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# Motion definitions

### AQA A level physics

Term | Definition |
---|---|

Efficiency | The ratio of useful energy transferred by a device to the energy supplied |

Centre of gravity | The point through which all of a bodies weight acts |

Acceleration – Equation and definition | a=(v-u)/t, Change in velocity per unit time |

Displacement | Distance travelled in a given direction |

Equilibrium | State of an object at rest or in uniform motion |

Conditions for equilibrium | Resultant force zero & resultant torque zero. |

Inertia | Resistance of a body to changes in its motion |

Kinetic energy | Energy of a body due to its motion, 1/2 mv^2 |

Newton’s first law | An object remains at rest of in uniform motion unless it is acted upon by a force |

Newton’s second law (constant mass) | Resultant force = mass x acceleration |

Principle of moments | Object in equilibrium: sum of clockwise moments about any point equals the sum of the anticlockwise moments |

Terminal speed | Max speed. The resultant force (of drag and weight) is equal to zero |

Torque of a couple | The total force x perpendicular distance between the line of action of the forces |

Moment of a force about a point | Force x perpendicular distance between the line of action of the force and the point |

Work | Force x distance moved in the direction of the force |

Free fall | The force due to gravity is the only force acting on an object |

Assumptions for projectiles | Air resistance is said to be negligible and the horizontal velocity is constant |

Significance of a closed triangle of vectors | Resultant force is zero / Forces in equilibrium |