Net force and Newton's first law

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CHAPTER 2 DYNAMICS

• Net force and Newton's first law
• Newton's second law
• Newton's third law
• Frictional forces
• Gravitation
• Circular motion
• Centripetal force
• Static equilibrium and reference frames

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Who are they??? Universal Laws of Classical
Mechanics
Inertia
Equal Areas in Equal Times
F GMm/R2
Force
F m a
Mass
Action Reaction

• a v2/R

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What is the difference between Kinematics and
Dynamics ?
Answer is Forces
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Part 1
Net force and Newton's first law
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Review Vector sum of Net force

• Net force System of many vectors act to one
  mass point O.
• Example

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Learning Check

• Can you explain all forces acting on a bus when
  it is at rest ??
• Draw the arrows for them.

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Newtons First Law

• _at_ The motion of an object does not change unless
  it is acted upon by a net force.
• If v 0, it remains at rest.
• If v C, it till move at that value.
• _at_ Another way to say the same thing
• No net force ? call freely moving object
• Velocity is constant or zero ? acceleration is
  zero

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Learning check

• Suppose you are on an airplane travelling at
  constant velocity with a speed of 500 miles per
  hour (roughly 200 m/s)
• If you throw a ball straight up, does it return
  to you?

• How long does it appear to you?
• How does the path of the ball look
• to an observer on the ground?
• Can you think of any experiment done inside the
  airplane that would detect the motion of the
  airplane at constant velocity?

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Valuable Solution
1.25 m
To person on airplaneTime 1 sec
1.25 m
200 m
To person on ground - Time 1 sec
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What about pouring coffee?
(We exaggerate and assume the coffee is poured
1.25 meters above the cup!)
To person on airplaneTime 1/2 sec
To person on groundTime 1/2 sec
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Question 1

• An airplane flying Many forces act on the plane,
  including weight (gravity), drag (air
  resistance), the thrust of the engine, and the
  lift of the wings. At some point during its trip
  the velocity of the plane is measured to be
  constant (which means its altitude is also
  constant). At this time, the total force on the
  plane 1. is pointing upward2. is pointing
  downward 3. is pointing forward 4. is pointing
  backward5. is zero

Velocity is constant implies there is no net
force acting on it. All components of the net
force are zero. Forces are balanced in all three
dimensions.
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Question 2
An object is held in place by friction on an
inclined surface. The angle of inclination is
increased until the object starts moving. If the
surface is kept at this angle, the object 1.
slows down 2. moves at uniform speed 3.
speeds up 4. none of the above
When the object is at rest net force on it is
zero. When the object starts to move, there is
change in velocity - i.e., there is acceleration
or a net force due to gravity The force remains
constant when inclination is kept at that angle
leading to constant acceleration - continuous
speed up.
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Contact and Field Forces
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MCQ Test

• Forces that appear in contact
• A. Friction
• B. Electron acts to another electron
• C. Gravity (you and the Earth)
• D. Two mobiphones at distance of 5m.

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Mass or Inertia

• Inertia is the tendency of an object
• to remain at rest or in motion with
• constant speed along a straight line.
• Mass (m) is the quantitative measure of inertia.
• Mass is the property of an object that measures
  how hard it is to change its motion.
• Units M kg

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Learning check

• Look at all people in this ficture, who has
  smallest inertia ?

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Question 3
A passenger in a car and is not wearing your seat
belt. Without increasing or decreasing its speed,
the car makes a sharp left turn, and He find
yourself colliding with the right-hand side door.
Which of the following is correct of the
situation? 1. Before and after the collision
there is a rightward force pushing him into the
door 2. Starting at the time of collision, the
door exerts a leftward force on him 3. Both of
the above 4. Neither of the above
Law of inertia his body tends to move in a
straight line forward. It collides with the door
which being part of the car is beginning to
curve leftward. When the contact happens he feel
the doors force on him.
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Mass vs. Weight

• Mass is an intrinsic property of an object.
• A rock has same mass whether it is on the moon or
  on Earth
• Weight is the force exerted on an object by
  gravity.
• This is different depending upon the strength of
  the gravitational force.
• On the surface of Earth, gravitational force is
  constant so we can easily convert from mass to
  weight.

