Download Chapter 6 Forces in Motion

Chapter 6
Forces in Motion
Section 1:
Gravity and Motion
Gravity and Motion
• All objects fall with the same acceleration
(remember that acceleration is the rate at which
velocity changes… Acceleration = Δ v
time)
Objects fall to the ground at the same rate
because the acceleration due to gravity is the
same for all objects.
Accelerating at a constant rate
• All objects accelerate toward Earth at a
rate of 9.8 m/s/s (For every second an
object falls, the objects downward velocity
increases by 9.8 m/s) regardless of the
mass
Velocity of falling objects
• To find the change in velocity (Δ v) of a
falling object, multiply the acceleration due
to gravity (g or 9.8 m/s/s) by the time it
takes that object to fall in seconds.
Δv=gxt
Let’s look at an example.
Problem 1
A stone at rest is dropped from a cliff, and it
takes 3 seconds to hit the ground. Its
downward velocity when it hits the ground
is?
Δv=gxt
Δ v = 9.8 m/s x 3s =29.4 m/s
s
Problem 2
A penny at rest is dropped from the top of a
tall stairwell.
What is the penny’s velocity after it has
fallen for 2 seconds?
9.8 m/s x 2 s = 19.6 m/s
s
The penny hits the ground in 4.5 seconds.
What is its final velocity?
9.8 m/s x 4.5 s = 44.1 m/s
s
Problem 3
A boy standing on a high cliff dives into the
ocean below and strikes the water after 3
seconds. What is the boys velocity when
he hits the water?
29.4 m/s downward
Problem 4
A rock falls from a high cliff and hits the
ground in 6.5 seconds. What is it’s final
velocity?
63.7 m/s downward
Problem 5
A brick falls from the top of a building and
strikes the ground with a velocity of 19.5
m/s downward. How long does the brick
fall?
2 seconds
Accelerating at a constant rate
Air Resistance Slows down
Acceleration
• Fluid friction is also known as air
resistance. Air resistance occurs between
the surface of falling objects and the air
that surrounds it.
Air Resistance Affects Some
Objects More Than Others
• The amount of air resistance acting upon
an object depends upon the size and
shape of the object
Acceleration Stops at the Terminal
Velocity
• As an object falls, air resistance continues
to increase until it exactly matches the
downward force of gravity. The object has
then reached its terminal velocity…or a net
force of zero
Free fall occurs when there is no
air resistance
• Free fall only occurs if there is no gravity
pulling it down and no other forces are
acting upon it.
Orbiting Objects Are In Free Fall
• Free fall can be either ascending or
descending. If you could toss a ball
upward with no forces other than gravity
acting upon it (no air resistance), its entire
path (up and down) would be in free fall.
Two Motions Combine to Cause
Orbiting
• An object is orbiting when it is traveling in
a circular or nearly circular motion around
another object.
• While a space ship orbits the Earth, it is
also in free fall toward the Earth.
Remember the Space Station example?
How an Orbit is Formed
Why doesn’t the space shuttle fall to Earth if gravity is
pulling it downward?
What would happen if the shuttle started moving much
faster or much slower?
Projectile Motion and Gravity
• Projectile motion: the curved path an
object follows when thrown or propelled
near the surface of the Earth.
• Projectile motion has 2 components
horizontal and vertical and one has no
impact on the other (independent). When
combined, they formed a curved path.
Projectile Motion and Gravity
Horizontal Motion
• Horizontal motion is motion parallel to the
ground
Vertical Motion
• Vertical motion is motion that is
perpendicular to the ground
Chapter 6
Forces in Motion
Section 2:
Newton’s Laws of Motion
Newton’s First Law of Motion
• Newton’s first law of motion states that the
motion of an object will not change if no
unbalanced forces act upon it.
Inertia
• Inertia is the tendency of matter to resist a
change in motion.
• Mass is a measure of inertia
Newton’s Second Law
• Newton’s second law of motion states that
the acceleration of an object depends on
its mass and the force exerted upon it.
Expressing Newton’s Second Law
a= F/ m OR F = m x a OR m = F/a
a = acceleration, m = mass, F = force
Let’s try a few
Second Law Problems
You are hitting an object of unknown mass
with a force of 15 N and that object
accelerates at 5 m/s/s. What is the mass
of the object?
