Physics Unit 1 Mechanics

Physics

Unit 1- Mechanics

Module 1- Motion

-Physical Quantities and Units

Below are the SI Units used across the world:

Quantity Unit Abbreviation

Mass Kilogram kg

Length Metre m

Time Second s

Temperature Kelvin K

Electrical Current Ampere A

Amount of Substance mole mol

Module 1- Motion

-Physical Quantities and Units

Below are the Unit prefixes:

Prefix Name Abbreviation

10-12 pico p

10-9 nano n

10-6 micro μ

10-3 milli m

10-2 centi c

103 kilo k

106 mega M

109 giga G

1012 tera T

Module 1- Motion

-Scalar and Vector Quantities

A scalar quantity is one that has magnitude (size) but not a direction.

A vector quantity is one that has magnitude (size) and direction.

Scalar Vector

Density Displacement

Temperature Velocity

Pressure Acceleration

Potential Difference Force

Frequency Impulse

Wavelength Momentum

Power Electric Current

Magnetic Field

Electric Field

Module 1- Motion

-Vector Component Forces

Here is a triangle which trigonometry can be used to find unknowns:

Fcos𝜃

𝜃

F

Fsin𝜃

Module 1- Motion

-Definitions in kinematics

Speed is distance per unit time.

Displacement is distance

moved in a stated direction.

Speed is the distance

travelled per unit time- it is

a scalar

Velocity is the displacement

per unit time- it is a vector

Instantaneous speed is the speed at a given instant of time (it is the gradient

of the graph of displacement against the against time at that instant)

Acceleration is the rate of

change of velocity.

Average speed = 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒

𝑡𝑖𝑚𝑒

Average acceleration = 𝑖𝑛𝑐𝑟𝑒𝑎𝑠𝑒 𝑖𝑛 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦

𝑡𝑖𝑚𝑒=

𝑣;𝑢

𝑡

Module 1- Motion

-Graphs of Motion

Displacement/ Time graphs:

• A straight line indicates constant velocity

• The gradient of a straight line gives the velocity

• The gradient at any point is the velocity, and this is called

instantaneous velocity

Velocity/ Time graphs:

• The gradient represents acceleration

• The area beneath a velocity/ time graph represents the

displacement

Module 1- Motion

-Equations of Motion

Summary of the equations of motion for constant

acceleration:

v = u + at Term not included: s

v2= u2 + 2as Term not included: t

s = ( 𝑢:𝑣

2) t Term not included: a

s = ut + ½ at2 Term not included: v

s = vt - ½ at2 Term not included: u

Symbol Quantity Alternative Quantity SI Unit

S Distance Moved Displacement Metre

U Speed at the start Velocity at the start Second

V Speed at the end Velocity at the end m s-2

A Acceleration m s-1

T Time interval m s-1

Module 1- Motion

-Free Fall

An object undergoing free fall on the Earth has an acceleration of g =

9.81 m s-2. Acceleration is a vector quantity- and g acts vertically

downward.

Remember, when answering questions on free fall, make sure you deal

with the horizontal and vertical components separately, and watch out for

negative values.

Module 1- Motion

-Measurement of g

Below is a diagram on the ‘trap door and electromagnet method for

determining g’.

There will be a degree of uncertainty

in this experiment because:

1. If the electromagnet’s current is too

strong there will be a delay in

releasing the ball after the current is

switched off and the clock is

triggered.

2. If the distance of fall is too large, or

the ball is too small, air resistance

might have a noticeable effect on its

speed.

3. You need you make sure you

measure from the bottom of the ball

when it is held by the electromagnet.

Module 2- Forces in Action

-Force and the Newton

-Types of Force

Generally, a force is push or pull, but can be others such as drag, tension, friction,

weight and thrust. Thrust, for example, is the term used for the driving force

provided by a jet engine.

Outside the nucleus of an atom, there is just three types of force, which are:

• Gravitational force between two objects with mass. (Only one I will need is

between the object and the Earth: The weight).

• Magnetic force between two magnetic objects. At an atomic level this is a force

between moving charges, and will only concern you in examples using

magnetised forces.

• Electrical force between charged objects, which is responsible for all

interactions between objects. When two atoms collide, they exert an electrical

force on one another, and may chemically bond as a result of the electrical

attraction between them.

This list of the three basic forces outside a nucleus can be reduced to two by

treating the electrical and magnetic forces as a single electromagnetic force. This

is because the theory of electromagnetism establishes the connection between

electrical and magnetic effects.


