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Physics Revision - 1. Forces and Motions., Physics Revision - 2.…
Physics Revision - 1. Forces and Motions.
1. Speed
Speed is a measure of how far an object has moved in a certain time. Speed is a
scaler.
While velocity is a
vector.
Equation for speed =
v = s/t
v
= Velocity
s
is the distance
t
is the time
Speed can also be measured in units of kilometres per hour
(km/h)
or miles per hour
(mph)
2. Distance-time Graphs
A distance-time graph can be used to see when an object is stationary or at a constant speed
The steeper the slope, the faster the speed. The gradient (slope) of a distance-time graph shows the speed. This is because of the equation for speed:
v=s/t or speed= distance/time
If the distance-time graph is a straight line then the object is moving at a
constant velocity.
A steeper gradient on a straight line means a faster
constant velocity.
An upwards curve means that the object is accelerating and a curve that's levelling off shows an object is
decelerating
A flat line shows an object is
stationary
as its not travelling any distance. A line with a negative gradient shows that the object is
travelling back in the reverse direction
3. Acceleration
Acceleration
is a measure of how much the
velocity
changes by in a given
time
Equation for acceleration=
Acceleration = change in velocity/time taken
The equation can be re written like this:
a = ( v - u ) / t
a= acceleration
v = final velocity
u = initial velocity
t = time taken
4. Motion
Speed = distance/ time
Velocity is the same as speed in a specific direction.
The gradient of a distance-time graph is speed.
Scalers
have a size but not a direction
But vectors have a size
and
a direction.
5. Stopping distance
A car's
stopping distance
is the sum of the
breaking distance
and
thinking distance
The time taken by the driver to react and press the brake pedal is called the
thinking distance
The
Stopping distance
is increased if the roads are wet or icy
In an emergency, the distance travelled by a vehicle after the brakes are applied is called the
breaking distance
In an emergency, the distance travelled by a vehicle before the brakes are applied is called the
thinking disance
Factors that can effect the
thinking distance
Drugs
Alcohol
High speeds
Drinking
Phones/ distractions
Tiredness
Factors that can effect braking distances
Icy roads
High speeds
Worn breaks
Weather
Worn tyres
Heavy car
6. Forces
A force is a
push
or a
pull
It can cause objects to
turn, change shape
or
accelerate
Forces is measured in
newtons (N)
Newtons are measured using a device called a
newton meter
Forces are
vectors
- they have a
size
and a
direction
The
length
of the arrow represents the size of the force
Examples of
contact forces
Friction, Lift, Thrust, Upthrust, Normal contact Force, Tension & Compression
Examples of
Non- contact forces
Weight, Electrostatic & Magnetic
7. Newtons Laws
Newtons
first law
An object will remain at rest or a constant speed unless acted upon by an unbalanced force (resultant force more than zero)
Newtons
second law
The rate of change of momentum is equal to the unbalanced force (resultant force). This translates into the
acceleration
of an object
is proportional to
the size of the
resultant force
acting on it
This then gives us the equation: F= m x a (Force = mass x acceleration)
8. Mass and Weight
Mass
Mass is a measure of how much matter an object is made from
Mass is measured in
kilograms (kg)
In space, mass stays the
same
Weight
Weight is the name given to the forces caused by
gravity
Weight is measured in
Newtons (N)
In space, the weight changes depending on the
gravitational field strength
On, Earth, your weight always acts towards
the centre of the Earth.
9. Terminal velocity
Falling objects
There are two main forces which affect a falling object at different stages of its fall:
:point_right: The
WEIGHT
of the object- this is a force acting downwards, caused by the Earth's gravitational field acting on the object's mass
:point_right:
DRAG/ AIR RESISTANCE
- This is a frictional force acting in the opposite direction to the movement of the object.
Three stages of falling
At the start, the object accelerates downwards because of its weight. There is a very little air resistance. There is a resultant force acting downwards. The acceleration is constant when the object is close to Earth
As the object gains speed, its weight stays the same but the air resistance on it increases. There is a smaller resultant force acting downwards
Eventually, the object's weight is balanced by the air resistance. There is no resultant force and the object reaches a steady speed- this is known as the terminal velocity
10. Hooke's Law
Hooke's law states that:
The extension of a stretched material is proportional to the force applied to it
This means that if you double the force the extension of the material is doubled
If the material does not obey
Hooke's Law
then the graph does not have a straight line through the origin.