If F = m x a, and 1N = 1kg x m/s/s, then
15 kg x m/s/s = m x 5 m/s/s
3 kg = m
Second Law Problems
What is the acceleration of a 7 kg mass if a
force of 68.6 N is used to move it toward
the Earth? (Hint: 1N = 1 kg x m/s/s)
a = F/m
a = 68.6 N / 7 kg OR a = 68.6 N
7N x m/s/s
a = 9.8 m/s/s
Second Law Problems
• What force is necessary to accelerate a
1,250 kg car at a rate of 40 m/s/s?
F=mxa
F = 1,250 kg x 40 m/s/s OR (1,250 N x m/s/s) x 40 m/s/s
F = 50,000 N
Second Law Problems
• What is the mass of an object if a force of
34 N produces an acceleration of 4 m/s/s?
m = F /a
m = 34 N / 4 m/s/s OR (34 kg x m/s/s) / 4 m/s/s
m = 8.5 kg
Complete Math Skills WS
“Newton: Force and Motion”
Newton’s Third Law of Motion
Gravity and Motion
• All objects fall with the same acceleration
(remember that acceleration is the rate at which
velocity changes… Acceleration = Δ v
time)
Objects fall to the ground at the same rate
because the acceleration due to gravity is the
same for all objects.
Weird Science
If a penny fell from the top of the Empire
State Building (about 385 m), it would be
traveling with enough velocity to dent
almost anything it struck at ground level
Accelerating at a constant rate
• All objects accelerate toward Earth at a
rate of 9.8 m/s/s (For every second an
object falls, the objects downward velocity
increases by 9.8 m/s) regardless of the
mass
Velocity of falling objects
• To find the change in velocity (Δ v) of a
falling object, multiply the acceleration due
to gravity (g or 9.8 m/s/s) by the time it
takes that object to fall in seconds.
Δv=gxt
Let’s look at an example.
Problem 1
A stone at rest is dropped from a cliff, and it
takes 3 seconds to hit the ground. Its
downward velocity when it hits the ground
is?
Δv=gxt
Δ v = 9.8 m/s x 3s =29.4 m/s
s
Problem 2
A penny at rest is dropped from the top of a
tall stairwell.
What is the penny’s velocity after it has
fallen for 2 seconds?
9.8 m/s x 2 s = 19.6 m/s
s
The penny hits the ground in 4.5 seconds.
What is its final velocity?
9.8 m/s x 4.5 s = 44.1 m/s
s
Problem 3
A boy standing on a high cliff dives into the
ocean below and strikes the water after 3
seconds. What is the boys velocity when
he hits the water?
29.4 m/s downward
Problem 4
A rock falls from a high cliff and hits the
ground in 6.5 seconds. What is it’s final
velocity?
63.7 m/s downward
Problem 5
A brick falls from the top of a building and
strikes the ground with a velocity of 19.5
m/s downward. How long does the brick
fall?
2 seconds
Accelerating at a constant rate
Air Resistance Slows down
Acceleration
• Fluid friction is also known as air
resistance. Air resistance occurs between
the surface of falling objects and the air
that surrounds it.
Air Resistance Affects Some
Objects More Than Others
• The amount of air resistance acting upon
an object depends upon the size and
shape of the object
Acceleration Stops at the Terminal
Velocity
• As an object falls, air resistance continues
to increase until it exactly matches the
downward force of gravity. The object has
then reached its terminal velocity…or a net
force of zero
Free fall occurs when there is no
air resistance
• Free fall only occurs if there is no gravity
pulling it down and no other forces are
acting upon it.
Orbiting Objects Are In Free Fall
• Free fall can be either ascending or
descending. If you could toss a ball
upward with no forces other than gravity
acting upon it (no air resistance), its entire
path (up and down) would be in free fall.
Two Motions Combine to Cause
Orbiting
• An object is orbiting when it is traveling in
a circular or nearly circular motion around
another object.
• While a space ship orbits the Earth, it is
also in free fall toward the Earth.
Remember the Space Station example?
How an Orbit is Formed
Why doesn’t the space shuttle fall to Earth if gravity is
pulling it downward?
What would happen if the shuttle started moving much
faster or much slower?
Projectile Motion and Gravity
• Projectile motion: the curved path an
object follows when thrown or propelled
near the surface of the Earth.
• Projectile motion has 2 components
horizontal and vertical and one has no
impact on the other (independent). When
combined, they formed a curved path.
Projectile Motion and Gravity
Horizontal Motion
• Horizontal motion is motion parallel to the
ground
Vertical Motion
• Vertical motion is motion that is
perpendicular to the ground