.8m/s/s x 10 s
Velocity= 98m/s down
Remember air friction is negligible!!
Air Resistance and Terminal
• In air…
Velocity
– A stone falls
faster than a
feather
• Air resistance
affects stone
less(shape)
• In a vacuum
– A stone and a
feather will fall at
the same speed.
So, what happens when there is significant air
friction?
Air Resistance Happens!!!
It is the force that opposes motion of objects
through the air. (pushes in the opposite
direction of the motion)
Air resistance is dependent on size,
shape, and speed of the object.
Factors Affecting Air Resistance
Larger Surface Area= more air friction or
resistance to fall
As speed increases air resistance
increases
(air molecules are packing up as the object
moves faster and faster)
Eventually the upward force of the air will
equal or balance the downward force of
gravity (Net force will equal 0)
When this happens you have
TERMINAL VELOCITY
Terminal Velocity: When gravity and air
friction are balanced. Inertia continues
to make the object fall, but it is no longer
accelerating. ItFalls at a constant speed will
fall at a constant speed!!
Terminal Velocity is not
the same for all objects.
– A person falling reaches a
terminal velocity of around
54 m/s which means the
person stops accelerating b/c:
– Force of air resistance =
gravitational force
– With a parachute,
terminal velocity is only
6.3 m/s
• Allows a safe landing
Falling Continued
A ball thrown
horizontally will
fall at the same
rate as a ball
dropped directly.
The horizontal
motion is not
affecting the rate
of vertical
acceleration!!!
More Free Fall
A ball thrown into the air
will slow down, stop, and
then begin to fall with the
acceleration due to
gravity.
Guess What?
The rate at which it slows
per second on the upward
vertical motion is also
9.8m/s/s!!
What about Orbiting Objects?
They are in True FREE FALL!!!!
No other forces are acting on these objects.
Objects cannot be weightless anywhere in
the universe because there is always some
amount of gravity pulling on them.
Astronauts seem to float, but they are
really falling around the planet.
SPACE MAN
N.A.S.A
Free Falling Orbits
2 motions combine to cause orbiting
Forward motion and a free fall
As objects move forward, they are also
falling this combination makes the result a
fall around the gravitational object such as
Earth.
Ex. Moon orbiting Earth
Satellites orbiting Earth
Planets orbiting the Sun
Projectile Motion
•An object thrown upward at an angle to the
ground follows a curved path called a
parabola.
• combines vertical and horizontal motion
•Orbiting objects- forward motion combines
with free fall and object follows a curved path
Projectile motion
Acceleration due to Gravity
Review
• Dropping something from a resting
position (gains speed as it falls so it
accelerates)
• Gravity pulls objects down at the same
rate (air resistance can affect how fast) (no
air resistance- in a vacuum)
• When gravity is the only thing that affects
falling objects → FREE FALL
• Acceleration due to gravity is 9.8 m/s2 on
Earth
• Orbiting= objects falling around an object