Impulse and Momentum Review Mr. Horton

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Impulse and Momentum ReviewMr. Hortons AP
Physics

• This is whats on the test

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Basic Relations , momentum

• Momentum a vector that is mv
• Find the momentum of a 40kg bike that is going
  10m/s South
• If the momentum of a go cart is 2500kgm/s east
  when it is going 10m/s east, what is its mass?
• If the momentum of a 100kg go cart is 2500kgm/s
  east, find its velocity

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Basic Relations , momentum

• Find the momentum of a 40kg bike that is going
  10m/s South 400kgm/s South
• If the momentum of a go cart is 2500kgm/s east
  when it is going 10m/s east, what is its mass?
  250kg
• If the momentum of a 100kg go cart is 2500kgm/s
  east, find its velocity 25m/s East

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Basic Relations , momentum

• If the momentum of a go cart is 2500kgm/s east,
  what would its momentum be if it doubled its
  speed?
• If the momentum of a go cart is 2500kgm/s east,
  what would the momentum of another go cart be if
  its mass is 25 greater but it has the same
  velocity?

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Basic Relations, momentum

• If the momentum of a go cart is 2500kgm/s east,
  what would its momentum be if it doubled its
  speed? Double, 5000kgm/s east
• If the momentum of a go cart is 2500kgm/s east,
  what would the momentum of another go cart be if
  its mass is 25 greater but it has the same
  velocity? 25 greater, 3250kgm/s

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Basic Relations, impulse

• Impulse Ft
• Find the impulse of a 4000N average force to the
  west that acts for 50s.
• Find the impulse of a 4000N average force to the
  west that acts for half the time as above.
• Will the change in momentum be the same, greater,
  or less for the 50s impulse?

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Basic Relations, impulse

• Impulse Ft
• Find the impulse of a 4000N average force to the
  west that acts for 50s. 200,000Ns west
• Find the impulse of a 4000N average force to the
  west that acts for half the time as above.
  100,000Ns west
• Will the change in momentum be the same, greater,
  or less for the 50s impulse? Greater (2x greater)

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Basic Relations, impulse is the change in
momentum

• Impulse Ft change in momentum
• If (and only if) the momentum change is the same,
  force will decrease if time increases.
• An object has 400kgm/s of momentum to the north
  when a 10N force to the north is applied for 2s.
  Find the change in momentum and the final
  momentum.
• An object has 400kgm/s of momentum to the north
  when a 10N force to the south is applied for 2s.
  Find the change in momentum and the final
  momentum.

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Basic Relations, impulse is the change in
momentum

• An object has 400kgm/s of momentum to the north
  when a 10N force to the north is applied for 2s.
  Find the change in momentum and the final
  momentum. 20Ns north 420kgm/s north
• An object has 400kgm/s of momentum to the north
  when a 10N force to the south is applied for 2s.
  Find the change in momentum and the final
  momentum. 20Ns south 380kgm/s north

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Basic Relations, impulse is the change in
momentum

• An object has 400kgm/s of momentum to the north
  when a 10N force to the east is applied for 20s.
  Find the change in momentum and the final
  momentum.
• An object has 400kgm/s of momentum to the north
  . Find the impulse required to stop it.

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Basic Relations, impulse is the change in
momentum

• An object has 400kgm/s of momentum to the north
  when a 10N force to the east is applied for 20s.
  Find the change in momentum and the final
  momentum. Pythag 44.7m/stan-1 (40/20) 63
  deg North of E

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Basic Relations, impulse is the change in
momentum

• An object has 400kgm/s of momentum to the north.
  Find the impulse required to stop it. Dmv -
  400kgm/s -400Ns 400Ns south

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Impulse and Momentum at an angle

• An object has 400kgm/s of momentum 30deg east of
  south. Find the components in the north and
  east.
• An object has 400kgm/s of momentum 30deg east of
  south when a 20N force is applied to it for 20s
  at 30deg east of south. Find its final momentum
  and velocity.

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Impulse and Momentum at an angle

• An object has 400kgm/s of momentum 30deg east of
  south. Find the components in the north and
  east.
• 400 sin(30deg) 200kgm/seast
• 400 cos(30deg) 346.4kgm/seast
• Vel (400kgm/s)/mass 30deg east of south.

