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Net force and Newton's first law

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The motion of an object does not change unless it is acted upon by a net force. ... 1 pound = 1 slug - ft/s2. 1 Newton = 1 kg - m/s2. 9.8 Newtons = 2.2 pounds ... – PowerPoint PPT presentation

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Title: Net force and Newton's first law


1
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

2
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

3
What is the difference between Kinematics and
Dynamics ?
Answer is Forces
4
Part 1
Net force and Newton's first law
5
Review Vector sum of Net force
  • Net force System of many vectors act to one
    mass point O.
  • Example

6
Learning Check
  • Can you explain all forces acting on a bus when
    it is at rest ??
  • Draw the arrows for them.

7
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

8
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9
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?

10
Valuable Solution
1.25 m
To person on airplaneTime 1 sec
1.25 m
200 m
To person on ground - Time 1 sec
11
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
12
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.
13
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.
14
Contact and Field Forces
15
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.

16
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17
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18
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

19
Learning check
  • Look at all people in this ficture, who has
    smallest inertia ?

20
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.
21
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
22
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.)
23
Part 2
  • Newtons second law

24
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25
Learning Check
  • Can you explain all forces acting on a bus when
    it is at rest ??
  • Draw the arrows for them.

26
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).

27
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28
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
29
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30
Newtons Second Law
  • This law tells us how motion changes when a net
    force is applied.
  • acceleration (net force)/mass

31
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

32
Example Newtons 2. Law
a Fnet/M 200N/10kg 20 m/s2
a Fnet/M (200N-100N)/10kg 10 m/s2
33
Part 3
  • Newton s third law

34
Newton's Third Law...
  • FA ,B - FB ,A. is true for all types of
    forces

35
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36
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
37
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 ?

38
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

39
  • 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

40
Let try to familier with Interactive Physics
software
41
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

42
  • Please
  • Make friend with DVD experiments
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