-Force and the Newton

The link between these three terms was first established by Newton, when he

discovered that when an object has no resultant force on it, the object won’t

accelerate; it will stay at a constant velocity. Once Newton established this, he

found that:

• Acceleration is proportional to force, if the mass is constant

• Acceleration is inversely proportional to mass, if the force is constant.

Putting this algebraically:

a ∝ F and a ∝ 1

𝑚, so F=ma

A resultant force always causes

acceleration.

Zero resultant force implies a constant

velocity, which may also be zero (it will

be in equilibrium).

Remember, forces cause

acceleration, and not the

other way round!

One Newton is the force that causes a mass of one kilogram to have an

acceleration of one metre per second every second.


-Motion with non-constant acceleration

Weight is the gravitational

force on a body

Weight:

• Weight is a force, so is measured in newton's.

• The mass of an object is measured in kg.

To work out mass or weight, we can use the

equation W=mg

Non-constant (non-linear) acceleration

When an object travels through a fluid (liquid or gas), it experiences a resistive force,

known as drag, which depends on several factors, such as velocity, roughness of

surface, cross-sectional area and shape (how it is streamlined)

Terminal Velocity:

This is when the drag (upwards) becomes equal to the weight of the object

(downwards) so the resultant force is zero, so it is travelling at a constant velocity.

This is called terminal velocity.


-Equilibrium

The triangle of forces:

Here are some examples of triangular forces:

4N

2.8N

R= 5.7N 4N

3N

R=5N 4N 4N

R=1.4N

5N 5N

Resultant

(almost)

zero A

B

C D

E resultant A+B+D

A+B=C

C+D=E

A+B+D=E

Equilibrium: When there is

zero resultant force acting on

an object.


-Centre of Gravity

Whenever mass is used, the position of the weight of the object has to be

considered. For all objects there is a point where the entire weight of the object

can be considered to act as a single force, and this is called the centre of gravity

of an object. Although the weight of an object does not act through just the centre

of gravity, it does simply calculations.

Finding the centre of gravity-

Support the piece freely on a wire passed

through a small hole.

Hang a string with a small weight at the bottom.

Repeat the procedure with a different hole, and

the centre of gravity is where the lines meet.


-Turning Forces

This is needed when doing things like designing building, to make sure it

can support itself and will not collapse.

Loading forces are usually vertically downwards, and need to be

balanced by vertically upward support forces. We need to establish

equilibrium when working with forces that are parallel.


-Turning Forces

Terms associated with Turning Forces:

Couple- A couple occurs when two forces are equal and

opposite to each other, but are not in a straight line. No

linear acceleration can be produced, as the upward and

downward forces cancel. The resultant of theses forces is

zero, however they can produce rotation.

Torque- This can be applied to a couple and describes a

turning effect of the couple. The formula for torque is:

X

Y

Torque = one of the forces x perpendicular distance between the forces

So torque is measured in newton metres, and

produces rotation rather than linear motion, so

the term is used in drills etc.

A couple is a pair of equal

and parallel but opposite

forces, which tends to

produce rotation only.


-Turning Forces

Moment of a force:

The moment of a force is the turning effect of a single

force shown to the right. Moments are also measured

in Newton metres. The principle of moments states

that: For a body in rotational equilibrium, the sum of the

clockwise moments equals the sum of the

anticlockwise moments. (CW=ACW).

P F

X

Moment of force = Fx

P

F

X The moment of a force is

the force multiplied by the

perpendicular distance

from the stated point.

Equilibrium of an extended object

A large object may have many forces acting on it.

These forces may provide a resultant force, which will

cause acceleration, and a resultant moment, which will

cause rotation. For a large object to be in equilibrium,

both the resultant force and the resultant moment

must be zero.


-Density

Density = 𝑚𝑎𝑠𝑠

𝑣𝑜𝑙𝑢𝑚𝑒, and density has the SI unit kg m-3.

1m3 = (100 cm)3 = 1 000 000 cm3.

The volume of water has a mass of 1000kg, so the density is 1000 kg m-

3. Material Density

kg m-3

Material Density

kg m-3

Hydrogen 0.0899 Silicon 2300

Helium 0.176 Concrete 2400

Oxygen 1.33 Iron 7870

Air 1.29 Copper 8930

Ethanol 789 Silver 10500

Olive oil 920 Gold 19300

Water 1000 Platinum 21500

Mercury 13600 Osmium 22500

Aluminium 2710

Density is defined as

mass per unit volume.


