Notes

6.4.2 Atoms and nuclear radiation

6.4.2.1 Radioactive decay and nuclear radiation

Some atomic nuclei are unstable. The nucleus gives out radiation
as it changes to become more stable. This is a random process
called radioactive decay.

Activity is the rate at which a source of unstable nuclei decays.
Activity is measured in becquerel (Bq)

Count-rate is the number of decays recorded each second by a
detector (eg Geiger-Muller tube).

The nuclear radiation emitted may be:

• an alpha particle (α) – this consists of two neutrons and two
protons, it is the same as a helium nucleus

• a beta particle (β) – a high speed electron ejected from the
nucleus as a neutron turns into a proton

• a gamma ray (γ) – electromagnetic radiation from the nucleus

• a neutron (n).

Required knowledge of the properties of alpha particles, beta
particles and gamma rays is limited to their penetration through
materials, their range in air and ionising power.

Students should be able to apply their knowledge to the uses of
radiation and evaluate the best sources of radiation to use in a
given situation.

6.4.2.2 Nuclear equations
Content Key opportunities for
skills development

Nuclear equations are used to represent radioactive decay.
In a nuclear equation an alpha particle may be represented by the
symbol:

and a beta particle by the symbol:

The emission of the different types of nuclear radiation may cause a
change in the mass and /or the charge of the nucleus. For example:
So alpha decay causes both the mass and charge of the nucleus to
decrease.

So beta decay does not cause the mass of the nucleus to change
but does cause the charge of the nucleus to increase.
Students are not required to recall these two examples.

Students should be able to use the names and symbols of common
nuclei and particles to write balanced equations that show single
alpha (α) and beta (β) decay. This is limited to balancing the atomic
numbers and mass numbers. The identification of daughter
elements from such decays is not required.

The emission of a gamma ray does not cause the mass or the
charge of the nucleus to change.

6.4.2.3 Half-lives and the random nature of radioactive decay

Radioactive decay is random.
The half-life of a radioactive isotope is the time it takes for the
number of nuclei of the isotope in a sample to halve, or the time it
takes for the count rate (or activity) from a sample containing the
isotope to fall to half its initial level.

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Students should be able to explain the concept of half-life and how
it is related to the random nature of radioactive decay.

Students should be able to determine the half-life of a radioactive
isotope from given information.

(HT only) Students should be able to calculate the net decline,
expressed as a ratio, in a radioactive emission after a given number
of half-lives.

(HT only)
6.4.2.4 Radioactive contamination

Radioactive contamination is the unwanted presence of materials
containing radioactive atoms on other materials. The hazard from
contamination is due to the decay of the contaminating atoms. The
type of radiation emitted affects the level of hazard.

Irradiation is the process of exposing an object to nuclear radiation.
The irradiated object does not become radioactive.

Students should be able to compare the hazards associated with
contamination and irradiation.

Suitable precautions must be taken to protect against any hazard
that the radioactive source used in the process of irradiation may
present.

Students should understand that it is important for the findings of
studies into the effects of radiation on humans to be published and
shared with other scientists so that the findings can be checked by
peer review.

6.5 Forces
Engineers analyse forces when designing a great variety of machines and instruments, from road
bridges and fairground rides to atomic force microscopes. Anything mechanical can be analysed in
this way. Recent developments in artificial limbs use the analysis of forces to make movement
possible.

6.5.1 Forces and their interactions

6.5.1.1 Scalar and vector quantities

Scalar quantities have magnitude only.

Vector quantities have magnitude and an associated direction.
A vector quantity may be represented by an arrow. The length of
the arrow represents the magnitude, and the direction of the arrow
the direction of the vector quantity.

6.5.1.2 Contact and non-contact forces
Content Key opportunities for
skills development

A force is a push or pull that acts on an object due to the interaction
with another object. All forces between objects are either:

• contact forces – the objects are physically touching
• non-contact forces – the objects are physically separated.

Examples of contact forces include friction, air resistance, tension
and normal contact force.

Examples of non-contact forces are gravitational force, electrostatic
force and magnetic force.

Force is a vector quantity.

Students should be able to describe the interaction between pairs
of objects which produce a force on each object. The forces to be
represented as vectors.

