The kinetic energy is converted from mass and velocity into heat energy when it strikes the growing body. This energy, and therefore heat, is considerable, and if accretion occurs fast enough, then the larger body can be heated all the way to melting by accretional kinetic energy. If the larger body is melted even partially, it will differentiate.
How is energy transfigured into heat, and how is heat transformed into melting? To transfer energy into heat, the type of material has to be taken into consideration. Heat capacity describes how a material's temperature changes in response to added energy. Some materials increase in temperature easily in response to energy, while others need more energy to get hotter. Silicate minerals have a heat capacity of 245.2 cal/°lb (1,256.1 J/°kg), meaning that 245.2 calories of energy are required to raise the temperature of one pound of silicate material one degree. Here is a sample calculation. A planetesimal is about to impact a larger body, and the planetesimal is one kilometer in radius. It would weigh roughly 3.7 X 1013pounds (1.7 X 1013kg) if its density were about 250 pounds per cubic foot (4,000 kg/m3). If it was traveling at six miles per second (10 km/sec), then its kinetic energy would be
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