-Pressure

Pressure = 𝑓𝑜𝑟𝑐𝑒

𝑎𝑟𝑒𝑎, and the SI unit for pressure is the Pascal (Pa). 1 pascal represents

the force of 1N spread uniformly over an area of 1 m2 and is a comparatively small

unit of pressure.

Pressure in a liquid is given by hpg, where

h is height, p is density and g is 9.81m s-2.

Pressure is defined as

force per unit area.

Eg) An oil tanker has a total mass of 400 000 tonnes (ship + oil). It has a width of

40m and a length of 500m.

Force upward = Weight downward = mg = 400 000 000 kg x 9.81m s-2= 3.92 x 109N.

Upward force due to water = pressure x area of base of ship, so

3.92 x 109 = hpg x 40 x 500

p = density of sea water = 1030 kg m-3.

h= distance from the bottom of the ship to the surface, so

h= 3.92×109

1030 ×9.81 ×40 ×500 = 19.4m


-Car stopping distances

Force x distance gives the work done by a vehicle against its braking force. This

quantity is called the kinetic energy of a vehicle. The table below shows a car

(which including passengers and luggage is 1200kg) and its breaking distance.

Braking force/ N Braking Distance/m at

15m/s (k.e. = 135000J)

Braking Distance/m at

30m/s (k.e. = 540000J)

100 1350 5400

1000 135 540

10 000 13.5 54.0

100 000 1.35 5.4

1 000 000 0.135 0.54

If you double the speed, the kinetic energy quadruples. So, for every given braking

force, the braking distance is always four times larger when the car is travelling at

twice the speed.



Thinking Distance + Braking Distance = Stopping Distance

Thinking Distance = speed x reaction time

Reaction time is increased by tiredness, alcohol/ other drug use, illness, and

distractions such as children and phones.

Braking distance depends on the braking force, friction between the tyres

and the road, the mass and the speed.

• Braking force is reduced by reduced friction between the brakes and the

wheels (worn or badly adjusted brakes)

• Friction between the tyres and the road is reduced by wet or icy roads,

leaves or dirt on the road, worn out tyre treads, etc

• Mass is affected by the size of the car and what you put in it.



Thinking, braking and stopping distances:

Stopping distance = thinking distance + braking distance

Thinking distance = Time taken to see the need to stop and apply the brakes

Braking distance = The time taken from hitting the brakes to coming to a stop

Eg) A car of mass 1000kg has brakes that are 75% efficient. It is travelling at 40ms-1

and it’s daylight and the road is dry. The driver takes 0.25 seconds to respond to an

incident that requires an emergency stop. What’s the shortest possible distance for

stopping?

Thinking distance= 40𝑚

𝑠 × 0.25s = 10m

Acceleration while braking= −75

100 × 9.8 = −7.35𝑚 𝑠;2

Since, for braking: v2 = u2 + 2as

02 = 402 + 2 x (-7.35)s

s = 1600

14.7= 109𝑚 109m + 10m = 119m


-Car Safety

You can stop a moving vehicle with less braking force if you increase the braking

distance, because kinetic energy = braking force x breaking distance. This becomes

more relative when thinking of someone involved in a car crash. In a crash, you want

to reduce the force, and you can do this by increasing the crash time, or the distance

your body moves in a crash. A good car does this with crumple zones, seat belts and

airbags.

Crumple zones: These are meant to collapse during a

collision (usually the front end). The crumple zones slightly

decrease collision speed, which increases the collision

time, so the average force you endure is less.

Seat Belts: The distance in which a force can act is also

increased by wearing a seatbelt, as it stretches during an

incident. However, the main advantage of a seatbelt is to

keep you kept in the car, as without one your body would

be most likely stopped by the windscreen or another rigid

part of the car.


-Car Safety

Airbags: These work well with seatbelts, as they should

be fully inflated when you hit them, which they most likely

won’t be without the aid of seatbelts. Airbags are

designed to inflate in 0.05 s, and deflate in 0.3s, which is

sufficient to slow you down. An airbag consists of three

parts:

• A flexible nylon bag that is folded into the steering

wheel or dashboard

• A sensor know as an accelerometer. When the front

end of the spring is suddenly stopped, the mass on

the end of the spring continues to move forward and

makes contact with a switch, starting a chemical

reaction. This occurs when the acceleration is

around -10g, an acceleration that only occurs during

an incident.

