Inelastic CollisionsSection 7.6

Conservation of Momentum

• How do we apply conservation of energy and momentum to inelastic collisions?

Inelastic Collisions

• Inelastic collisions: collisions in which kinetic energy is not conserved

• Some of the initial KE can be transformed to other types of energy

• Heat, friction….

• In this case, final KE is less than initial KE

Inelastic Collisions

• Or, potential energy can be released

• Then KE final is larger than KE initial

• Explosions!

Totally Inelastic

• Most collisions are at least partially inelastic

• If the two items stick together, the collision is completely inelastic

Railroad Cars

• Cars are 10,000 kg• How much of the initial KE is transformed to other forms

of energy?

Ballistic Pendulum

• Used to measure the speed of a projectile, m (usually a bullet)

• Shot into a block of wood, M, suspended as a pendulum

• M starts at rest• After the collision, the

masses stick together

Ballistic Pendulum

• After the collision, the block moved up a height, h

• Goal is to find v initial

Ballistic Pendulum

• For these problems, use two steps

• Step one is conservation of momentum

• Conservation only holds for the instant before and after the collision

• po = pf

• mv = (m + M)v’

Ballistic Pendulum

• Step 2 is conservation of kinetic energy

• E1 = E2• Solve this for v’• Insert into momentum equation• v = ?• Use this equation for first part of

Problem # 32• Use Pythagorean Theorem for

part 2

Hints for Homework

• Problem 34: Use same procedure as example problem• Step 1: Conservation of momentum: Solve for v2’

• Step 2: Conservation of energy• Insert v2’

• Keep the ½ from the ½ mv2 separate from the other fractions

• Problem 35: Use same procedure as example problem• Step 1: Conservation of momentum: Solve for vA• Step 2: Conservation of energy• Work done by friction equals change in kinetic energy

Homework

• Read Section 7.6• Do Problems # 32, 34, and 35, Page 189