Physics AS notes OCR

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Physics AS ­ Level Summary
*Points to help remember definitions etc.
SCALARS AND VECTORS:
A SCALAR quantity has magnitude only and is completely described by a number and
appropriate units. *'scale' is a measure so magnitude only.
A VECTOR quantity is completely described only if both its magnitude and direction are
stated. *mathematical vectors in translation tell you where to go (direction) and by how
many units (magnitude)
To determine whether a quantity is scalar or vector, think about if you are given a direction in
your answer. *If it can be a negative value it is usually a vector quantity.
VECTOR TRIANGLES:
Join the forces head to tail, join the ends to find the resultant;
Resultant
If it is a right angled triangle, use Phythagoras' Theorem (a2+b2=c2)
If not, use Trigonometry (SOHCAHTOA)
You can also use SCALE DRAWING to find the resultant.
1. State the scale e.g. 1N = 1cm
2. Draw accurately to scale (both length and angle)
3. Use the diagram to measure the size and direction of the resultant.
NOTE: Force is a vector quantity so remember to measure both magnitude (force) and
direction (angle)
RESOLVING VECTORS:
If you are given one force and an angle, use Trigonometry (SOHCAHTOA)
NOTE: You can use this to find horizontal and vertical components.
DISPLACEMENT, SPEED AND VELOCITY:
DISPLACEMENT is the straight line distance travelled in a stated direction.
NOTE: displacement is a vector quantity whereas distance is scalar.
INSTANTANEOUS SPEED is the speed at a given instant of time.
AVERAGE SPEED = DISTANCE / TIME *Speed is scalar so use distance (because it is also
scalar)
VELOCITY = DISPLACEMENT / TIME *Velocity is a vector so displacement is used.
ACCELERATION = CHANGE IN VELOCITY / TIME *this is the gradient on a velocity/time graph.
Conventionally, upwards or forwards is positive and so downwards or backwards is
negative. For example, acceleration due to gravity would be, g = -9.81ms-2

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GRAPHS OF MOTION:
Displacement-Time graphs:
The gradient is velocity so a straight line shows constant velocity and a curve shows
acceleration *m = y / x which is m/s or ms-1 which is what velocity is measured in.
Velocity-Time graphs:
The gradient shows acceleration *m = y / x which is m/s/s or ms-2 which is acceleration.
The area under a v-t graph gives the displacement.…read more

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Equilibrium is when the resultant forces are equal (and three forces make a closed triangle)
CENTRE OF GRAVITY:
The centre of gravity of an object is the single point through which the entire weight of the
object seems to act.
TEST TO FIND THE C.O.G.
1. The body is allowed to dangle vertically, swinging from a horizontal pivot.
2. Mark the position of the plumb line and draw a line from this point across to the
pivot.
3.…read more

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CAR SAFETY:
BREAKING DISTANCE is the distance travelled by the car from when the driver starts to brake
to when the car comes to a stop.
THINKING DISTANCE is the distance travelled while the driver is reacting. This can be
calculated by the SPEED OF THE VEHICLE X REACTION TIME.
STOPPING DISTANCE = THINKING DISRTACE + BREAKING DISTANCE
*When asked about factors that affect braking distance, if worn tyres are mentioned make
sure you say `in wet weather conditions'.…read more

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The LAW OF CONSERVATION OF ENERGY is that energy cannot be created or destroyed but
may be converted from one form to another.
SANKEY DIAGRAMS can be used to represent this conversion of energy.
ENERGY AND WORK DONE:
Work done = force x distance moved in the direction of the force
W = Fx or W = Fxcos when the angle between the force and the distance is .…read more

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F = kx
On a graph when the graph is no longer a straight line the spring is permanently deformed.
Force
Work done or strain energy
Distance
The ELASTIC LIMIT is the maximum load up to which the spring regains its original length when
the load is removed.
STRAIN ENERGY:
Strain energy = work done = 1/2kx2
Stain energy = area under graph
RUBBER:
There is no Hooke's Law region for rubber *toothpaste shape...…read more

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Q, the wire, is usually ~2.0m and ~0.1mm2 thin to get a large extension.
ELASTIC AND PLASTIC BEHAVIOUR:
BRITTLE MATERIALS are elastic right up to the breaking point e.g. glass or cast iron.
DUCTILE MATERIALS have elastic behaviour initially, and then beyond a certain point the wire
stretches until it won't return to the original length when unloaded. This permanent change is
called PLASTIC DEFORMATION.
These materials can be drawn into thin wires e.g.…read more

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