Physics 2211: Lecture 25 Todays Agenda

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Physics 2211 Lecture 25Todays Agenda

• Collisions in two dimensions

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Two blocks that initially move as shown undergo a
1-D collision. After the collision, the 2.0 kg
block recoils to the left with a speed of 9.0
m/s. What is the velocity of the 4.0 kg
block after the collision? (1) 0 m/s (2)
1.0 m/s left (3) 2.0 m/s left (4) 3.0 m/s
left (5) 4.0 m/s left (6) 5.0 m/s left
4.0 kg
2.0 kg
3.0 m/s
6.0 m/s
17514
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PART 2 For the collision we just discussed,
compute the change in kinetic energy and describe
the type of collision. (1) 30 J
lost---inelastic collision (2) 48 J
lost---inelastic collision (3) 0 J
lost---elastic collision (4) 54 J
lost---totally inelastic collision (5) 81 J
lost---totally inelastic collision
4.0 kg
2.0 kg
3.0 m/s
6.0 m/s
17514
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Momentum Conservation

• The concept of momentum conservation is one of
  the most fundamental principles in physics.
• This is a component (vector) equation.
• We can apply it to any direction in which there
  is no external force applied.
• You will see that we often have momentum
  conservation even when energy is not conserved.

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Inelastic collision in 2-D

• Consider a collision in 2-D (cars crashing at a
  slippery intersection...no friction).

V
v1
m1 m2
m1
m2
v2
before
after
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Inelastic collision in 2-D...

• There are no net external forces acting.
• Use momentum conservation for both components.

v1
V (Vx,Vy)
m1 m2
m1
m2
v2
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Inelastic collision in 2-D...

• So we know all about the motion after the
  collision!

V (Vx,Vy)
Vy
?
Vx
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2-D Elastic Collision of 2 objects

• Suppose we know what the pre-collision
  velocities are.
• We want to find out about the motion of both
  objects after the collision.
• We want v1x,f , v1y,f , v2x,f , v2y,f
• What else do we know
• In an elastic collision, kinetic energy is
  conserved as well as momentum. This leads to 3
  equations
• Ef Ei
• Px,f Px,i (where Px p1x p2x m1v1x
  m2v2x etc)
• Py,f Py,i
• We have 3 equations and 4 unknowns
• We need more information (scattering angle,
  masses).

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