Download Motion Graphs

Motion Graphs
Distance-Time Graphs
Velocity-Time Graphs
Velocity
(m/s)
Distance
10
5
Time
The gradient gives you the
speed/velocity of the object
d=sxt
Straight lines means the object is
travelling at a constant speed.
Curved lines means the object is
accelerating or decelerating
Time (s)
The gradient gives you the
acceleration of the object. The
steeper the line the greater the
acceleration.
If the lines gradient is negative then
deceleration is occurring.
a=v–u
t
The area below the graph gives the
distance travelled by the object
Forces
Different types of forces include: Air resistance, friction, weight, motive,
drag, upthrust, reaction
300N
500N
Resultant Force
When the forces are
added together the
resultant force is given.
When the resultant force
is 0 the object is either
stationary of moving at a
constant speed.
If it is not zero the
object is either
decelerating of
accelerating.
When two objects exert a
force on each other, the
forces are opposite but
equal.
The mass
An object will
accelerate in the
direction of the
resultant force. A force
on a large mass will
accelerate it less than
the same force on a
smaller mass.
Key Equations:
Force = Mass x Acceleration
Weight = Mass x Acceleration due to gravity
Forces and Braking
Motion of the car
The car has kinetic energy when it is moving.
There is friction between the tyres and roads
generates heat energy.
Thinking Distance
This is the distance travelled by the car
before the driver reacts and presses the
brakes.
Drugs, alcohol, speed of car etc. can affect the
thinking distance.
Braking distance
This is the distance travelled once the cars
brakes are applied.
Total stopping distance = thinking distance + Braking distance
Braking
The faster the car is travelling, the greater
the braking force needed to stop in a certain
distance. When the car brakes, work is done
against the kinetic energy and hence it
decreases.
Falling and Terminal Velocity
Velocity-Time graph for a parachutist
When an object starts to fall, the resultant force is in the downwards direction
due to the weight causing the object to accelerate. As the object accelerates
the resistive force increases. Eventually the two become equal but opposite
(there is no resultant force) so the object starts to travel at a constant velocity,
this is known as the objects terminal velocity.
What would the weight of the parachutist be if he weighed 70kg?
Take g=10m/s2
At terminal velocity, what would the value of the resistance forces
be? How is this force different to the parachutists weight
Stretching and Squashing
A force applied to an object can cause it to
stretch or squash – it changes shape.
An elastic object is one which can return to
its original shape once the force applied is
removed.
Energy changes
Work is done to change an objects shape.
The energy transfer that occurs in this
process results in elastic potential energy
being stored in the stretched object.
Hooke’s Law:
The extension of an elastic object is
directly proportional to the force applied,
provided its limit of proportionality is not
exceeded.
Force applied = spring constant x extension
(N)
(N/m)
(m)
Notes:
1.
The spring constant is a measure of how stiff the
spring is. The stiffer the spring, the larger the
constant.
2. Extension of a spring is the difference between the
original length of the spring and the length of the
spring when a force is applied
Work, Energy, GPE and KE
When a force causes an object to move work is
done. Work done and energy transferred are
often used interchangeably.
Work done = Force applied x Distance moved
(J)
(N)
(m)
In symbol form:
W=Fxd
The gravitational potential energy is the
energy that an object has due to its position in
a gravitational field.
Ep = m x g x h
(J) (kg) (m/s2) (m)
As an object falls it loses gravitational
potential energy and gains kinetic energy. If
there is friction present work is done against
the frictional forces. This causes heating.
Power is the rate at which work is done
or energy is transferred.
Power = Energy Power = Work Done
Time
Time
The kinetic energy of an object is
dependent on its mass and velocity.
Ek = ½ x m x v2
(J)
(kg) (m/s)
Momentum and Impacts
Momentum
Momentum is a property all moving
objects have.
Momentum = Mass x Velocity
(kg m/s)
(kg) (m/s)
p=mxv
The Conservation of Momentum
In a closed system, the momentum
before an event must be equal to
that of the momentum after the
event.
mbefore x vbefore = mafter x vafter
Impacts
Air bags and crumple zones increase the amount
of time an impact takes place in. The longer the
impact time, the more the impact force is
reduced.
Why is this the case?
We know acceleration = velocity/time. If we
increase the time the acceleration therefore
decreases.
We also know that force = mass x acceleration.
As the acceleration has decreased this
therefore means that the force decreases.
Momentum
As stated previously, momentum before a
collision is equal to that of the momentum after
the collision. If a car hits a stationary object
then the car decelerates and the object
accelerates (conservation of momentum).