Where the line of the graph stops being straight it is said to be the limit of proportionality
Where the line stops is the point where the material snaps
Physics Revision - 2. Electricity
1. Circuits
The properties of a circuit
Current
is the rate of
flow of charge
round the circuit.
Electrons
usually carry the charge - they're
negatively charged
particles. Current will
only flow
through a component if there is a
voltage
across that component.
Voltage
is what drives the current round the circuit. Kind of like '
electrical pressures'
. It can also be called as potential difference.
Resistance
is anything in the circuit which
slows the flow down
. If you add more components to the circuit there will be a
higher overall resistance.
There's a
balance
. The
voltage
is trying to
push
the current round the circuit, and the
resistance
is
opposing
it - the
relative
sizes
of the voltage and resistance decide how big the current will be
A standard test circuit
The Ammeter
An
ammeter
measures the current (in amps) flowing through the component
It must be placed in series anywhere in the main circuit, but never in
parallel
like the voltmeter
The voltmeter
A
voltmeter
measures the voltage (in volts) across the component.
Must be placed
in parallel
around the component under test -
NOT around the variable resistor or bettery!
A.Cs & D.Cs
The UK mains electricity supply is approximately 230 volts
It is an
a.c supply
(alternating current), which means the current is
constantly changing direction
By contrast, cells and batteries supply direct current (d.c). This just means that the current keeps flowing in the
same direction
2. Resistance and V= I x R
The
voltage
across and
current
through a component are linked by
resistance
- if you plot them against each other, you can see how the resistance
changes
.
Voltage = Current x Resistance or V = I x R
You can use this formula to work out the resitance for a pair of values from an
I-V Graph
, by sticking them in the formula R= V/I.
The gradient of an
I-V Graph
shows you how the resistance of the component behaves. The steeper the graph the lower the resistance
A
straight line graph
has a constant gradient and shows a constant resistance. If the graph
curves
, it means the resistance is
changing
3. LDRs, Thermistors and LEDs
LEDs
Light-emitting diodes
(LEDs) emit light when a current flows through them into the forward direction
They have a lot of practical applications
They are used for modern energy saving bulbs, in traffic lights and in phone and TV screens. Unlike older light bulbs they don't have a filament that can burn out.
LRDs
A
light-dependent resistor
(LDR) is a special type of resistor that changes its resistance depending on how much light falls on it.
In bright light, the resistance falls and in darkness, the resitance is at its highest,
This makes it a useful device for various electronic circuits, e.g. burgler dectectors
Thermistors
A thermistor is a
temperature-dependent resistor.
In hot conditions, the resistance drops and in cool conditions, the reisitance goes up.
Thermistors make useful temperature detectors, e.g. car engine temperature sensors, thermostats and fire alarms
4. Series and Parallel circuits
Series circuits
In
series
circuits
, the different componesnts are connected in a line, end to end, between the +ve and -ve of the power supplt
If you remove or disconnect one component, the circuit is broken and they all stop working. This is generally not very handy, and in practice only a few things are connected in series
There is a bigger supply in
potential difference
when more cells are in series
The current is the same everywhere. The size of the current depends on the total potential difference and the total resistance of the circuit
For a
series
circuit
:
The total
potential
difference
of the supply is
shared
between components. The potential difference for each component depends of its
resistance
.
The
total resistance
of the circuit depends on the
number
of
components
and the type of components used. The
total
resistance
is the
sum
of the resistance of
each
component
in the circuit.
Parallel circuits
In
parallel circuits
, each component is separately connected to the +ve and -ve of the supply (except ammeters, which are always connected in series)
If you remove or disconnect one component, it will hardly affect the others at all.
Everyday circuits often contain a mixture of series and parallel parts - when looking at components on the same branch the rules for series apply
For a
Parallel circuit:
The
potential **
difference** is the same across all branches.
Current
is shared between
branches
. The
total
current
flowing around the circuit is equal to the
total
of all the currents through the
separate
components
In a parallel circuit, there are
junctions
where the current either splits or re-joins. The total current going into a junction equals the total current leaving it, as charge can't just appear or disappear.
The
current
through a branch depends on the
resistance
of the branch - the higher the reistance, the harder it is for charge to low, and so the lower the current in that branch. If two
identical
components
are connected in parallel then the
same
current
will flow through each component
The
total resistance
of the circuit
decreases
if you add a second resistor in parallel
5. Charge, Voltage and Energy Change
