Download Section 2: Gravity

Section 2: Gravity
 Law of Universal Gravitation
▪ States that every object in the universe exerts an ______________ force on
every other object in the universe.
▪ Two factors affect the strength of the gravitational force between two objects.
1. Masses of the Objects—the greater the masses of the objects, the
___________ the force of gravity between them.
2. Distance Between the Objects—the greater the distance between the
centers of the objects, the ___________ the force between them.
 Free Fall and Weight
▪ When gravity is the only force acting on an object, it is said to be in ________
__________.
▪ When objects near the earth’s surface are in free fall, they accelerate toward
the ground at a rate of ___________.
▪ The acceleration due to gravity is abbreviated using the symbol ____.
▪ All objects, regardless of their mass, accelerate at the same rate when they
are in free fall. Why? Newton’s 2nd Law of Motion explains it: an object’s
acceleration increases if the force on it increases, but its acceleration
decreases if its mass increases. So, the more massive an object is, the
greater the force due to gravity, but at the same time, it resists changes in its
motion more due to its mass. Therefore, the two effects cancel each other
out, and the object’s acceleration is unaffected by its mass.
▪ Weight—this is just a term that refers to the force on an object due to
__________. Since it is a type of force, weight has units of __________. The
force of an object’s weight is always in the __________________ direction.
 To calculate an object’s weight, just ____________ its __________ (in
kilograms) by the ________________ due to _____________.
 Example 1: How much does a 2 kg bottle of Pepsi weigh?
m = 2 kg
w = mg
g = 9.8 m/s²
w = (2 kg)(9.8 m/s²)
w=?
w = 19.6 N
 Example 2: A new-born possum weighs 2.6 × 10-3 N. What is its mass
in grams?
w = 2.6 × 10-3 N
m = w/g
g = 9.8 m/s²
m = (2.6 × 10-3 N)/(9.8 m/s²)
m=?
m = 2.65 × 10-4 kg = 0.265 g
♦ Weight and mass are not the same!!!! Weight is a ___________, while
mass is the measure of the _____________ ____ ____________ in an
object.
▪ Air Resistance—the frictional force that results when an object moves through
the air. It is caused by the collisions of air molecules with the surface of the
object. The air resistance on a particular object can be reduced by:
♦ reducing the ____________ ________ of the object.
♦ reducing the ____________ of the object as it moves through the air.
♦ making the object more ______________________ in shape.
▪ Terminal Velocity—the constant velocity of a falling object when the force of
______ ________________ is _____________ ____ __________________
and ______________ ____ _________________ to the ____________ ____
________________.
♦ Example—a 650 kg gorilla falls from a tree after being shot with a
tranquilizer dart. When the gorilla reaches a terminal velocity, how
much force of air resistance is on him?
Since the force of air resistance equals the weight of the object when the object is falling at
terminal velocity, we need to find the weight of the gorilla.
m = 650 kg
w = mg
g = 9.8 m/s²
w = (650 kg)(9.8 m/s²)
w=?
w = 6370 N, downward
Therefore, the force of air resistance on the gorilla is 6370 N upward.
 Free Fall and Motion
▪ Since the force of air resistance acts on a skydiver as she falls, she is not
technically in ________ ________.
▪ Since there is no air in space and ___________ is the only force acting on
them, astronauts orbiting the Earth are in free fall.
▪ Why do astronauts appear to be weightless when they are inside an orbiting
space station? It is because they are in _______ _______ around the Earth.
▪ Orbiting motion is the result of a combination of ____________ motion and
______________ motion caused by Earth’s gravity.
 Projectile Motion and Gravity
▪ Projectile Motion is the curved path an object follows when __________,
____________, or otherwise _______________ near the ____________ of
the __________.
▪ Projectile motion has 2 components: ______________ and ___________.
▪ The 2 components are ________________—meaning they have no effect on
each other. The horizontal motion is not affected by the vertical force of
gravity, and vice versa.
▪ Projectiles have some horizontal motion
○ After you have thrown a ball, there are no ________________ __________
acting on the ball (if you neglect air resistance). Therefore there are no
forces to change the ball’s horizontal motion, and the ball’s horizontal
velocity is ___________ after it leaves your hand.
▪ Projectiles have some vertical motion
○ After throwing a ball, the Earth’s ______________ force begins to pull the
ball downward. This downward net force causes the ball to ____________
in the downward direction (as Newton’s 2nd Law explains).
○ When this vertical acceleration is combined with the constant horizontal
velocity discussed above, a projectile will follow a curved path through the
air, called a trajectory.