AS-Level Mechanics OCR G481 Flashcards Preview

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Flashcards in AS-Level Mechanics OCR G481 Deck (55)
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define scalar and vector quantities and give examples; 

Scalar: Magnitude without direction
Density, volume, temperature etc
Vector: A quantity that has (both) magnitude / size and direction
acceleration, displacement and weight  etc


define displacement

Displacement = (net) distance moved in a particular direction.


define instantaneous speed

Instantaneous speed = speed of a body at a specific instance or at a specific point


define average speed

distance travelled / time taken


define velocity

speed in a given direction


define acceleration

Acceleration is the rate of change of velocity (gradient of a velocity vs time graph, change in velocity/time taken)


Define average velocity

(net) displacement / time taken


define the newton;

The (net) force which gives a mass of 1kg an acceleration of 1 ms-2


define the torque of a couple;

one of forces × perpendicular distance (between forces) 

(Not force x perpendicular distance)


define the moment of force;

moment = force x perpendicular distance from pivot / axis / point 


define thinking distance

Thinking distance: The distance travelled (by the car) from when the driver sees a problem and the brakes are applied 


Define braking distance 

The distance travelled (by the car) whilst the brakes are applied and the car stops


define stopping distance

stopping distance: Thinking distance + braking distance. The total distance travelled in the interval between a driver spotting a hazard, and the vehicle coming to a complete stop.


define work done by a force;

work done = force x distance moved in the direction of the force 


define the joule;

Energy required  to move a weight of 1N (through) a distance of 1 m


define power 

The rate at which work is done:

Power = Work done / Time


Power = Energy / Time


Define the Watt

Power required to move 1N through a distance of 1m in 1 sec (Rate of doing work)


Define Stress

force/(cross-sectional) area 


Define Strain

extension/original length 


Define Young's modulus 

Young modulus = stress/strain / Young modulus is equal to the gradient from stress-strain graph (in the linear region) 


Define ultimate tensile strength

Ultimate tensile strength = Maximum stress material can withstand (before fracture) 


define the term elastic deformation 

Elastic extension (or compression) is proportional to force (as long as elastic limit is not exceeded) 


define plastic deformation of a material

Plastic: Material does not return to original length / shape/ size (is permanently deformed / longer) when the force / stress is removed.


Define density

Density = mass/volume or mass per (unit) volume


Derive the equations of motion for constant
acceleration in a straight line from a velocity
against time graph;

Area of triangle    = ½ (v-u) t    [(v-u) = at]
                             = ½ at2 
Area of rectangle = ut    add the two together


Apply the definition of work done to derive the equation for the change in gravitational
potential energy;

Work done = force x distance
Force = mass x acceleration
Weight = mass x gravitational field strength
G.P.E. = m x g x h


apply the equations of constant acceleration to describe and explain the motion of an object due to a uniform velocity in one direction and a constant acceleration in a perpendicular direction

With a uniform velocity in one direction the distance travelled per unit time will remain constant

s = ut + ½at2

For a = 0

s = ut which is linearly proportional to "t"

With a constant acceleration in another direction the distance travelled per unit time in that direction increases.

s = ut + ½at2 which is not linearly proportional to "t"

This means that the object traces a parabollic path.


If work is done on a system, what can we say about transfer of energy in that system.

If work is done energy must be transfered from one type to another. The total amount of energy transferred into other forms is equal to the work done


describe an experiment to determine the
acceleration of free fall g using a falling body;


1) Height (distance)

2) Time (of fall)


1) Ruler/tape (measure)

2) Stop watch/timer/clock/video


g = 2s/t2 or g = twice the gradient  of s-t2 graph

Why is this not accurate?

air resistance / drag
  parallax (landing time)
      starting/stopping the clock

Derivation of g = 2s/t2

s=ut+½at2 (ut=0),

s = ½at2,


For a falling body a = g, so g = 2s/t2 


Describe the motion of bodies falling in a
uniform gravitational field with drag;

1) Initially the body has no speed. The resultant force acting on it is 9.81N/kg and it is accelerating at 9.81m/s2. Because the body has no velocity, air resistance is zero.

2) Then as speed increases, air resistance increases, so the resultant force is now less than 9.81N/kg and therefore its acceleration is decreasing.

3) Eventually the object will reach a speed where the force of air resistance balances the force of gravity. At this point there is no acceleration, we say the object has reached terminal velocity.