Download 13-2 Notes - westscidept

13-2 Notes: Gravity and Motion
Galileo Galilei argued that the mass of an object does not affect the time it takes to
fall to the ground. Objects fall to the ground at the same rate because the
acceleration due to gravity is the same for all objects near Earth’s surface.
Acceleration depends on both force and mass.
Acceleration is the rate at which velocity changes over time. An object accelerates
when the forces on it are unbalanced. Gravity exerts a downward, unbalanced force
on falling objects. So, the objects accelerate. Falling objects accelerate toward Earth
at a rate of 9.8 meters per second per second (9.8 m/s/s). So, for every second that
something falls, its downward velocity increases by 9.8 m/s/s.
Air resistance is the force that opposes the motion of objects through air. The
magnitude of air resistance acting on an object depends on the size, shape, and speed
of the object.
As the speed of a falling object increases, air resistance increases. The upward force
of air resistance continues to increase until it is equal to the downward force of
gravity. At this point, the net force is 0 N, and the object stops accelerating. The
object then falls at a constant velocity called the terminal velocity.
Something is in free fall only if gravity is pulling it down and no other forces are
acting on it. Therefore, it can only happen where there is no air. For example, there
is no air resistance in outer space or in a vacuum.
Projectile motion is the curved path an object follows when it is thrown or propelled
near the surface of Earth. Projectile motion is made of two different motions –
horizontal movement and vertical movement. A force gives the object its horizontal
movement, which is movement parallel to the ground. Gravity gives the object
vertical movement, which is movement perpendicular to the ground.
An object is orbiting when it is moving around another object in space. A spacecraft
orbiting Earth is moving forward. But the spacecraft is also in free fall toward Earth.
You can think of something in orbit as being in projectile motion but never reaching
the ground.
Gravity provides the centripetal force that keeps things in orbit. Gravity also plays
an important role in maintaining the shape of the solar system.
Unit 5 (Ch. 13) – Motion & Forces
02/22/10
Bach
Section 13-2 Review
1. More force is needed to accelerate an object of greater mass.
Why, then, does a heavier object fall at the same rate as a lighter
object?
The gravitational force on the more massive object is greater.
2. Which two forces combine to determine the net force on a falling
object? In which direction do these two forces act?
Gravitational force and air resistance determine the net force on a
falling object. Gravitational force pulls the object downward, and air
resistance pushes upward on the object.
3. What is the difference between terminal velocity and free fall?
Terminal velocity is the constant velocity of a falling object when the
force of air resistance is equal in magnitude and opposite in direction
to the force of gravity.
Free fall is the motion of a body when only the force of gravity is acting
on the body.
4. Why can an object falling in a vacuum never reach terminal
velocity?
There is no air resistance in a vacuum, so the falling object will not
have the upward force of air resistance opposing the downward force
of gravity. As a result, the object will not stop accelerating downward
and will not have a terminal velocity.
5. What is the unbalanced centripetal force that keeps the planets in
orbit around the sun?
The force of the sun’s gravity.
6. A squirrel drops a nut over a cliff. What is the velocity of the nut
after 3 seconds? After 5 seconds? After 10 seconds? (Ignore air
resistance. Remember that the acceleration due to gravity is 9.8
m/s².)
After 3 seconds, the velocity of the nut is 29.4 m/s.
After 5 seconds, the velocity of the nut is 49.0 m/s.
After 10 seconds, the velocity of the nut is 98.0 m/s.
Unit 5 (Ch. 13) – Motion & Forces
02/22/10
Bach
13-2 Notes: Gravity and Motion
Galileo Galilei argued that the ______ of an object does not affect the time it takes
to fall to the ground. Objects fall to the ground at the same ______ because the
acceleration due to _________ is the same for all objects near Earth’s surface.
_______________ depends on both force and mass.
Acceleration is the rate at which __________ changes over time. An object
accelerates when the forces on it are _____________. _________ exerts a
downward, unbalanced force on falling objects. So, the objects _____________.
_________ objects accelerate toward Earth at a rate of 9.8 meters per second per
second (9.8 m/s/s). So, for every second that something falls, its downward
__________ increases by 9.8 m/s/s.
_____ _______________ is the force that opposes the motion of objects through
air. The ______________ of air resistance acting on an object depends on the size,
_________, and speed of the object.
As the speed of a falling object increases, air resistance ___________. The
__________ force of air resistance continues to increase until it is equal to the
downward force of __________. At this point, the net force is 0 N, and the object
stops ________________. The object then falls at a constant velocity called the
___________ ____________.
Something is in ______ ______ only if gravity is pulling it down and no other
forces are acting on it. Therefore, it can only happen where there is no ______. For
example, there is no air resistance in outer space or in a ___________.
_____________ motion is the curved path an object follows when it is thrown or
propelled near the surface of Earth. Projectile motion is made of ______ different
motions – horizontal movement and ___________ movement. A force gives the
object its horizontal movement, which is movement ____________ to the ground.
__________ gives the object vertical movement, which is movement perpendicular
to the ground.
An object is ___________ when it is moving around another object in space. A
spacecraft orbiting Earth is moving __________. But the spacecraft is also in _____
______ toward Earth. You can think of something in orbit as being in
____________ motion but never reaching the ground.
___________ provides the centripetal force that keeps things in orbit. Gravity also
plays an important role in maintaining the shape of the ________ ___________.
Unit 5 (Ch. 13) – Motion & Forces
02/22/10
Bach
Section 13-2 Review
1. More force is needed to accelerate an object of greater mass.
Why, then, does a heavier object fall at the same rate as a lighter
object?
2. Which two forces combine to determine the net force on a falling
object? In which direction do these two forces act?
3. What is the difference between terminal velocity and free fall?
4. Why can an object falling in a vacuum never reach terminal
velocity?
5. What is the unbalanced centripetal force that keeps the planets in
orbit around the sun?
6. A squirrel drops a nut over a cliff. What is the velocity of the nut
after 3 seconds? After 5 seconds? After 10 seconds? (Ignore air
resistance. Remember that the acceleration due to gravity is 9.8
m/s².)
Unit 5 (Ch. 13) – Motion & Forces
02/22/10
Bach