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Impulse and Momentum at an angle

• An object has 400kgm/s of momentum 30deg east of
  south. Find the components in the north and
  east.
• 400 sin(30deg) 200kgm/seast
• 400 cos(30deg) 346.4kgm/seast

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Impulse and Momentum at an angle

• An object has 400kgm/s of momentum 30deg east of
  south. If it strikes another object that is at
  rest, what must the total momentum be after the
  collision in the north and east directions? What
  condition are we assuming for this to be true?

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Impulse and Momentum at an angle

• Since momentum must be conserved (if there are no
  outside forces), its the same as before
• 400 sin(30deg) 200kgm/seast
• 400 cos(30deg) 346.4kgm/seast

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Conservation of Momentum

• The total linear momentum of an isolated system
  remains constant. An isolated system is one for
  which the vector sum of the external forces
  acting on the system is zero. All of the momentum
  before an event must be the same after the event.
  

m1v1f m2v2f m1v1i m2v2i
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Conservation of Momentum

• If a 80kg guy is rolling on his skates very fast
  towards his girlfriend at 8m/s while she is
  rolling towards him at 2m/s. Find their final
  velocity if they hold on to one another after the
  collision assuming that she is only half his
  mass.
• m1v1f m2v2f m1v1i m2v2i
• 8okgv1f 40kgv2f 80kg8m/s 40kg(-2m/s)
• Since they move together, v1f v2f vf

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Conservation of Momentum

• 8okgv1f 40kgv2f 80kg8m/s 40kg(-2m/s)
• Since they move together, v1f v2f vf
• 8okgvf 40kgvf 80kg8m/s 40kg(-2m/s)
• 8okgv1f 40kgv2f 560kgm/s
• 120kg(v1f) 560kgm/s
• 4.67m/s in the boyfriends initial direction
  (which was made the pos direction above)

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Conservation of Momentum

• If he pushed her fast enough so that he stops,
  how fast would the girlfriend have to be going?
• m1v1f m2v2f m1v1i m2v2i
• 8okg0m/s 40kgv2f 80kg8m/s 40kg(-2m/s)
• 0 40kgv2f 560kgm/s
• v2f (560kgm/s)/40kgv2f 14 m/s in the orig
  direction of the boyfriend

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Conservation of Momentum

• If a superball is 0.4kg and going 25m/s east
  before striking a wall and returning at 25m/s,
  find the force that the wall exerted if the time
  of contact was 0.20 sec.
• Dmv Ft 0.4kg(-25m/s 25m/s) Ft
  0.4kg(-50m/s) -20kgm/s Ft -20kgm/s
  F(0.20s)
• F -100N

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Conservation of Momentum

• The drawing shows a collision between two pucks
  on an air hockey table. Puck A has a mass of
  0.25 kg and is moving along the x-axis with a
  velocity of 5.5 m/s. It makes a collision with
  puck B, which has a mass of 0.5 kg and is
  initially at rest. After the collision, the two
  pucks fly apart with the angles shown in the
  drawing. Find the final speed of (a) puck A and
  (b) puck B.

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• p initial, total 5.5(.25) 1.375kgm/s to the
  right.
• After collision in the y directionmvAsin(65) -
  mvBsin(37) 0
• After collision in the x direction mvAcos(65)
  mvBcos(37) 1.375kgm/s right.
• After collision in the y direction.25vA(.9063)
  .5vB(-.60) 0
• vB 0.753vA.
• After collision in the x direction.25vA(.4226)
  .5vB(.7986) 1.375kgm/s r

• .25vA(.4226) .5(0.753vA)(.7986) 1.375kgm/s r
• 0.4063vA 1.375kgm/s rt
• vA 3.4m/s Now plug back into vB 0.753vA.
  0.753(3.4m/s)
• vB 0.753vA. 0.753(3.4m/s) 2.55m/s

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Impulse again

• If two objects collide, Newtons 3rd law tells us
  that the forces must be equal but opposite
  directions. Common sense tells us that if A is
  in contact with B for some time, t, then B is
  also in contact with A for time, t.
• Therefore, the impulse between two interacting
  bodies will also be equal but opposite (if there
  are no outside forces)

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Test Taking Skills

• Tip 1 Take everything one step at a time and
  include units for each step.
• Tip 2 Watch the signs/directions
• Tip 3 When adding vectors, dont forget to add
  head-to-tail
• Tip 4 If you put the correct answer, dont
  change it!! ?
• Tip 5 Ask yourself if the answer makes sense.