Docstoc

Impulse-Momentum

Document Sample
Impulse-Momentum Powered By Docstoc
					Impulse-Momentum

   January 6, 2009
              Newton’s 3rd law
• Equal and opposite forces – what does this
  mean?
  – 4 conceptual questions
                3rd law demos
• Tug of war
• Tug of war on roller skates
              Momentum
• Momentum: mass in motion

• p = mv
• Momentum: mass in motion

• p = mv

• Change momentum by exerting a force over a
  period of time (impulse)
         Impulse-Momentum
• Impulse = Ft
• Impulse = m∆v
  Examples of Impulse-Momentum
• Determine the direction of the impulse
  – Cart on a track
  – Projectile motion
  – Circular Motion
• Determine the direction of momentum change
  – Cart on a track
  – Projectile motion
  – Circular Motion
            Impulse-Momentum
• Impulse and momentum change are the same
  thing!!!
• Impulse is always in the same direction as the
  force (so is momentum change)
  – Impulse is a vector (it has direction)
              Happy/sad balls
• If I drop two identical balls from the same
  height, which will exert the larger force on the
  ground: the one that bounces or the one that
  does not bounce? Why?
• Impulse = F*t

• Impulse = m∆v
  Rank the impulse from most positive to most negative (negative
               numbers are smaller than positive)




10 m/s    20 m/s       20 m/s    20 m/s        30 m/s      20 m/s




10 m/s    0 m/s        10 m/s    -10 m/s       -10 m/s   -20 m/s
         Rank the change in momentum from most positive to most negative
                    (negative numbers are smaller than positive)




10 m/s         20 m/s       20 m/s     20 m/s         30 m/s      20 m/s




10 m/s        0 m/s         10 m/s     -10 m/s        -10 m/s   -20 m/s
      Rank the following situations in order
     of greatest to least momentum change
     V = 2m/s          V = 4m/s
                   C
A


       V = -2m/s          V = -4m/s




    V = 2m/s           V = 4m/s

B
                   D

        V =0               V=0
    Rank the following situations in order of
           greatest to least impulse
    V = 2m/s         V = 4m/s
                C
A


    V = -2m/s       V = -4m/s




    V = 2m/s        V = 4m/s

B
                D

     V =0            V=0
           Impulse Investigation
• A cart holding a force sensor is pulled from rest
  across a frictionless surface.
  – If we want to determine the impulse the person will
    experience, what measurements should we take?
    What equation will you use?
          Impulse Investigation
• A person holding a force sensor is pulled from
  rest across a frictionless surface.
  – If we want to determine the impulse the person will
    experience, what measurements should we take?
    What equation will you use?
  – Is there more than one answer to the previous
    question?
            Force-time Graphs
• Jumping on a force plate
 Two methods of determining impulse
• Force Sensor – area under curve
• Motion sensor – m∆v
     Impulse from Multiple Forces
• Ex – book sitting on a table
     Impulse from multiple forces
• A man weighing 800 N gets in an elevator. As
  the elevator is accelerating upward, the scale
  reads 850 N.
  – Draw a FBD of the man
  – Determine the net impulse he experiences during
    the first 2 seconds of motion.
  – How fast is the man moving after 2 seconds?
     Conservation of momentum
• Equal and opposite impulses (just like equal
  and opposite forces)

• Yesterday’s collision simulation
Calculating final velocity (inelastic
             collisions)

        mava + mbvb = (ma + mb) v’
                  Example
• A car with a mass of .5 kg and a velocity of .5
  m/s collides with a car of mass 1 kg and
  velocity -.25 m/s. Determine the final velocity
  of the carts after they collide, assuming they
  stick together after the collision.
         Thursday, January 14
• Conservation of momentum principles
 What do you think will happen to
the speed of the cart when a mass
     is dropped on top of it?
    Conservation of momentum
• http://www.physicsclassroom.com/mmedia/
  momentum/cbb.cfm
• http://www.physicsclassroom.com/mmedia/
  momentum/fcb.cfm
• http://www.physicsclassroom.com/mmedia/
  momentum/creti.cfm
          “reverse” collisions
• When you shoot a gun, why does it “recoil”?
  Explain using the law of conservation of
  momentum.
                    Hint:
• When objects START together and separate
  (shooting a gun, throwing a baseball, pushing
  off from someone on ice skates) simply flip
  the conservation of momentum equation

           (ma + mb) v = mava + mbvb
                    Practice
• A 15-kg medicine ball is thrown at a velocity of
  20 m/s to a 60-kg person who is at rest on ice.
  The person catches the ball and subsequently
  slides with the ball across the ice. Determine
  the velocity of the person and the ball after
  the collision.
• Suppose that you have joined NASA and are
  enjoying your first space walk. You are outside
  the space shuttle when your fellow astronaut
  of approximately equal mass is moving
  towards you at 2 m/s (with respect to the
  shuttle). If she collides with you and holds
  onto you, then how fast (with respect to the
  shuttle do you both move after the collision?
• http://www.physicsclassroom.com/mmedia/
  momentum/ast.cfm
               Tomorrow’s lab
• Predict what velocity-time graphs will look like
  for:
  – Two cars pushed toward each other, sticking
    together after the collision
  – Two carts pushed toward each other, bouncing off
    of each other after the collision
  – One cart initially at rest, while the other is pushed
    toward it. The carts stick together.
  – One cart initially at rest, while the other is pushed
    toward it. The carts bounce off of one another.
      Tuesday, January 19, 2010
• TEST tomorrow!
• Go over last week’s homework
• On the test:
  – Impulse (m∆v, Ft, change in momentum)
  – Direction of impulse = direction of force
  – Force-time graphs
  – Conservation of momentum
Describing the motion….
  “reverse” collisions (explosions)
• You are standing at rest on a frictionless sheet
  of ice. You throw a .25-kg baseball forward at
  a speed of 18 m/s. If you have a mass of 55
  kg, at what speed (and in what direction) are
  you moving after you throw the baseball?

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:15
posted:8/13/2012
language:
pages:36