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# OCR A level Mec 1

### Definitions Unit 1Module 1

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%
Created by: Susan E Casey

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