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Forces and Newton

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Chapter 4 Forces and Newton s Laws of Motion – PowerPoint PPT presentation

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Title: Forces and Newton


1
Chapter 4
  • Forces and Newtons Laws of Motion

2
Isaac Newton (1642-1727)
3
Isaac Newton (1642-1727)
  • IsaacNewton is without a doubt one of the most
    influential men in history.
  • Just a few of his accomplishments
  • Built the first practical reflecting telescope
  • Developed a theory of color including the idea
    that white light is composed of all colors of the
    rainbow
  • Studied the speed of sound
  • Developed calculus from scratch!
  • Defined the 3 Laws of Motion that govern all
    objects
  • Studied the effects of gravity (story about the
    apple)

4
Force
  • A force is a push or a pull.
  • Force is not a thing in itself, but rather an
    interaction between two objects.
  • Force is a vector quantity direction matters in
    the answer!

5
Common Forces
6
Common Forces
7
Newtons First Law
  • The Law of Inertia
  • A body remains at rest or moves in a straight
    line at a constant speed unless acted upon by a
    net force.
  • Objects do not accelerate unless a net force is
    applied.

8
Newtons First Law
  • Inertia is a property of an object most closely
    related to its mass (not to be confused with
    momentum) that explains why objects with greater
    mass resist a change in motion more than those
    with a lesser mass.

9
Net Force
  • Net force is the vector sum of ALL forces acting
    on an object.
  • If there is zero net force, then there is zero
    acceleration (constant velocity), this is a
    special case called equilibrium.
  • If there is a net force, there will be an
    acceleration. That means that the object will be
    speeding up, slowing down, or changing direction.

10
Free Body Diagrams
  • A Free Body Diagram is a simple drawing that
    shows the magnitude and direction of all of the
    force vectors acting on an object.
  • The length of the arrows in relation to each
    other is VERY important
  • Each arrow must point away from the free body
    and be labeled appropriately
  • The system, the object the force is applied to,
    is drawn as a shaded circle

11
Free Body Diagrams
  • Here is an example of a FBD of a book at rest on
    a table top.
  • Fg is acting downward but is balanced by FN
    acting upward.
  • Results in no net force and zero acceleration

FN
The book is drawn as a ball
Fg
12
Free Body Diagrams
  • Here is an example of a FBD of a box being pulled
    by a rope at a constant speed on a flat surface.
  • Fg and FN are still opposite and equal.
  • FT and Ff are also opposite and equal.

FN
FT
Ff
Fg
Object is in motion, but not accelerating
13
Balanced Forces (zero net)
14
Free Body Diagrams
  • Here is an example of a FBD of a ball under free
    fall conditions.
  • Fg is the only force acting on this object.
  • The net force is down and the object is
    accelerating.

Fg
Object is in motion and accelerating
15
Unbalanced Forces (non-zero net)
16
HW Questions P. 89 1-5
  • Draw a FBD for the following situations
  • A flowerpot falls freely from a windowsill.
    (Ignore any forces due to air resistance.)
  • A sky diver falls downward through the air at
    constant velocity. (The air exerts an upward
    force on the person.)
  • A cable pulls a crate at a constant speed across
    a horizontal surface. The surface provides a
    force that resists the crates motion.
  • A rope lifts a bucket at a constant speed.
    (Ignore air resistance.)
  • A rope lowers a bucket at a constant speed.
    (Ignore air resistance.)

17
Answers to HW
1
2
18
Answers to HW
3
4
19
Answers to HW
5
20
  • Draw a free body diagram for the following
    situations
  • A car accelerates from rest on a flat road (there
    is both friction from the air and the ground).
  • The space shuttle just after launch is
    accelerating upward (include friction from the
    air)

21
Newtons Second Law
  • The accel. of an object is directly proportional
    to the net force acting on the object, and
    inversely proportional to the mass of the object.

22
Newtons Second law of Motion mathematically
Net Force (mass)(accel) Fnet ma
23
NEWTON'S 2nd LAW
a
24
Mass
  • Mass is the amount of matter in an object (not to
    be confused with weight)
  • Also considered a measure of the inertia of an
    object
  • measured in SI unit of kilograms (kg) if mass is
    given in grams you must convert!

