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# Leaving cert Physics

### Definitions - Mechanics

Question | Answer |
---|---|

Vector | A quantity which has both magnitude and direction. |

Scalar | A quantity which has magnitude only. |

Distance | A measure of the separation between two points. |

Displacement | The separation between two points in a given direction. |

Speed | The distance traveled per unit time. |

Velocity | The rate of change of displacement. |

Acceleration | The rate of change of velocity. |

Equations of motion | v = u + at, s = ut +1/2at², v² = u² + 2as |

Momentum | The product of the mass and velocity of a body p = mv |

The principle of the conservation of momentum | In any closed system of colliding bodies, the total momentum remains constant |

Force | That which changes or tends to change the velocity of an object |

Newton | A force of 1 N gives an acceleration of 1m/s-² to a mass of 1kg |

Weight | The force with which a force is attracted to a planet W = mg |

Friction | The force that opposes motion between two surfaces in contact |

Newtons 1st Law of motion | A body remains at rest or in uniform motion unless acted upon by a resultant force. |

Newtons 2nd Law of motion | The rate of change of an objects momentum is directly proportional to the force that caused it, and takes place in the direction of that force. F = ma |

Newtons 3rd Law of motion | For every action there is an equal and opposite reaction. |

Work | When a force causes a body to be displaced W =Fd |

Energy | The ability to do work |

Potential energy | The energy that a body has due to it's position or state P.E = mgh |

Kinetic energy | The energy a body has due to it's motion K.E = 1/2mv² |

1 Joule | The work done when a force of 1N displaces an object by 1m |

The principle of conservation of energy | Energy can neither be created or destroyed , only converted from one form to another |

Power | The rate at which work is done or energy is converted P = W/t |

1 Watt | 1 watt of power is equal to doing 1 joule of work per second |

A renewable source of energy | an energy source that will not be exhausted e.g, wind , tidal , solar |

The efficiency of a machine | The energy output as a percentage of energy input |

Lever | Any rigid body that is free to move around a fixed point called a fulcrum |

The centre of gravity | The point at which the weight of the object appears to act |

The moment of a force (Torque) | The force applied multiplied by the perpendicular distance from the fulcrum M = Fd |

Conditions for equilibrium | 1. The vector sum of the forces in any direction must be zero. 2. The sum of the moments about any point must be zero. |

A Couple | Two parallel forces that act in opposite directions. The moment of a couple is the magnitude of one force multiplied by the perpendicular distance between them |

Hooke's law | The extension of an elastic body is directly proportional to the force extending it |

Simple Harmonic Motion (SHM) | Motion where an objects acceleration is directly proprtional to it's displacement from a mean position and is always directed towards it |

Newton's law of universal gravitation | The force between two bodies is directly proportional to the product of their masses. And inversely proportional to the square of the distance between them |

Centripetal Force | The force acting towards a centre of a body moving in a circular path |

Angular Velocity | The angle, in radians, swept out per second |

Periodic time | The time taken to complete one revolution |

Kepler's 3rd Law | The square of the periodic time of a satellite is directly proportional to the cube of it's radius of orbit and inversely proportional to the mass of the planet it orbits |

Density | The mass per unit volume of a substance p = m/v |

Pressure | The force per unit area P = F/A |

Archimedes' principle | When a body is partially or fully immersed in a fluid, it experiences an upthrust which is equal to the weight of the liquid displaced |

Law of flotation | A floating body displaces it's own weight of fluid |

Boyle's Law | For a fixed mass of gas at constant temperature, its volume is inversely proportional to it's pressure |