Therefore the car has lost momentum
Electric Circuits
Static Electricity
When two insulators are
rubbed against each
other, electrons
transfer from one
object to the other,
making both electrically
charged.
The material that gains
electrons becomes
negatively charged and
the material that loses
electrons become
positively charged.
Like charges repel.
Opposite charges
attract.
Current
In metals, there are
free electrons that are
not attached to the
positive ions. When the
electrons flow, a
current is produced.
Current is defined as
the rate of flow of
electric charge.
Resistance
Electrons need to push
themselves past vibrating ions.
The faster these ions vibrate
(they vibrate faster when the
temperature increases leading to
an increase in their kinetic
energy), the harder it is for
electrons to do this. This means
there is a higher resistance.
Charge = Current x Time
(C)
(A)
(s)
Resistance = P.d / Current
(Ω)
(V)
(A)
Potential difference
P.d is defined as the
work done between two
points per coulomb of
charge that passes the
points.
P.d = Work done/Charge
(V)
(J)
(C)
Current and potential
difference graphs
Resistor/Wire
Filament Lamp
Diode
Thermistor
LDR
Red
Blue
Ohm’s law states that, for a
resistor, the current that flows
through it is directly proportional
to the potential difference across
the resistor, providing it is at
constant temperature.
A diode only
allows current
to flow in one
direction.
LED’s are
replacing
filament lamps
as they can
operate at
lower currents.
Red
Blue
The resistance in a thermistor
increases as the temperature
decreases. (High temp = red line
low temp = blue line).
The resistance in a LDR
decreases as the light intensity
increases. (Bright light = red line
low temp = blue line).
Resistors get hot as current
flows through them.
Series and Parallel Circuits
A series circuit is one in
which current only has one
path to flow through.
There is the same
current through each
component. A1=A2=A3
The total resistance
of the circuit is the
sum of the resistance
of each component.
The potential
difference is shared
by each components.
E.g. A 12V power
supply could supply 8V
to the resistor and 4V
to the bulb.
A parallel circuit is one in
which there is more than one
path/junction, current can
flow through.
When the current
reaches a junction is
‘splits’. However the
total current is the
sum of the current
through the
components.
A1=A2+A3=A4
The potential
difference across
each branch is the
same. It would then
be shared by the
components in that
branch. V1=V2=V3
Mains Electricity
Cells and batteries supply
current that always passes in
the same direction. This is
known as a direct current
(d.c).
An alternating current is one
that is constantly changing
direction. Mains electricity
has a frequency of 50Hz and
is about 230V
figure 1
peak voltage
time period
The frequency is given by the
number of cycles per second
frequency = 1/time for one
cycle
Cables
Some appliances only have two
wires, they are missing the
earth wire. This is because
they have a plastic casing.
The cables are said to be
two-core cables. This is also
known as double insulation.
Metal cased appliances tend
to be earthed.
The fuse
A fuse contains a thin
wire which heats up and
melts if too much
current is passed
through it. The fuse is
always placed in series
with the live wire.
Circuit breakers
A circuit breaker is an electromagnet switch that opens
when there is a fault, that stops the current in the live
wire flowing. They are quicker to reset than fuses and
work faster. An RCCB works even faster than an
ordinary circuit breaker, as it is more sensitive. It
‘trips’ when the current in the live wire is different to
that of the neutral wire.
Mains Electricity
The rate at which energy is transferred by an
appliance is called the power.
The equations from this
chapter:
Power (W) = Energy (J)
Time (s)
Q=Ixt
P=IxV
Power, potential different and current are related by
the following equation:
Power (W) = Current (A) x Potential difference (V)
Higher tier only
Energy, transferred, potential difference and charge
are related by the equation:
Energy (J) = Potential difference (V) x Charge (C)
Prefixes
Remember to always check the units of the numbers
given. If there is a prefix, you will need to convert
the number into standard form first:
M = 1x106 k = 1x103 m = 1x10-3 μ = 1x10-6
V=W/Q
Note:
These two
are the
same
equation!
V=IxR
P=E/t
P=IxV
E = V x Q (HT only)
Radioactivity
The nuclear model
Rutherford’s scattering experiment determine
todays nuclear model of the atom, which replaced
the plum pudding model. Rutherford fired positive
alpha particles towards gold foil. He found that most
of the alpha particles passed straight through, thus
concluding that the atom is mostly empty space.
He found that some alpha particles were deflected,
which showed that the entre of the atom was
positively charged. Very few alpha particles bounced
straight back, leading Rutherford to conclude that
the nucleus contained positive protons and neutrons
which had electrons which orbit it.