6.5.1.3 Gravity

Weight is the force acting on an object due to gravity. The force of
gravity close to the Earth is due to the gravitational field around the
Earth.

The weight of an object depends on the gravitational field strength
at the point where the object is.

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The weight of an object can be calculated using the equation:
weight = mass × gravitational field strength

W = m g

weight, W, in newtons, N

mass, m, in kilograms, kg

gravitational field strength, g, in newtons per kilogram, N/kg (In any
calculation the value of the gravitational field strength (g) will be
given.)

The weight of an object may be considered to act at a single point
referred to as the object’s ‘centre of mass’.

Students should be able to
recall and apply this
equation.

The weight of an object and the mass of an object are directly
proportional.

Weight is measured using a calibrated spring-balance (a
newtonmeter).

Students should recognise
and be able to use the
symbol for proportionality, ∝

6.5.1.4 Resultant forces

A number of forces acting on an object may be replaced by a single
force that has the same effect as all the original forces acting
together. This single force is called the resultant force.
Students should be able to calculate the resultant of two forces that
act in a straight line.

(HT only) Students should be able to:

• describe examples of the forces acting on an isolated object
or system

• use free body diagrams to describe qualitatively examples
where several forces lead to a resultant force on an object,
including balanced forces when the resultant force is zero.

(HT only) A single force can be resolved into two components
acting at right angles to each other. The two component forces
together have the same effect as the single force.

(HT only) Students should be able to use vector diagrams to
illustrate resolution of forces, equilibrium situations and determine
the resultant of two forces, to include both magnitude and direction
(scale drawings only).

6.5.2 Work done and energy transfer

When a force causes an object to move through a distance work is
done on the object. So a force does work on an object when the
force causes a displacement of the object.

The work done by a force on an object can be calculated using the
equation:

work done = force × distance

moved along the line of action of the force
W = F s

work done, W, in joules, J
force, F, in newtons, N
distance, s, in metres

Students should be able to
recall and apply this
equation.

One joule of work is done when a force of one newton causes a
displacement of one metre.

1 joule = 1 newton-metre

Students should be able to describe the energy transfer involved
when work is done.

Students should be able to convert between newton-metres and
joules.

Work done against the frictional forces acting on an object causes a
rise in the temperature of the object.

6.5.3 Forces and elasticity

Students should be able to:
• give examples of the forces involved in stretching, bending or
compressing an object

• explain why, to change the shape of an object (by stretching,
bending or compressing), more than one force has to be
applied – this is limited to stationary objects only

• describe the difference between elastic deformation and
inelastic deformation caused by stretching forces.

The extension of an elastic object, such as a spring, is directly
proportional to the force applied, provided that the limit of
proportionality is not exceeded.

f orce = s pring constant × extension
F = k x e

force, F, in newtons, N

spring constant, k, in newtons per metre, N/m
extension, e, in metres, m

Students should be able to
recall and apply this
equation.
This relationship also applies to the compression of an elastic
object, where ‘e’ would be the compression of the object.
A force that stretches (or compresses) a spring does work and
elastic potential energy is stored in the spring.

Provided the spring
is not in-elastically deformed, the work done on the spring and the
elastic potential energy stored are equal.

Students should be able to:
• describe the difference between a linear and non-linear
relationship between force and extension

• calculate a spring constant in linear cases

• interpret data from an investigation of the relationship
between force and extension

• calculate work done in stretching (or compressing) a spring
(up to the limit of proportionality) using the equation:

elastic potential energy = 0.5 × s pring constant × extension 2
Ee =1/2 x k x e (squared)

Students should be able to
apply this equation which is
given on the Physics
equation sheet.

This equation is also given
in Changes in energy (page
122).

Students should be able to calculate relevant values of stored
energy and energy transfers.

6.5.4 Forces and motion

6.5.4.1 Describing motion along a line

6.5.4.1.1 Distance and displacement

Distance is how far an object moves. Distance does not involve
direction. Distance is a scalar quantity.

Displacement includes both the distance an object moves,
measured in a straight line from the start point to the finish point
and the direction of that straight line. Displacement is a vector
quantity.

Students should be able to express a displacement in terms of both
the magnitude and direction.


Throughout this section
(Forces and motion),
students should be able to
use ratios and proportional
reasoning to convert units
and to compute rates.