• An inflation system in which a spark ignites a violent

chemical reaction in which nitrogen gas is produced (it

may sometimes be air, but usually Nitrogen gas)


-Car Safety

Global Positioning System (GPS): A GPS in cars enable you to know

where you are on the worlds surface within a distance of about 1m,

using satellites orbiting Earth at the height of about 20 000km. At any

one point, there will always be at least four satellites available for any

GPS receiver. The system relies on accurately measuring time

differences between the arrival of signals sent simultaneously from

several satellites, and on the precise position of these satellites. The

satellites clocks are synchronised with clocks on the ground and are

accurate to one second in 100 million years.


-Car Safety

Global Positioning System (GPS): The method used for determining the position

of the GPS receiver in a car is called trilateration. If satellite A sends out a signal

and it arrives after a known time ay the GPS receiver then, given the speed of travel

of electromagnetic radiation, the distance of the receiver from the satellite can be

found. We now repeat this for the other satellites, which gives your current location;

where all the spheres meet! The in-car computer then plots this position on its map,

and can guide the car along a suitable route to the requested destination. Although

trilateration only needs 3 satellites, GPS systems actually use at least four

satellites.

You are

here

Module 3- Work & Energy

-Work and the joule

Work, is defined by the equation:

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

Since the definition has a direction for the force, you would think it is a vector but in

fact it is a scalar. It defies the general rule of Vector x Scalar = Vector.

The SI Unit for work is the joule, and 1 joule = 1 newton metre.

Eg) Picking up a pen = 0.2N x 0.1m = 0.02Nm = 0.02J

1 joule is the work done when a force

of 1 newton moves its point of

application 1 metre in the direction of

the force.


-Work and the joule

Force at an angle to the direction of movement:

Eg) A barrel of weight 200N is raised by a vertical distance of 1.8m by being moved

along the ramp. The work done against gravity will be 200N x 1.8m = 360J

If the ramp is at an angle of 25o to the horizontal, then the force required will be less

but the total work done must, if the friction is negligible, be the same, so:

Distance moved along the ramp = 1.8𝑚

𝑠𝑖𝑛25° = 4.26

Force required = 360𝐽

4.26𝑚 = 84.5N

A simpler way is to use the vertical component of the distance moved along the

slope:

Work done = 200N x 4.26m x cos65o

= 360J

65o is the angle between the force and the distance moved. In other words:

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

= F d cos𝜃

Where d is the distance travelled and 𝜃 is the angle between the force and the

direction of travel.


-Work and the joule

The picture that was used in the example previously:

25o

200N

1.8m


-Work and the joule

Note that if the force and direction of travel are at right angles to one

another, then no work is done as cos 90o is zero. This may seem rather

irrelevant, as at first sight a force at right angles to the direction of travel

seems impossible, however the force of gravity on the Moon as it orbits

Earth is at right angles to the Moon’s direction of travel. So, despite the

large gravitational force the Earth is exerting on the Moon, the Earth is not

doing any work on the Moon, and so the Moon moves at a constant speed

for a very long time.


-The conservation of Energy

Energy is the stored ability to do work.

• Energy cannot be created or destroyed.

• Energy can be transferred from one form to another but the total amount of

energy in a closed system will not change.

Total energy in = Total energy out

At a basic level, energy is either kinetic energy or potential energy.

• Kinetic Energy: where movement is taking place

• Potential Energy: Regions where electric, magnetic, gravitational and nuclear forces

exist. Regions such as these are called fields.

Below are different forms of energy together with some details of how the energy is

stored:

Chemical Energy: energy can be released when the arrangement of atoms is altered

Electrical potential energy: Eg) A positive charge is pushed close to another positive

charge. This will often be called electrical energy.

Electromagnetic energy: includes all the waves that travel at the speed of light in a

vacuum (gamma rays, X-rays, ultraviolet, light, infrared, microwaves, radio waves).

These waves hold their energy in electric and magnetic fields.


-The conservation of Energy

Gravitational potential energy: where an object is at a high level in the Earth’s

gravitational field.

Internal energy: the molecules in all objects have random movement and have

some potential energy when they are close to one another.

Kinetic energy: when an object has speed.

Nuclear energy: energy can be released by reorganising the protons and neutrons

in an atom’s nucleus. This form of energy is also known as atomic energy.

Sound energy: in the movement of atoms

Conservation of energy describes the

situation in any closed system, where

energy may ne converted from one from

into another, but cannot be created or

destroyed.


