Mechanics Definitions Scalar quantity- a quantity that is completely specified by a magnitude e.g. distance, time, speed and energy Vector quantity- a quantity that is completely specified by a magnitude and direction e.g. displacement, velocity, force and momentum Displacement- the distance is a given direction Velocity- the rate of change of displacement Acceleration- the rate of change of velocity Force- the rate of change of momentum Impulse- the change in momentum Resultant force- a single force which has the combined effect of two or more forces (vector sum) Equilibrium- the resultant force acting on an object is equal to zero - forces form a ‘closed triangle’ (forces are balanced) - object remains stationary or moves at a constant velocity Moment- the turning effect of a force - moment = force x perpendicular distance from the line of action of the force to the pivot Principle of Moments- for an object in equilibrium, the sum of all anticlockwise moments about any point must equal the sum of all the clockwise moments about that point Couple- a pair of equal but opposite coplanar forces Torque- the total turning effect of a couple Centre of mass- the point where all the mass of an object can be thought to be concentrated - use lines of symmetry to find centre of mass of a regular shape Displacement-time graph- a graph of displacement (y-axis) against time (x-axis) - the gradient equals instantaneous velocity Velocity-time graph- a graph of velocity (y-axis) against time (x-axis) - the area under the graph equals displacement - the gradient equals acceleration Acceleration-time graph- a graph of accleration (y-axis) against time (x-axis) - the area equals change in velocity Average velocity- the velocity of an object can be thought to be travelling at over a given time period - average velocity = total displacement/ total time the a level physicist Newton’s First Law- an object remains at rest or remains in uniform motion unless acted upon by a force Newton’s Second Law- the acceleration of an object is directly proportional to the resultant force Newton’s Third Law- for every action, there is an equal but opposite reaction Conservation of Momentum- for a system of interacting particles, the total momentum is constant provided no external force acts upon the system Elastic collision- a collision where there is no loss of kinetic energy Inelastic collision- a collision where kinetic energy is lost Terminal speed- the maximum velocity of a falling object - there is no resultant force as the force air resistances is equal to the weight of the object Air resistance- the frictional force acting on an object moving through air, in the opposite direction to the motion of the object - air resistance increases with speed - Acceleration due to gravity, g- 9.81ms-2 Impact time- the duration for which the impact force acts Car safety features- crumple zone, airbag and seatbelt - safety features increase the impact time, which decreases the deceleration of the vehicle to decrease the impact force experienced by the occupants - airbag also increases contact area to spread force over a greater area, which reduces the pressure on the body Power- rate of energy transfer Efficiency- proportion of total energy input transferred to useful energy output the a level physicist
