Chapter 2-2: Forces and Motion
Gravity and Motion
Effects of Gravity on Matter
• Gravity – a force of _____________
between objects that is due to their masses
• Gravity can affect the position of an object
or the direction of the object’s motion.
• All __________, because it has mass, is
affected by gravity.
Size of Earth’s Gravitational
Force
• Earth has a ______ mass
in comparison to all other
objects around you.
• Earth has a ________
gravitational force.
Newton and Gravity
• Law of Universal Gravitation – all
objects in the universe attract each
other through gravitational force. The
size of the force depends on the
____________ of the objects and the
___________ between them.
• As mass increases, gravitational force
increases.
Weight
• Weight – a measure of the gravitational
force on an object – changes with position
• Mass – the amount of matter in an object
• If an object has more mass, then
gravitational force is larger and they weigh
more.
• Where do you weigh more, on Earth or on
the moon?
Units of Weight and Mass
• Weight is a force so it is measured in
__________.
• Mass is measured in _________ or
kilograms
Galileo
Gravity and Acceleration
• The acceleration on Earth due to gravity is
the same for _______ objects.
• Acceleration depends on both _________
and _________.
• Heavier objects experience more gravity
but they have more inertia, so they are
more difficult to accelerate.
Acceleration Due To Gravity
• Acceleration is the rate at which velocity
changes.
• All objects accelerate toward Earth at a rate
of 9.8 m/s2.
• For every second an object falls, its velocity
increases by 9.8 m/s.
• In the 1st second, a falling object’s velocity
is 9.8 m/s. In the 2nd second, it is traveling
at a rate of 19.6 m/s.
Velocity of Falling Objects
• The following equation is used to find the
change in velocity (Δv):
• Δv = gravity • time
• Try the practice problems in the Math
Focus section on page 151.
Air Resistance
• Air resistance is the force that opposes the
motion of objects through the air.
• The amount of air resistance depends on the size,
shape, and speed of the object.
• The larger the __________ ________ of an
object, the more air resistance acting on it.
• How do parachutes help sky divers land safely?
• They increase air resistance and slow the sky
divers to a safe terminal velocity.
Acceleration and Terminal
Velocity
• When the upward force of air resistance
balances the downward force of gravity,
the object stops accelerating.
• The object then falls at a constant velocity
called terminal velocity.
• The terminal velocity of a human without a
parachute is about 120 miles per hour.
Free Fall with no Air Resistance
• An object is in free fall if gravity is pulling
it down and no other forces are acting on it.
• Free fall can occur in a vacuum (a place
where there is no matter) or in space.
• Weightlessness is when objects are in a state
of free fall. Astronauts float in orbiting
spacecrafts because they are in a state of
free fall. They have a sensation of
weightlessness.
Orbiting Objects
• Two motions combine to cause orbiting motion:
forward due to inertia and downward (free fall)
due to gravity.
• The moon stays in orbit around Earth because of
the moon’s forward motion and Earth’s
gravitational pull.
• The unbalanced force that causes orbiting objects
to move in a circular path is centripetal force.
• Centripetal means “center seeking”.
Projectile Motion and Gravity
• Projectile motion – the curved path an
object follows when it is thrown or
propelled near the surface of the Earth.
• Projectile motion has two components:
horizontal motion and vertical motion,
which are independent of each other.
Projectiles
• Gravity does not effect the horizontal
motion of a projectile.
• When you throw a ball, the horizontal
velocity is constant.
• Gravity does have an effect on the vertical
motion of a projectile.
• Gravity causes the vertical velocity to
increase.