Download Newton`s Second Law Power Point

Newton’s Second Law
Section 2 p. 556-562
Force and Acceleration
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When the motion of an object
changes, the object is accelerating
Acceleration occurs anytime an
object speeds up, slows down, or
changes direction.
Newton’s Second Law
The second law connects the net
force on an object, its mass and its
acceleration.
 The second law saysAn object acted upon by an
unbalanced force will accelerate in
the direction of the force
a=Fnet/m
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Newton’s Second Law“Simplified”
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A moving object speeds up if the net
force is in the direction of the
motion.
A moving object slows down if the
net force is the direction opposite the
motion
A moving object turns if the net force
is at an angle to the direction of
motion.
Example of Law

Newton's Second Law of Motion
Units of Force
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Force is measured in newtons (N)
1N=1 kg·m/s2
Gravity
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Gravity is a force
Gravity is also the force that causes
Earth to orbit the Sun and the Moon
to orbit the Earth.
What is gravity?

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The force of gravity exists between
two objects that have mass.
Gravity is always attractive and pulls
objects between each other.
Force of Gravity

The force of gravity depends on the mass
of objects and the distance between
them.
Weight
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Weight is the gravitational force
exerted on an object.
Your weight on Earth is the
gravitational force between you and
the Earth. W=m(9.8m/s2)
Weight and Mass
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Weight and mass are different.
Weight is a force and is measured in
newtons.
Mass is the amount of matter in an object
and it does not depend on location.
Weight will vary with location, but mass
remains constant. The weight of a book on
Earth and on Mars would be different, but
mass would be the same on both planets.
Calculating Acceleration
Suppose you pull a 10kg box so that the
net force on the box is 5N. What is the
acceleration?
a=Fnet/m a= 5N ÷10kg a=0.5m/s2

The box keeps accelerating as long as you
keep pushing on it.
Acceleration does not depend on how fast
the box is moving. It depends only on the
net force and mass of the box.
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This is an example of how Newton's Second Law
works:
Mike's car, which weighs 1,000 kg, is out of gas.
Mike is trying to push the car to a gas station,
and he makes the car go 0.05 m/s2. Using
Newton's Second Law, you can compute how
much force Mike is applying to the car.
Answer = 50 newtons
Practice
A book with a mass of 2.0kg is pushed
along a table. If the net force on the book
is 1.0 N, what is the acceleration of the
book?
a=Fnet/m

F
a
m
Answer- 1.0 n ÷ 2.0 kg = 0.5 m/s2
Practice

A baseball has a mass of 0.15kg.
What is the net force on the ball if it
has an acceleration of 40.0 m/s2?
F
a m
Answer- 0.15kg x 40m/s2 = 6 N
NASA Video on Second Law

This NASA video segment explores how
Newton's second law of motion applies to
aerospace. Viewers watch an instructor at
NASA's National Test Pilot School as he
defines the second law and demonstrates
how to calculate a person's mass using
the law. There is also a discussion about
how people experience different g forces
at the top and bottom of a roller coaster
hill. Footage of the instructor in a fighter
jet illustrates what it means to pull 2 and
4 g.
NASA
Air Resistance
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Whether you are walking, running or
biking, air is pushing against you.
This push is air resistance.
Air resistance is a form of friction that acts
to slow any moving object down moving in
air.
Air resistance gets larger as an object
moves faster.
Air resistance depends on the shape of the
object.
Air Resistance (cont.)
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When air resistance force equals the
weight, the net force on the object is zero.
By Newton’s second law, the object’s
acceleration is then zero, and its speed no
longer increases.
When air resistance balances the force of
gravity, the object falls at a constant
speed called terminal velocity.
Terminal velocity is the fastest velocity
with which an object falls.
Terminal Velocity of a Penny

Click on Penny to see video
Terminal Velocity
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Video of Falcon
1.How fast is the fall of a typical skydiver?
A normal skydiver in free-fall hits a speed of about 120 miles per
hour (mph).
2. What materials are used in the lures the falconer, Franklin, makes
to train his falcons?
Franklin uses small cotton bags filled with lead, and topped with
a piece of meat.
3. How long does it take the peregrine falcon, Frightful, to catch the
lure?
It takes Frightful only 43 seconds to capture the lure.
4. What was Frightful's top speed?
Frightful was clocked at diving 242 mph.
5. Speed-skydivers adapt their clothing to emulate the peregrine
falcon in free-fall. What clothes are designed to mimic the bird's
feathers?
The skydiver's pants expand near the ankle, allowing him much
greater control in free-fall.
6. What was the top speed of Mark, the skydiver?
Mark has hit speeds of just over 300 mph
Examples
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By spreading larger sails on a
sailboat, it increases the force of the
wind against the sails
The maximum speed of a skydiver is
120mph
What happens to the velocity if you
increase the size of the parachute?
Center of Mass
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The center of gravity is the point in
an object that moves as it all the
object’s mass were concentrated at
that point.
The object moves as if the net force
is being applied there.
Load Transfer
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NASCAR corners are divided into three parts
because the car's grip changes in different parts
of a turn. The higher center of gravity in the new
car challenges crew chiefs to minimize weight
shift around a turn. Equipment like the sevenpost rig helps, but the ultimate test is on the
track.
NASCAR
Principals at Work in Skateboarding
Aerodynamics of Flight
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http://www.sciencekids.co.nz/videos
/engineering/flightaerodynamics.htm
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