If you’re unfamiliar with the concept of kinetic energy (KE), take a few minutes to skim Chapter 14 right
now.
When the satellite and the UFO bounced off each other, they experienced a perfectly elastic collision.
If kinetic energy is lost to heat or anything else during the collision, it is called an inelastic collision.
The extreme case of an inelastic collision is called a perfectly inelastic collision.
The second collision between the satellite and the UFO was a perfectly inelastic collision. BUT,
MOMENTUM IS STILL CONSERVED, EVEN IN A PERFECTLY INELASTIC COLLISION!
Two-Dimensional Collisions
The key to solving a two-dimensional collision problem is to remember that momentum is a vector, and as
a vector it can be broken into x and y components. Momentum in the x -direction is always conserved, and
momentum in the y -direction is always conserved.
Maggie has decided to go ice-skating. While cruising along, she trips on a crack in the ice and goes
sliding. She slides along the ice at a velocity of 2.5 m/s. In her path is a penguin. Unable to avoid the
flightless bird, she collides with it. The penguin is initially at rest and has a mass of 20 kg, and
Maggie’s mass is 50 kg. Upon hitting the penguin, Maggie is deflected 308 from her initial path, and the
penguin is deflected 608 from Maggie’s initial path. What is Maggie’s velocity, and what is the
penguin’s velocity, after the collision?We want to analyze the x -component of momentum and the y -component of momentum separately. Let’s
begin by defining “right” and “up” to be the positive directions. Now we can look at the x -component.