6.5.4.1.2 Speed

Speed does not involve direction. Speed is a scalar quantity.
The speed of a moving object is rarely constant. When people walk,
run or travel in a car their speed is constantly changing.

The speed at which a person can walk, run or cycle depends on
many factors including: age, terrain, fitness and distance travelled.
Typical values may be taken as:

walking ̴ 1.5 m/s

running ̴ 3 m/s

cycling ̴ 6 m/s.

Students should be able to recall typical values of speed for a
person walking, running and cycling as well as the typical values of
speed for different types of transportation systems.

It is not only moving objects that have varying speed. The speed of
sound and the speed of the wind also vary.

A typical value for the speed of sound in air is 330 m/s.
Students should be able to make measurements of distance and
time and then calculate speeds of objects.

For an object moving at constant speed the distance travelled in a
specific time can be calculated using the equation:

distance travelled = s peed × time
s = v x t
distance, s, in metres, m

speed, v, in metres per second, m/s

time, t, in seconds, s

Students should be able to
recall and apply this
equation.

Students should be able to calculate average speed for non-uniform
motion.


6.5.4.1.3 Velocity

The velocity of an object is its speed in a given direction. Velocity is
a vector quantity.

Students should be able to explain the vector–scalar distinction as it
applies to displacement, distance, velocity and speed.

(HT only) Students should be able to explain qualitatively, with
examples, that motion in a circle involves constant speed but
changing velocity.

6.5.4.1.4 The distance–time relationship

If an object moves along a straight line, the distance travelled can
be represented by a distance–time graph.

The speed of an object can be calculated from the gradient of its
distance–time graph.

(HT only) If an object is accelerating, its speed at any particular time
can be determined by drawing a tangent and measuring the
gradient of the distance–time graph at that time.

Students should be able to draw distance–time graphs from
measurements and extract and interpret lines and slopes of
distance–time graphs, translating information between graphical
and numerical form.

Students should be able to determine speed from a distance–time
graph.

6.5.4.1.5 Acceleration

The average acceleration of an object can be calculated using the
equation:

acceleration = change in velocity
time taken a = ∆ v/t

acceleration, a, in metres per second squared, m/s2
change in velocity, ∆v, in metres per second, m/s
time, t, in seconds, s

An object that slows down is decelerating.

Students should be able to estimate the magnitude of everyday
accelerations.


Students should be able to
recall and apply this
equation.

The acceleration of an object can be calculated from the gradient of
a velocity–time graph.

(HT only) The distance travelled by an object (or displacement of an
object) can be calculated from the area under a velocity–time
graph.

Students should be able to:

• draw velocity–time graphs from measurements and interpret
lines and slopes to determine acceleration

• (HT only) interpret enclosed areas in velocity–time graphs to
determine distance travelled (or displacement)

• (HT only) measure, when appropriate, the area under a
velocity–time graph by counting squares.
The following equation applies to uniform acceleration:

Students should be able to
apply this equation which is
given on the Physics
equation sheet.

An object falling through a fluid initially accelerates due to the force
of gravity. Eventually the resultant force will be zero and the object
will move at its terminal velocity.

6.5.4.2 Forces, accelerations and Newton's Laws of motion

6.5.4.2.1 Newton's First Law

Newton’s First Law:

If the resultant force acting on an object is zero and:

• the object is stationary, the object remains stationary

• the object is moving, the object continues to move at the
same speed and in the same direction. So the object
continues to move at the same velocity.

So, when a vehicle travels at a steady speed the resistive forces
balance the driving force.

So, the velocity (speed and/or direction) of an object will only
change if a resultant force is acting on the object.

Students should be able to apply Newton’s First Law to explain the
motion of objects moving with a uniform velocity and objects where
the speed and/or direction changes.

(HT only) The tendency of objects to continue in their state of rest
or of uniform motion is called inertia.

6.5.4.2.2 Newton's Second Law

Newton’s Second Law:

The acceleration of an object is proportional to the resultant force
acting on the object, and inversely proportional to the mass of the
object.

As an equation:

Students should recognise
and be able to use the
symbol for proportionality, ∝

resultant f orce = mass × acceleration
F = m a
force, F, in newtons, N
mass, m, in kilograms, kg
acceleration, a, in metres per second squared, m/s2

Students should be able to
recall and apply this
equation.

