Inelastic Collisions and
Explosions
Lesson 4
Monday, October 17, 2011
Announcements

Physics Labs this week

Waves, Lesson 1
HW Quiz on Tuesday (HW 1-4)
 Optics lab due Tuesday
 M&E HW #4 due Tuesday

AP Physics B Standards
Inelastic Collisions and Explosions
LESSON 4:

I.D.3.a. Students should understand linear momentum
conservation, so they can:
(2) Identify situations in which linear momentum, or a
component of the linear momentum vector, is conserved.
(3) Apply linear momentum conservation to onedimensional elastic and inelastic collisions and twodimensional completely inelastic collisions.
Lesson Objectives
Students will be able to
1.
2.
use conservation of momentum to perform related
calculations.
use conservation of momentum to describe inelastic
collisions and explosions.
Collisions


Collisions occur when two or more objects meet
simultaneously.
Collisions are distinguished by kinetic energy.

Elastic collisions
• Colliding bodies bounce off each other with no deformation or
conversion of kinetic energy to other forms of energy.

Inelastic collisions
• Deformation occurs, kinetic energy is converted to other forms
of energy.

Perfectly Inelastic
• Objects stick together and become one object. Kinetic energy
is converted to other forms of energy.
Inelastic Collisions

Inelastic collisions



Deformation occurs.
Kinetic energy is converted to other forms of energy.
Perfectly Inelastic


Objects stick together and become one object.
Kinetic energy is converted to other forms of energy.
o Sample Problem 4.1:
An 80-kg roller skating grandma
collides with and picks up a 40-kg
kid. What is their velocity after the
collision?
o Sample problem 4.2:
A fish moving at 2 m/s swallows a
stationary fish which is 1/3 its mass.
What is the velocity of the big fish
after dinner?
Explosions



A perfectly inelastic collision in reverse.
Explosions obey conservation of
momentum, since all forces in an explosion
are internal to the system.
There is a definite conversion of energy
from one form to another in an explosion!

Sample Problem 4.3:
An exploding object breaks into three fragments. A 2.0 kg fragment
travels north at 200 m/s. A 4.0 kg fragment travels east at 100 m/s. The
third fragment has mass 3.0 kg. What is the magnitude and direction of
its velocity?
Recoil



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Recoil is the backward movement of a gun when
it is fired.
The gun moves backward with the same
momentum with which the bullet moves forward.
All forces are internal.
Momentum is conserved.
o Sample Problem 4.4:
A 5.0-kg projectile launcher shoots a 209 gram projectile at
350 m/s. What is the recoil velocity of the projectile launcher?