Atoms
In an atom there are the same number of protons as
there are electrons. The number is denoted by the
atomic number. Chlorine has 17 of each. If an atom
loses of gains an electron it becomes an ion.
The number of protons and neutrons in an atom is
the mass number. Chlorines mass number is 35.
Radioactivity
An unstable atom becomes more
stable by releasing radiation.
There are 3 types of radiation:
Alpha:
An alpha particle is two protons
and two neutrons. It is a helium
nucleus.
Beta:
A beta particle is a fast moving
electron emitted by a nucleus
which has too many neutrons.
Gamma:
Gamma radiation is an electromagnetic wave, therefore has no
charge.
Particle
Relative
mass
Relative
charge
proton
1
+1
neutron
1
0
electron
0.0005
−1
Alpha, Beta & Gamma
Nuclear reactions
Penetrating and Ionisation
We can test absorbed materials
by placing the material between
the source and a Geiger-Muller
counter. As you add the absorber
material the count rate will drop
to 0 i.e. all the radiation has been
absorbed.
Alpha emission:
Beta emission:
Deflection in a magnetic or electric field
The charged particles
deflect in a magnetic
or electric field. The
beta particle deflects
more than the alpha
particle
as
it
is
lighter.
You can find the penetrating
power in air by moving the source
further from the counter. You
find that alpha only travel a few
centimetres, due to it being
highly ionising, beta travels about
1m and gammas range is unlimited.
Radioactivity
Half-life
There are two definitions for the half-life of a
radioactive substance, either is just as valid as the
other:
1. The time it takes for the count rate to half
2. The time it takes for half of the radioactive
nuclei to decay
Uses of radioactivity
Here are some examples of how radiation is used:
• in smoke detectors
• for sterilising medical instruments
• for killing cancer cells
• for dating rocks and materials such as
archaeological finds
• in chemical tracers to help with medical diagnosis
• for measuring the thickness of materials in, for
example, a paper factory
Evaluating the uses
- Medical Tracers
Doctors may use radioactive
chemicals called tracers for
medical
imaging.
Radiation
detectors placed outside the body
detect the radiation emitted and,
with the aid of computers, build up
an image of the inside of the body.
When a radioactive chemical is
used in this way it is not normally
harmful, because:
• it has a short half-life and so
decays before it can do much
damage
• it is not poisonous
Emitters of beta radiation or
gamma radiation are used because
these types of radiation readily
pass out of the body, and they are
less likely to causes ionisation of
the cells compared to alpha
radiation.
Fission and Fusion
Fission
In a nuclear reactor, fission is sued to generate energy
and hence electricity.
The two most common fissionable substances used in
nuclear reactors are urnaium-235 and plutonium-239.
When either absorbs a neutron, the fissionable
substance, uranium in the example below, splits into two
smaller nuclei. When it does so, further neutrons are
also released which can then be absorbed by other
uranium nuclear, this creating a chain reaction. Control
rods in a nuclear reactor absorb neutrons to control the
rate of fission.
A
moderating
material
slows down neutrons so
that they can be absorbed.
Fast moving electron have
little effect.
Energy is also released
when the uranium nuclei
splits which is what is
harnessed
to
produce
electricity in a nuclear
power station.
Fusion
Nuclear fusion is the joining
together of smaller nuclei to form
larger nuclei. This is the process
which energy is released in stars.
When the smaller nuclei fuse
together there is a drop in mass
compared to their separate masses.
The difference in mass is released as
H (with
He
energy.
H
neutron)
The future
We are trying to create fusion
generators on Earth but it is proving
difficult to generate the conditions
needed (high temperature and
pressure).
Background radiation and radiation
dose
Background radiation stems from
nuclear fallout, rocks, medical,
cosmic rays.
Air travel and occupation such as
medicine increases the exposure.
Stars and Element Formation
A main sequence star is a
stable star where the radiation
pressure, caused by fusion,
balances the gravitational
force of the star.
Hydrogen nuclei form to
become helium nuclei in a main
sequence star. This process
continues up until, and
including, iron. After this too
much energy is required for
heavier elements to be formed.
Stars form when dust and gas is pulled together
by gravitational attraction. Once enough matter is
pulled together a protostar forms which continues
collecting dust and gas, becoming hotter and
hotter. Eventually a star is born and fusion can
begin in a main sequence star.
The heavier elements form
when a star collapses and then
explodes as a supernova, where
there is enough energy
available for heavier elements
to form.