(HT only) Students should be able to explain that:

• inertial mass is a measure of how difficult it is to change the
velocity of an object

• inertial mass is defined as the ratio of force over acceleration.
MS 3a

Students should be able to estimate the speed, accelerations and
forces involved in large accelerations for everyday road transport.
Students should recognise and be able to use the symbol that
indicates an approximate value or approximate answer, ̴


Required practical activity 19: investigate the effect of varying the force on the acceleration of an
object of constant mass, and the effect of varying the mass of an object on the acceleration
produced by a constant force.

6.5.4.2.3 Newton's Third Law

Newton’s Third Law:
Whenever two objects interact, the forces they exert on each other
are equal and opposite.

Students should be able to apply Newton’s Third Law to examples
of equilibrium situations.


6.5.4.3 Forces and braking

6.5.4.3.1 Stopping distance

The stopping distance of a vehicle is the sum of the distance the
vehicle travels during the driver’s reaction time (thinking distance)
and the distance it travels under the braking force (braking
distance). For a given braking force the greater the speed of the
vehicle, the greater the stopping distance.

6.5.4.3.2 Reaction time

Reaction times vary from person to person. Typical values range
from 0.2 s to 0.9 s.

A driver’s reaction time can be affected by tiredness, drugs and
alcohol. Distractions may also affect a driver’s ability to react.
Students should be able to:

• explain methods used to measure human reaction times and
recall typical results

• interpret and evaluate measurements from simple methods to
measure the different reaction times of students

• evaluate the effect of various factors on thinking distance
based on given data.

Measure the effect of
distractions on reaction
time.

6.5.4.3.3 Factors affecting braking distance 1

The braking distance of a vehicle can be affected by adverse road
and weather conditions and poor condition of the vehicle.
Adverse road conditions include wet or icy conditions. Poor
condition of the vehicle is limited to the vehicle's brakes or tyres.
Students should be able to:

• explain the factors which affect the distance required for road
transport vehicles to come to rest in emergencies, and the
implications for safety

• estimate how the distance required for road vehicles to stop in
an emergency varies over a range of typical speeds.

6.5.4.3.4 Factors affecting braking distance 2

When a force is applied to the brakes of a vehicle, work done by the
friction force between the brakes and the wheel reduces the kinetic
energy of the vehicle and the temperature of the brakes increases.

The greater the speed of a vehicle the greater the braking force
needed to stop the vehicle in a certain distance.

The greater the braking force the greater the deceleration of the
vehicle. Large decelerations may lead to brakes overheating and/or
loss of control.

Students should be able to:
• explain the dangers caused by large decelerations WS 1.5

• (HT only) estimate the forces involved in the deceleration of
road vehicles in typical situations on a public road.
(HT only)

6.5.5 Momentum (HT only)

6.5.5.1 Momentum is a property of moving objects

Momentum is defined by the equation:
momentum = mass × velocity
p = m v

momentum, p, in kilograms metre per second, kg m/s
mass, m, in kilograms, kg
velocity, v, in metres per second, m/s

Students should be able to
recall and apply this
equation.

6.5.5.2 Conservation of momentum

In a closed system, the total momentum before an event is equal to
the total momentum after the event.

This is called conservation of momentum.

Students should be able to use the concept of momentum as a
model to describe and explain examples of momentum in an event,
such as a collision.

Investigate collisions
between laboratory trollies
using light gates, data
loggers or ticker timers to
measure and record data.

6.6 Waves

Wave behaviour is common in both natural and man-made systems. Waves carry energy from one
place to another and can also carry information. Designing comfortable and safe structures such
as bridges, houses and music performance halls requires an understanding of mechanical waves.
Modern technologies such as imaging and communication systems show how we can make the
most of electromagnetic waves.

6.6.1 Waves in air, fluids and solids

6.6.1.1 Transverse and longitudinal waves

Waves may be either transverse or longitudinal.
The ripples on a water surface are an example of a transverse
wave.

Longitudinal waves show areas of compression and rarefaction.
Sound waves travelling through air are longitudinal.
Students should be able to describe the difference between
longitudinal and transverse waves.


Students should be able to describe evidence that, for both ripples
on a water surface and sound waves in air, it is the wave and not
the water or air itself that travels.