Physics 140 Fall 2004 by liuqingyan


									Racquetball Striking a Wall                                   Copyright: Loren M. Winters

       AP Physics C                           Prof. Tim McKay
                                            University of Michigan

                              Andrew Davidhazy         Mt. Etna
                 Quick recap
• 1st Law: If motion          • 2nd Law: Quantifies
  doesn‟t change, no            forces as
                                         
                                    F  ma
  net force is acting
  – Really a statement of
    what has to be
    explained                 • Mass is a measure of
• 1st Law: If motion            inertia: how hard it is
  changes, it is                to change the motion
  because of an                 of an object
  unbalanced force          Pay attention to the forces
                            and you can predict motion
  Another Newtonian insight: forces
    always come from interaction
• Forces don‟t come            Flift
  from nowhere
• In fact, all forces                  
  emerge from                          v
  interactions between
  two objects
• In our suitcase:
  – Earth and suitcase (W)
  – You and suitcase (Flift)
           Newton‟s third law
• Forces always appear     • 3rd Law: “For every
  in pairs, in               action there is an
  interactions               equal and opposite
• When you pull up on        reaction”
  the suitcase, it pulls   • If object A exerts a
  down on you                force on object B
• When the Earth pulls       (FAB), then object B
  down on the suitcase,      exerts and equal and
  the suitcase pulls up      opposite force on
  on the Earth               object A (FBA)
                                    
                             FAB   FBA
 Third law thought experiments
• Horse pulls a stone        • Newton: two magnets
  with a rope. The rope        with paper in between
  stretches and will do
  anything to be
                                       A   B
  shorter. So it pulls the
  stone forward, and
  the horse back.
  These pulls are equal      • If FAB was bigger than
  and opposite because         FBA, the whole thing
  the rope doesn‟t know        would accelerate off
  which way to pull…           to the left….
   How can the 3rd law be true?
• Throwing a ball
               
        FPB   FBP
• Why does the ball fly
  off, and the person
  remain still?
• Free-body diagrams                                FBP
  help reveal this!
• Extra forces (friction)   NOT drawn after
  act on the person…        the throw, but
A locomotive pulls a series of wagons. Which
is the correct analysis of the situation?
     1. The train moves forward because the locomotive pulls
     forward slightly harder on the wagons than the wagons pull
     backward on the locomotive.
     2. Because action always equals reaction, the locomotive
     cannot pull the wagons the wagons pull backward just as hard
     as the locomotive pulls forward, so there is no motion.
     3. The locomotive gets the wagons to move by giving them a
     tug during which the force on the wagons is momentarily
     greater than the force exerted by the wagons on the locomotive.
     4. The locomotive‟s force on the wagons is as strong as the
     force of the wagons on the locomotive, but the frictional force on
     the locomotive is forward and large while the backward frictional
     force on the wagons is small.
     5. The locomotive can pull the wagons forward only if it weighs
     more than the wagons.
In a tug-of-war two groups of students pull
  as hard as they can on one another in
  an effort to pull the other team across
  the line. If the physics students team
  wins over the history students team, it is

         1. The physics team pulled
           harder on the history team
         2. The history team pulled harder
           on the physics team
         3. Neither
More complicated free body diagram example


    Book                              WB=FEarth-on-Book




            FgBook-on-Earth              FgTable-on-Earth
 Some classifications for forces
• Two different              • Contact and non-
  dialectics…                  contact forces
• Active and Passive           – Contact: surfaces
  Forces                         actually touch (most
                                 forces we encounter;
  – Active have fixed
                                 normal force, friction)
    values (weight, a
    push)                      – Non-contact: objects
                                 are at a distance
  – Passive adjust to meet
                                 (gravity, magnetic
    a requirement (normal
 Weight: the attraction of the Earth
• Force of gravity
         
     W  mg
• Sensation of weight?
  – Think about jumping
    off a chair. What do
    you feel?
  – What if you stand?
    Sit? Stand on your
• Weightless: no
  sensation of weight…
  really freefall
An astronaut in the space shuttle orbits
 the Earth, floating freely with respect
 to the contents of the shuttle. Is the
 astronaut weightless?

      1. Yes
      2. No
      3. Depends on altitude
      4. Depends on mass
    Contact forces and the normal
• Useful to talk about         • Called “normal” force
  two parts to contact           (for perpendicular, not
  forces                         „usual‟)
• First prevents objects       • Classic passive force:
  from moving through            as big as it has to
  one another                    be…
• Acts  to interface          • How do inert objects
  between objects                exert forces?

     By bending: pillow, chair seat, plastic chair, a
     hard floor, all push back by distorting….
                            Normal force examples
                                Push down
                                                             Pull up

          N                         N                    N           Fpull

       Book on Table                                      Book on Table

          W                     Book on Table
                                   W       Fpush
     Fy = N – W = 0
          N=W                                          Fy = N – W + Fpull = 0
                                                            N = W - Fpull
                             Fy = N – W – Fpush = 0
To find the normal force,        N = W + Fpush
see how large it has to
be: a passive force…
Consider a car at rest. We can conclude that the
 downward gravitational pull of Earth on the car
 and the upward contact force of road on it are
 equal and opposite because:

     1: The two forces form an 3rd law pair
     2: The net force on the car is zero
     3: Neither of the above

    The normal force of the road pushing up on the
    car and the gravitational pull of the Earth down
    on the car can’t be 3rd law partners: they both
    act on the same object!
I hold a book which has a weight of -20N y ˆ
against the ceiling by pushing up on it with a
force of +30Ny. What is the normal force
exerted by the ceiling on the book?

                        +30N y

                 1.   +20 N yˆ
                 2.         ˆ
                      -30 N y
                 3.   +50 N yˆ
                 4.         ˆ
                      -50 N y
                 5.         ˆ
                      -10 N y

To top