The world is complex. Every situation has many possible factors to take into account. Often however many of these possible factors are irrelevant and can be ignored. For example, we would not expect the temperature to greatly influence the acceleration due to gravity so can ignore this factor. Even if a factor cannot be ignored, we can often gain insight into a phenomenon by ignoring it. A model can later be refined by taking more factors into account not making that simplifying assumption.

The table below lists some common assumptions.

Assumption | Examples |

Friction is negligible | Pulley The motion of a car down or up a slope |

No heat lost | Heat being transferred from one body to another when both are insulated together |

Mass of string or spring is negligible | Simple pendulum Mass on stretched spring |

Resistor of ammeter is zero | Electric circuits |

Resistance of voltmeter is infinite | Electric circuits |

Internal resistance of battery is zero | Electric circuits |

Conductor obeys Ohm's Law | Electric circuits |

100% efficiency | Transformers Projectiles |

Gas is ideal | Thermodynamics |

Collision is elastic | Collisions. In fact perfectly elastic collsions are only possible between atoms and subatomic particles. |