Energy during collision

kinetic energy is conserved in elastic collisions, but not in all collisions

momentum is always conserved in any collision in a closed system. some of the kinetic energy of colliding objects can be converted to other forms of energy, such as the loud sound of crash. kinetic energy- is conserved during- elastic collisions kinetic energy- is not conserved during- inelastic collisions- energy may be transferred as-heat-sound

energy and momentum conservation is used in analyzing forensic evidence

After a serious traffic accident, investigators may have to reconstruct the accident from forensic evidence such as skid marks on the road. A driver typically tries to avoid a collision by applying and locking the car's brakes. The car will then skid or slide, leaving prominent skid marks. From the length of the skid mark and the measurements of friction between the tires and the road surface, it is possible to estimate the work done by friction to bring the car to a stop. This tells investigators the initial kinetic energy and therefore the minimum speed it must have been moving. Tests are done continuously to model how collisions happen and how to make injury from them less likely. Here is a case in which physics theory and research are applied and save lives.

elastic collisions are those in which kinetic energy is the same for the system before and after the collision

An elastic collision is one in which the total kinetic energy before and after the collision are equal. Consider again a collision between two air-track gliders. The collision is elastic if the two gliders simply bounce off each other without producing thermal energy so that total kinetic energy is the same before and after the gliders collide.

collisions are characterized as elastic or inelastic depending on whether they conserve kinetic energy

Because kinetic energy is just one kind of energy, it can change into other forms of energy. Collisions between hard objects that do not deform much—such as billiard balls, steel and other hard objects—often nearly conserve kinetic energy and are nearly elastic collisions. Collisions between atoms, molecules, and collisions involving merely deflection of astronomical objects by gravity, are also nearly, if not completely, elastic. Collisions between deformable ordinary-size objects usually do not conserve energy and are inelastic.

Momentum is conserved, and kinetic energy is sometimes conserved, in a collision.

Cars collide. They exert force on each other. The force and acceleration that result can be extreme during the short collision time. The result can deform the cars. Kinetic energy is often converted into other forms of energy such as sound during the deformation. Thus, while momentum is always conserved in a collision, kinetic energy is not necessarily conserved. It would be difficult to apply Newton's laws to analyze what happens moment by moment in a collision. But by using the law of conservation of linear momentum, you can relate final velocities to initial velocities without knowing the intermediate details.

elastic collisions occur commonly in physics

Collisions between two objects such as billiard balls or air-track gliders that are barely deformed by the force of contact are nearly elastic. But an elastic collision between two objects need only involve interaction, not necessarily contact. A space probe sent to explore the solar system may use the deflection from gravitational force as it passes a planet orbiting the sun to increase the probe's speed for the rest of its mission. The total kinetic energy is conserved and thus we define the collision as elastic. For a different example, consider that the collisions between molecules in an ideal gas and also between the molecules and the walls of their container are nearly elastic. Chemists use this fact to calculate the relationships among volume, pressure, and temperature.

In an explosive collision, the kinetic energy before the collision is less than the kinetic energy after the collision

Consider a collision in which a 0.200 kg glider moving at 1.0 m/s 1.0 m per s bumps into a stationary 0.300 kg glider. What if the 0.200 kg glider then bounces backward at −0.230 m/s negative 0.230 m per s, and the 0.300 kg cart moves away at 0.820 m/s 0.820 m per s? The final kinetic energy in this case is larger than the initial kinetic energy. This would be impossible without a source for the extra energy. But if there were an initially compressed spring that can be released, pushing the two carts apart, then the potential energy in the spring could supply the missing kinetic energy. A collision in which the kinetic energy increases is an explosive collision . It occurs only if energy is supplied to provide for the kinetic energy increase.

consider what happens when objects lose their shape

Imagine a girl falling onto a surface from a significant height. She has kinetic energy that is transferred to whatever she comes in contact with. If the girl had fallen onto the elastic exercise ball, her kinetic energy would have been absorbed by the ball. How do you know this? The ball clearly changes its shape—it deforms. The ball will return to its original shape after the pressure is released, but the girl will not return to the same height because she has landed softly. Some of the kinetic energy has been transformed into other forms of energy in deforming the ball. In other cases, it may be transferred into a loud noise or into a small heating of the surface contacted—that is, it may turn into sound or heat. For a collision between fairly rigid objects, such as billiard balls, very little of the kinetic energy is lost, though some is, as sound. Thus, you can see that there are different kinds of kinetic energy relationships in collisions.

in an inelastic collision, the kinetic energy of the system before the collision is greater than the kinetic energy after the collision

In an inelastic collision, Ki is greater than Kf. This can result when some kinetic energy is converted to sound or heat. It can also occur in a completely inelastic collision, one in which the two objects stick together and continue to move as a single object whose mass is the sum of the two masses. Momentum is still conserved in such a collision. The total mass moving together after a collision is the sum of the two initial masses. This mass can be used to find the final velocity from the total momentum, allowing you to calculate if there is a loss of kinetic energy.

Conservation of both kinetic energy and momentum allows final velocities to be found

Some objects, such as billiard balls, hardly deform when they collide and have nearly elastic collisions. In an elastic collision along a straight line—that is, in one dimension—conservation of momentum and conservation of kinetic energy together provide enough information to calculate the final velocities from the initial velocities and masses without knowing other details of the collision.