25
Weight
  • Weight is the downward force upon an object due
    to acceleration caused by gravity
  • weight mass (kg) ?accel. due to gravity (m/s2)
  • Fg mg
  • measured in Newtons (N)

26
  • The weight of a 10 kg brick is...
  • A) 98 N
  • B) 10 kg
  • C) 9.8 kg
  • D) 10 N
  • E) 98 kg

27
Newtons 2nd Law Practice
  • Two horses are pulling a 100-kg cart in the same
    direction, applying a force of 50 N each. What is
    the acceleration of the cart?
  • 2 m/s2
  • 1 m/s2
  • 0.5 m/s2
  • 0 m/s2

28
  • Answer B

29
Practice 2
  • Two friends Mary and Maria are trying to pull a
    10-kg chair in opposite directions. If Maria
    applied a force of 60 N and Mary applied a force
    of 40 N, in which direction will the chair move
    and with what acceleration?
  1. The chair will move towards Mary with an
    acceleration of 2 m/s2.
  2. The chair will move towards Mary with an
    acceleration of 10 m/s2.
  3. The chair will move towards Maria with an
    acceleration of 2 m/s2.
  4. The chair will move towards Maria with an
    acceleration of 10 m/s2.

30
  • Answer C
  • ReasonSince the force is applied in opposite
    direction, if we consider Marias direction of
    pull to be positive direction then, net force
    60 N 40 N 20 N . Thus, the chair will move
    towards Maria with an acceleration.

31
Weight
Location
Mass
Earth
18.4 kg
180 N
Moon
1/6 of Earths
18.4 kg
30 N
Orbiting Earth
18.4 kg
0 N
32
Mass and Weight
  • On the Moon, the force of gravity is only 1/6 as
    strong as on the Earth. (approx. 1.63m/s2)
  • While orbiting, you are practically weightless
    but your mass remains unchanged.
  • Your mass does not depend on where your are.
  • e.g. Earth, Moon, or space

33
Falling with Air Resistance
  • Air resistance (drag force) increases with speed
    and increased cross-sectional area and can be
    effected by the size and shape of an object.

34
Terminal Velocity
Net Force
Acceleration g
Velocity 0 but motion is about to begin
mg
F
Acceleration lt g
v increasing downward
mg
F
Acceleration ltlt g
v still increasing downward just not as rapidly
as before
mg
F
Acceleration 0
Terminal velocity
mg
35
Terminal Velocity
  • Terminal velocity occurs when the drag force of
    air resistance becomes large enough to balance
    the force of gravity.
  • At this instant in time, there is no net force
    the object stops accelerating (see D below)
    terminal velocity has been reached.

36
Friction
  • The force that opposes the motion between two
    surfaces that are in contact.
  • Friction is the "evil monster" of all motion.
    Regardless of which direction something moves in,
    friction pulls it the other way.
  • Move something left, friction pulls right. Move
    something up, friction pulls down. It appears as
    if nature has given us friction to stop us from
    moving anything.
  • Friction is actually a force that appears when
    there is relative motion between two objects.
  • Although two objects might look smooth,
    microscopically, they're very rough and jagged.

37
Static (starting) Friction
  • The force that opposes the start of the motion.
  • Static means stationary ( not moving).

38
Kinetic (sliding) Friction
  • The force between surfaces in relative motion
  • For the same object, why is the force of kinetic
    friction less than the force of starting friction?

39
Newtons Third Law
  • Action-Reaction Law
  • Two forces that make up an interaction pair of
    forces are equal in magnitude, but opposite in
    direction and act on different objects.

40
Newtons Third Law
For every action, there is always an equal
(magnitude) and opposite (direction) reaction. By
action or reaction, we mean a
force. Action/reaction forces do not act on the
same object.
41
Reaction road pushes on tire
Action tire pushes on road
42
Reaction gases push on rocket
Action rocket pushes on gases
43
Identify at least six pairs of action-reaction
force pairs in the following diagram
44
Question 1
  • If a stone is hung from a mass-less rope, at
    which place on the rope will there be more
    tension?
  • The top of the rope, near the hook.
  • The bottom of the rope, near the stone.
  • The middle of the rope.
  • The tension will be same throughout the rope.

45
Answer 1
  • Answer D
  • Reason Because the rope is assumed to be without
    mass, the tension everywhere in the rope is equal
    to the stones weight .

46
Question 2
  • In a tug-of-war event, both teams A and B exert
    an equal tension of 200 N on the rope. What is
    the tension in the rope? In which direction will
    the rope move? Explain with the help of Newtons
    third law.

47
Answer 2
  • Team A exerts a tension of 200 N on the rope.
    Thus, FA on rope 200 N. Similarly, FB on rope
    200 N. But the two tensions are an interaction
    pair, so they are equal and opposite. Thus, the
    tension in the rope equals the force with which
    each team pulls (i.e. 200 N). According to
    Newtons third law, FA on rope ?FB on rope. The
    net force is zero, so the rope will stay at rest
    as long as the net force is zero.
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