What is the difference between an elastic collision and an inelastic one? How does an inelastic collision work and why?

When two objects collide with one another, they usually bounce. What distinguishes an elastic collision from an inelastic collision is the extent to which that bounce retains the objects’ total kinetic energy—the sum of their energies of motion. In an elastic collision, all of the kinetic energy that the two objects had before the collision is returned to them after the bounce, although it may be distributed differently between them. In an inelastic collision, at least some of their overall kinetic energy is transformed into another form during the bounce and the two objects have less total kinetic energy after the bounce than they had before it.

Just where the missing energy goes during an inelastic collision depends on the objects. When large objects collide, most of this missing energy usually becomes heat and sound. In fact, the only objects that ever experience perfectly elastic collisions are atoms and molecules—the air molecules in front of you collide countless times each second and often do so in perfectly elastic collisions. When the collisions aren’t elastic, the missing energy often becomes rotational energy or occasionally vibrational energy in the molecules. Actually, some of the collisions between air molecules are superelastic, meaning that the air molecules leave the collision with more total kinetic energy than they had before it. This extra energy came from stored energy in the molecules—typically from their rotational or vibrational energies. Such superelastic collisions can also occur in large objects, such as when a pin collides with a toy balloon.

Returning to inelastic collisions, one of the best examples is a head-on automobile accident. In that case, the collision is often highly inelastic—most of the two cars’ total kinetic energy is transformed into another form and they barely bounce at all. Much of this missing kinetic energy goes into deforming and heating the metal in the front of the car. That’s why well-designed cars have so called “crumple zones” that are meant to absorb energy during a collision. The last place you want this energy to go is into the occupants of the car. In fact, the occupants will do best if they transfer most of their kinetic energies into their airbags.

I am a huge figure skating fan and was wondering if you could explain to me the physics of a triple axle jump? My friends and I are always asking ourselves how it’s done. — AF

While I don’t know the details of the jump, there are some basic physics issues that must be present. At a fundamental level, the skater approaches the jump in a non-spinning state, leaps into the air while acquiring a spin, spins three times in the air, lands on the ice while giving up the spin, and then leaves the jump in a non-spinning state. Most of the physics is in spin, so that’s what I’ll discuss.

To start herself spinning, something must exert a twist on the skater and that something is the ice. She uses her skates to twist the ice in one direction and, as a result, the ice twists her in the opposite direction. This effect is an example of the action/reaction principle known as Newton’s third law of motion. Because of the ice’s twist on her, she acquires angular momentum during her takeoff. Angular momentum is a form of momentum that’s associated with rotation and, like normal momentum, angular momentum is important for one special reason: it’s a conserved physical quantity, meaning that it cannot be created or destroyed; it can only be transferred between objects. The ice transfers angular momentum to the skater during her takeoff and she retains that angular momentum throughout her flight. She only gives up the angular momentum when she lands and the ice can twist her again.

During her flight, her angular momentum causes her to spin but the rate at which she spins depends on her shape. The narrower she is, the faster she spins. This effect is familiar to anyone who has watched a skater spin on the tip of one skate. If she starts spinning with her arms spread widely and then pulls them in so that she becomes very narrow, her rate of rotation increases dramatically. That’s because while she is on the tip of one skate, the ice can’t twist her and she spins with a fixed amount of angular momentum. By changing her shape to become as narrow as possible, she allows this angular momentum to make her spin very quickly. And this same rapid rotation occurs in the triple axle jump. The jumper starts the jump with arms and legs widely spread and then pulls into a narrow shape so that she spins rapidly in the air.

Finally, in landing the skater must stop herself from spinning and she does this by twisting the ice in reverse. The ice again reacts by twisting her in reverse, slowing her spin and removing her angular momentum. She skates away smoothly without much spin.