Potential and Kinetic energies

Gravitational potential energy (GPE): this is the energy stored in an object (the work

an object can do) by virtue of its position in a gravitational field. The formulae is:

GPE = mgh

Kinetic energy (KE): this is the work an object can do by virtue of its speed. The

formulae is: kinetic energy (k.e.) = 1

2𝑚𝑣2. Also, the kinetic energy of a moving body

equals the work it can do as a result of its motion.

Falling objects: An object of mass m, falling from rest, loses gravitational potential

energy. From the principle of conservation of energy, it gains an exactly equivilant

amount of kinetic energy as a result of the work being done on it by gravity, so:

Mgh = 1

2𝑚𝑣2, where v is its speed and h is the distance fell, m cancels to give:

2gh = v2 or v= √2𝑔ℎ


Power and the Watt Power is the rate of doing work.

Power = 𝑤𝑜𝑟𝑘 𝑑𝑜𝑛𝑒

𝑡𝑖𝑚𝑒 𝑡𝑎𝑘𝑒𝑛

One watt (W) is equal to one joule per

second.

1kW = 1000 W

1MW = 1000kW = 1 000 000 W

To power a 100 watt light bulb, an

electric current must be flowing

through the filament of the bulb. It

supplies energy at the rate of 100

joules per second, so to power it

for one hour it would be:

100J s-1 x 3600s = 360 000 J. Electrical energy is sold to domestic users in units

called kilowatt- hours (kWh), which is equivalent to the use of 1000 W of power for

an hour.

Eg) 1kWh could be supplied to a 100W lamp over 10 hours.

1kWh = 1000J s-1 x 3600s = 3 600 000 J. Today one kWh of energy costs about

15p.


Power and the Watt

You need to be careful when distinguishing between rates and totals. For example, you

cannot buy a kW of power; you pay for energy. You can pay 1 kW used for 6 hours-

6kWh. Below is a table showing the relationship between rates and totals for several

units.

Rate Example of rate Time Total Example of total

Speed 80 km h-1 4h Distance 320km

Power 3 kW 200s Energy 600kJ

Current 25 mA 1000s Charge 25C

Human Power and Horse Power:

176W is a high rate of work that only a fit person could sustain for any length of

time. Most people would find it difficult to work continuously at a rate of 70W.

Horse power is still used to express some power ratings. 1 horse power is

equal to 746W- though this isn’t really what horses achieve.


Efficiency

Efficiency is expressed as:

Efficiency = 𝑢𝑠𝑒𝑓𝑢𝑙 𝑜𝑢𝑡𝑝𝑢𝑡 𝑒𝑛𝑒𝑟𝑔𝑦

𝑡𝑜𝑡𝑎𝑙 𝑜𝑢𝑡𝑝𝑢𝑡 𝑒𝑛𝑒𝑟𝑔𝑦 × 100%

Device Energy Input Energy Output Typical

Efficiency (%)

Electrical motor Electrical Kinetic/ Potential 85

Solar cell Light Electrical 10

Rechargeable battery Electrical Electrical 30

Electric radiator Electrical Internal 100

Power station Nuclear Electrical 40

Car (petrol) Chemical Kinetic/ Potential 45

Car (diesel) Chemical Kinetic/ Potential 55

Steam engine Chemical Kinetic/ Potential 8

To convert electrical energy into

heat, you just need resistance.


Sankey Diagrams

Input energy

Wasted output energy

split into different types

Useful output

energy

The width of the arrows should

relate to how much is wasted-

don’t use fat arrows for things

with small loss!

(Do it to scale)


Deformation of materials

The word elastic can be applied to a collision. In an elastic collision no kinetic energy

is lost. This can only happen when there is no permanent distortion of the objects

colliding, because if there is permanent distortion some energy must have been used

to create the distortion. Collisions which are not elastic collisions are not usually called

plastic collisions but inelastic collisions.

A stretch can be Elastic or Plastic…

Elastic

If a deformation is elastic, the material returns to

its original shape once the forces are removed.

1) When the material is put under tension, the

atoms of the materials are pulled apart from

one another.

2) Atoms can move small distances relative to

their equilibrium positions, without actually

changing position in the material.

3) Once the load is removed, the atoms return

to their equilibrium distance apart.

For a metal, elastic deformation happens as long

as Hooke’s law is obeyed.

Plastic

If a deformation is plastic, the

material is permanently

stretched.

1) Some atoms in the material

move position relative to one

another.

2) When the load it removed, the

atoms don’t return to their

original position.

A metal stretched past its elastic

limit shows plastic deformation.