1 pound 1 slug - ft/s2
1 kg x 9.8m/s2 9.8 Newtons
1 Newton 1 kg - m/s2
9.8 Newtons 2.2 pounds
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Principle of inertia

• Galileos Profound Contributions to Physics
  Include

Principle An object (Ball) moving on horizontal
surface will continue to move in the same
direction at constant speed unless it is
disturbed. (This becomes even more general in
the hands of Newton.)
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Part 2

• Newtons second law

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Learning Check

• Can you explain all forces acting on a bus when
  it is at rest ??
• Draw the arrows for them.

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How Does a Car Move?

• Each arrow represents a force.
• Your car is accelerating forward, meaning
• there is a net force in that direction.
• Identical concept for walking (or running).

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Which has more momentum?

• A bullet, mass 100 grams, speed 1000m/s
• A car, mass1000kg, speed 10m/s

100 grams 0.1kg pmv 0.1 kg x 1000m/s 100
kg.m/s
pmv 1000 kg x 10m/s 10,000 kg.m/s
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Newtons Second Law

• This law tells us how motion changes when a net
  force is applied.
• acceleration (net force)/mass

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Newtons Second Law

• Units
• Prove
• F M a
• F kg m/s2
• 1 Newton (N) ? 1 kg m/s2
• A scaler equation
• Fnet,x Max
• Fnet,y May

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Example Newtons 2. Law
a Fnet/M 200N/10kg 20 m/s2
a Fnet/M (200N-100N)/10kg 10 m/s2
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Part 3

• Newton s third law

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Newton's Third Law...

• FA ,B - FB ,A. is true for all types of
  forces

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Newtons 2. and 3. Laws

• Suppose you are an astronaut in outer space
  giving a brief push to a spacecraft whose mass is
  bigger than your own
• Compare the magnitude of the force you exert on
  the spacecraft, FS, to the magnitude of the force
  exerted by the spacecraft on you, FA, while you
  are pushing
• 1.FA FS 2. FA gt FS3. FA lt FS

2) Compare the magnitudes of the acceleration
you experience, aA, to the magnitude of the
acceleration of the spacecraft, aS, while you
are pushing 1. aA aS 2. aA gt aS 3. aA lt aS
aF/m F same ? lower mass gives larger a
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Exercise Action-Reaction

• Suppose a tennis ball (m 0.1 kg) moving at a
  velocity v 40 m/sec collides head-on with a
  truck (M 500 kg) which is moving with velocity
  V 10 m/sec.
• During the collision, the tennis ball exerts a
  force on the truck which is smaller than the
  force which the truck exerts on the tennis ball.
  TRUE or FALSE ?
• The tennis ball will suffer a larger acceleration
  during the collision than will the truck. TRUE
  or FALSE ?
• Suppose the tennis ball bounces away from the
  truck after the collision. How fast is the truck
  moving after the collision?
• lt 10 m/sec 10 m/sec
  gt 10 m/sec ?

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Summary

• Newtons First Law
• The motion of an object does not change unless it
• is acted on by a net force
• Newtons Second Law
• Fnet ma
• Newtons Third Law
• Fa,b -Fb,a

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• Learning check
• ---------------LAWS OF MOTION---------------------
  --
• write the good word to ..
• (I) .is constant absent external force
• (II) Force equals Mass x.
• (III) Action equals.

• Velocity
• Acceleration
• Reaction

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Let try to familier with Interactive Physics
software
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Short thesis

• Do the thesis about
• 1- Application of Crane truck
• 2- Combine harvester machine
• 3- Centrifuge machine
• 4- Automatic teller machine ATM
• 5- Meat grinder machine

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• Please
• Make friend with DVD experiments