Elastic collision: The kinetic energy of the system is ...



Elastic collision: The kinetic energy of the system is unchanged by the collision.

Inelastic collision: The kinetic energy of the system is changed by the collision.

In a collision, one moving object briefly strikes another object. During the collision, the forces the objects exert on each other are much greater than the net effect of other forces acting on them, so we may ignore these other forces.

Elastic and inelastic are two terms used to define types of collisions. These types of collisions differ in whether the total amount of kinetic energy in the system stays constant or is reduced by the collision. In any collision, the system’s total amount of energy must be the same before and after, because the law of conservation of energy must be obeyed. But in an inelastic collision, some of the kinetic energy is transformed by the collision into other types of energy, so the total kinetic energy decreases.

In both elastic and inelastic collisions occurring within an isolated system, momentum is conserved. This important principle enables you to analyze any collision.

[pic]

The picture and text above pose a classic physics problem. Two balls collide in an elastic collision. The balls collide head on, so the second ball moves away along the same line as the path of the first ball. The balls’ masses and initial velocities are given. You are asked to calculate their velocities after the collision. The strategy for solving this problem relies on the fact that both the momentum and kinetic energy remain unchanged.

Variables

|  |  |ball 1 (purple) |ball 2 (green) |

|  |mass |m1 = 2.0 kg |m2 = 3.0 kg |

|  |initial velocity |vi1 = 5.0 m/s |vi2 = 0 m/s |

|  |final velocity |vf1 |vf2 |

What is the strategy?

1. Set the momentum before the collision equal to the momentum after the collision.

2. Set the kinetic energy before the collision equal to the kinetic energy after the collision.

3. Use algebra to solve two equations with two unknowns.

|Physics principles and equations |

|Since problems like this one often ask for values after a collision, it is convenient to state the following conservation |

|equations with the final values on the left. |

|Conservation of momentum |

|m1vf1 + m2vf2 = m1vi1 + m2vi2 |

|Conservation of kinetic energy |

|½ m1vf12 + ½ m2vf22 = ½ m1vi12 + ½ m2vi22 |

[pic]

[pic]

Inelastic Collisions

Inelastic collision: The collision results in a decrease in the system’s total kinetic energy.

In an inelastic collision, momentum is conserved. But kinetic energy is not conserved. In inelastic collisions, kinetic energy transforms into other forms of energy. The kinetic energy after an inelastic collision is less than the kinetic energy before the collision. When one boxcar rolls and connects with another, as shown above, some of the kinetic energy of the moving car transforms into elastic potential energy, thermal energy and so forth. This means the kinetic energy of the system of the two boxcars decreases, making this an inelastic collision.

A completely inelastic collision is one in which two objects “stick together” after they collide, so they have a common final velocity. Since they may still be moving, completely inelastic does not mean there is zero kinetic energy after the collision. For instance, after the boxcars connect, the two “stick together” and move as one unit. In this case, the train combination continues to move after the collision, so they still have kinetic energy, although less than before the collision.

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download