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Physics of Technology PHYS 1800


Physics of Technology PHYS 1800 Lecture 10 Review for Test 1 – PowerPoint PPT presentation

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Title: Physics of Technology PHYS 1800

Physics of TechnologyPHYS 1800
  • Lecture 10
  • Review for Test 1

PHYSICS OF TECHNOLOGY Spring 2009 Assignment
Homework Handout
Physics of TechnologyPHYS 1800
  • Lecture 8
  • Mass Weight
  • Motion with Friction

Introduction and Review
Notes on Test
  1. Covers Chapters 1-4
  2. 8 short answer problems or questions (5 point
  3. 3 Numerical problems based heavily on the
    material from the homework and Lab/Demo sessions
    (20 points each). One problem each from Chapters
    2, 3 and 4.
  4. You will have a formula sheet just like the one
    in the handout.
  5. Test is Thursday January 29 130-245 in ESLC 46.

What Do We Need To Measure?
  • What is the minimum about things we need to know?
  • Where things area length, L
  • When things are therea time, T
  • How thing interact with gravitya mass, M
  • How things interact with EMa charge, Q
  • How thing inter act with weak nuclear force
  • How things interact with strong nuclear force

Scalars and Vectors
  • Scalar Measure of quantity or size
  • Sometimes called magnitude.
  • Examples Length, volume, mass, temperature,
  • Vectors Many measurements in physics require a
    knowledge of the magnitude and direction of
  • These are termed vector quantities.
  • Examples Velocity, acceleration, force,
    electric field
  • Direction is an essential feature of a vector
  • Example Flying at 1000 km/hr due North is
    quite different to the same speed due East!
  • Vectors require 2 pieces of information MAGNITUDE
    and DIRECTION.

Describing (Special) Motion
  • Positionwhere you are in space (L-meter)
  • Speedhow fast position is changing with time
    (LT-1 or m/s)
  • Accelerationhow fast speed is changing with time
    (LT-2 or m/s2)
  • We will focus on a special case of constant
    acceleration due to gravity, termed FREE FALL.

Constant Acceleration
Acceleration ?
Distance ?
Speed ?
The Math Approach
  • Lets put these ideas into math language

  • Let the initial velocity be 20 m/s upward.
  • It immediately starts experiencing a downward
    acceleration due to gravity, of approximately 10
  • Every second, the velocity decreases by 10 m/s.
  • After 2 s, the ball has reached its highest
  • Its velocity changes direction, from upward to
    downward, passing through a value of 0 m/s.
  • Now, the downward acceleration increases the
    downward velocity.

Which of these three balls would hit the floor
first if all three left the tabletop at the same
  1. The ball with initial velocity v1.
  2. The ball with initial velocity v2.
  3. The ball with initial velocity v3.
  4. They would all hit at the same time.

Since all three balls undergo the same downward
acceleration, and they all start with a vertical
velocity of zero, they would all fall the same
distance in the same time!
Projectile Motion
  • Treating the vertical motion independently of the
    horizontal motion, and then combining them to
    find the trajectory, is the secret.
  • A horizontal glide combines with a vertical
    plunge to produce a graceful curve.
  • The downward gravitational acceleration behaves
    the same as for any falling object.
  • There is no acceleration in the horizontal
    direction if air resistance can be ignored.
  • The projectile moves with constant horizontal
    velocity while it is accelerating downward.

Dennisons Laws of Motion
  1. Stuff happens (or not).
  2. The bigger they are the harder they fall.
  3. You get what you give.

Newtons Laws in Review
  • 1st Law a special case of the 2nd Law for
    statics, with a0 or Fnet0
  • An objects velocity remains unchanged, unless a
    force acts on the object.
  • 2nd Law (and 1st Law)How motion of a object is
    effected by a force.
  • The acceleration of an object is directly
    proportional to the magnitude of the imposed
    force and inversely proportional to the mass of
    the object. The acceleration is the same
    direction as that of the imposed force.
  • 3rd Law Forces come from interactions with other
  • For every action (force), there is an equal but
    opposite reaction (force).

Newtons First Law of Motion
An object remains at rest, or in uniform motion
in a straight line, unless it is compelled to
change by an externally imposed force.
Newtons Second Law of Motion
  • Note that a force is proportional to an
    objects acceleration, not its velocity.
  • Precise definitions of some commonly used
  • The mass of an object is a quantity that tells
    us how much resistance the object has to a change
    in its motion.
  • This resistance to a change in motion is called

Force has dimensions of (MLT-2)
Net Forces
  • It is the total force or net force that
    determines an objects acceleration.
  • If there is more than one vector acting on an
    object, the forces are added together as vectors,
    taking into account their directions.

Newtons Third Law (action/reaction)
For every action (force), there is an equal but
opposite reaction (force).
Mass, Weight, and Inertia
  • A much larger force is required to produce the
    same acceleration for the larger mass.
  • Inertia is an objects resistance to a change
    in its motion.
  • Mass is a measure of an objects inertia.
  • The units of mass are kilograms (kg).

Does a sky diver continue to accelerate?
  • Air resistance R is a force directed upward, that
    opposes the gravitational force W
  • R increases as the sky divers velocity increases
  • When R has increased to the magnitude of W, the
    net force is zero so the acceleration is zero
  • The velocity is then at its maximum value, the
    terminal velocity

Free Body Diagrams
  • Fancy Science Vector analysis of complex force
    problems is facilitated by use of a free body
  • Common Sense A picture is worth a 100 words. (A
    scale picture is worth an A!)
  • Key is to
  • Isolate a single body and draw all the forces
    acting on it.
  • Add up all the arrows (vectors).
  • Whats left is the net force.
  • Net force (and masses) ? a.
  • A plus initial conditions? motion!

Example Sea Lion splash!
mg sin?
mg cos?
Free body diagram
W m g
  • Resolve the weight force into two components
    parallel and perpendicular to ramp.

Result - Down slope force F m g sin ?
- Normal force N m g cos ?