## IB Physics Glossary

Browse the glossary using this index

Special | A | B | C | D | E | F | G | H | I | J |

**K**| L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | ALL

## K |
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## Kepler's 3rd law of planetary motion states the orbital period squared is proportional to the mean orbital radius cubed: $$T^2 \propto r^3$$. Derivation $$F = G \frac{Mm}{r^2}=\frac{mv^2}{r}$$. or $$ G \frac{M}{r}=v^2$$. Since the speed, $$v$$, is the distance travelled in one orbit, $$2 \pi r$$, divided by the orbital period $$T$$ we can write $$ G \frac{M}{r}=(\frac{2 \pi r}{T})^2$$ or $$ G \frac{M}{r}=\frac{4 \pi^2 r^2}{T^2}$$ then $$T^2 = \frac{4 \pi^2}{GM} \times r^3$$ or $$T^2 = constant \times r^3 => T^2 \propto r^3$$. | |

## Kinetic energyis the energy of a moving object. | |

## Kinetic theory of gasesis a theory which treats molecules in a gas as mechanical objects. We assume that 1. A gas contains a large number of molecules; 2. Molecules have a range of kinetic energies, and therefore a range of speeds; 3. A gas molecule occupies a volume much less than that of the volume of the gas; 4. Collisions of molecules with other molecules and with the walls of the container are elastic; 5. Molecules exert forces on other molecules or on container walls only when contact occurs; 6. Collisions times are small compared to time between collisions; 7. Molecules obey the laws of Newtonian mechanics; | |