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# OCR A level Mec 1

### Definitions Unit 1Module 1

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

Acceleration | The rate of change of velocity. |

scalar | A physical quantity with magnitude ( size) but not direction e.g. speed, distance, pressure, potential difference, density, energy |

vector | A physical quantity with magnitude ( size) and direction e.g. velocity, acceleration, force. |

Displacement (s) | Distance travelled in a particular direction. Therefore a vector. |

Instantaneous speed | The speed of an object at a given moment in time. |

Average speed | A measure of the total distance travelled in a unit of time |

Velocity (v) | Displacement per unit time. |

Average velocity = | Average velocity = displacement/ time= s/t |

Newtons second law | Net force = mass x acceleration |

The newton | Unit of force. 1N is the force that gives a mass of 1kg an acceleration of 1ms-2 |

Drag | The resistive force that acts on a body when it moves through a fluid |

Weight (W) | The gravitational force acting on an object measured in newtons.Weight = mass x acceleration due to free fall W=mg |

Terminal velocity | The velocity at which an object’s drag equals its accelerating force. Therefore there is no resultant force and zero acceleration |

Equilibrium | When the net force and net moment on an extended object is zero |

Centre of gravity | This is the point where the entire weight of an object appears to act |

Triangle of forces | If three forces acting at a point can be represented by the sides of a triangle, the forces are in equilibrium. |

Couple | This is a pair of forces that tends to produce rotation only. They are two forces that are equal in size but act in opposite directions but not in the same straight line. |

Torque of a couple | The turning effect due to a couple. Torque = one of forces x perpendicular distance between the forces. Torque = Fd |

Moment of a force | The turning effect due to a single force. Calculated from the force multiplied by the perpendicular distance from a given point. |

Principle of moments | For a body in rotational equilibrium the sum of the clockwise moments equals the sum of the anticlockwise moments. |

Density | The mass per unit volume |

Pressure(p) | Force per unit area. |

Thinking distance | The distance travelled from seeing the need to stop to applying the brakes |

Braking distance | The distance travelled by a vehicle whilst decelerating to a stop. |

Stopping distance | The sum of the thinking distance and braking distance. |

Crumple zone | An area of a vehicle designed to increase the time over which the vehicle decelerates and so reduce the average force acting. |

Work done by a force(W) | The product of the force and the distance moved in the direction of the force. |

The joule | This is a unit of energy. 1 J is the work done when a force of 1N moves its point of application 1m in the direction of the force. |

Conservation of energy | States that energy cannot be created or destroyed , just converted from one form to another or transferred from one place to another. |

power | Rate of work done |

The watt | A unit of power. 1 watt is 1J of energy transferred per second. |

efficiency | The ratio of useful output energy to total input energy. |

Tensile force | Usually two equal and opposite force acting on a wire in order to stretch it. |

Compressive force | Two or more forces that have the effect on reducing the volume of the object on which they are acting or reducing the length |

Extension(x) | The change in length of an object when subjected to a tension |

Elastic limit | The point at which elastic deformation becomes plastic deformation. |

Limit of proportionality | The point at which an object no longer obeys Hooke’s Law |

Hooke’s law | The extension of an elastic body is proportional to the force that causes it up to the limit of proportionality |

Force constant(k) | Force per unit extension or compression |

Elastic potential energy | The energy stored in a stretched or compressed object (e.g. a spring) |

stress | The force per cross-sectional area. |

strain | The extension per unit length |

Young’s modulus (Y) | The ratio between stress and strain |

Ultimate tensile strength | The maximum tensile force that can be applied to an object before it breaks. |

Breaking stress | The maximum stress that can be applied to an object before it breaks |

Elastic deformation | The object will return to its original shape when the deforming force is removed |

Plastic deformation | The object will not return to its original shape when the deforming force is removed, it becomes permanently deformed. |

Ductile material | Materials that have a large plastic region and can therefore can be drawn into a wire e.g. copper |

Brittle material | A material that distorts very little even when subject to a large stress and does not exhibit any plastic deformation e.g. concrete. |

Polymeric material | A material made of many smaller molecules bonded together, often making a tangle knot of chains e.g. rubber. These materials often exhibit very large strains e.g. 300% |