Deformation of materials

Tensile and compressive forces

Forces that stretch objects like wires, springs and rubber bands are called tensile

forces, because they cause tension in the object. Therefore, for there to be tension in

a fixed stretched wire, there must be equal and opposite forces on it at either end.

With a spring, it is possible to reduce its length by squeezing it, and in this instance

the forces applied are called compressive forces. Unless the spring is accelerating,

equal and opposite forces must be applied.

Once the elastic limit has been passed,

the stretch becomes permanent.

Plastic deformation- the object will not

return to its original shape when the

deforming force is removed, it becomes

permanently distorted.


Hooke’s Law

Hooke’s Law- the extension of

an elastic body is proportional

to the force that causes it.

The equation is F= kx,

where F is the force causing extension x, and k is known as the force constant

(stiffness constant). The force constant is expressed in newton's per metre. k tells us

how much force is required per unit of extension.

Eg) A k of 6N mm-1 means it takes 6N to cause an extension of 1mm. Note that the

force constant can only be used when the material is undergoing elastic deformation.

When deformation become plastic, the force per unit extension is no longer constant.

Graphs- When extension is plotted on the x-axis, the area beneath the line is equal to

the work required to stretch the wire.

Work done = area of triangle = ½ Fx And since F=kx…

Work done = 1

2𝑘𝑥2

In the case of elastic deformations, the elastic potential energy E equals the work

done, giving:

E = 1

2𝐹𝑥 =

1

2𝑘𝑥2.


Hooke’s Law

Energy stored in plastic deformation:

The graph shown below could be produced by stretching a copper wire beyond its

elastic limit. The work done stretching the wire is given by the area A + B. If the tension

is then reduced to zero, the wire behaves elastically, contracting to a permanent

extension x. As the tension is reduced, energy equivalent to area B is released from

the wire. The net result of the wire having work A + B done on it, but only releasing

energy B, is that the wire becomes hot to the touch.


Young’s modulus

Stress and Strain:

• Stress is force per unit cross-section area, therefore is

expressed in the SI Unit newton per square metre. N m-2.

This unit is called pascal (Pa), which is also used to

quantify pressure.

• Strain is extension per unit length. As a result, strain

does not have a unit, since it is length divided by length;

sometimes it is quoted as a percentage. A strain of 2% is

the same as a strain of 0.02 and implies that a material

has extended 2cm for each metre of its original length.

Stress is force per unit

cross-sectional area.

Strain is extension per

unit length.


Young’s modulus

Stress on a material causes strain. How much strain is caused depends on how

stiff it is. A stiff material, such as cast iron, will not alter its shape much when a

stress is applied to it, but a relatively small stress will cause a substantial strain in

a soft material, such as clay.

Young's Modulus is the ratio between stress and strain, measured in pascals

(Pa). The formulae is as follows:

Young Modulus (E) = stress

strain=

force

areaextension

length

= force × length

area ×extension

Where, F = force in N, A = cross-sectional area, l = initial length in metres and

e = extension in m


Categories of materials

Material variety:

There are many materials now, all with different strengths and weaknesses.

Some of the properties materials may have are: Ductility, brittleness, stiffness,

density, elasticity, plasticity, toughness, fatigue resistance, conductivity, and fire

resistance.

The properties of individual material types can be illustrated clearly by sketching

graphs of stress against strain.

Ductile- materials that have a large plastic region (therefore they can be drawn

into a wire); for example, copper. The strain on a ductile material may be around

50%

Brittle- A material that distorts very little even when subject to a large stress and

does not exhibit any plastic deformation; for example, concrete.

Polymeric material- A material made of many smaller molecules bonded

together, often making tangled long chains. These materials often exhibit very

large strains (e.g. 300%) for example rubber.


Interpreting Stress-Strain Graphs

Stress-Strain graphs for Ductile materials curve

Str

ess (

Nm

-2)

Strain Limit of Proportionality

Stops obeying Hooke’s

Law but would still return

to original shape

Elastic Limit

Starts behaving plastically, and would no

longer return to original shape once the stress

was removed.

Yield point

The material suddenly starts to stretch

without any extra load. The yield point is

the stress at which a large amount of

plastic deformation takes place with a

constant or reduced load.



Stress-Strain graphs for Brittle materials don’t curve

Str

ess (

Nm

-2)

Strain

Material

fractures

• Brittle materials obey Hooke’s

Law.

• When the stress reaches a certain

point, the material snaps (it does

not deform plastically).

• When stress is applied to a brittle

material any tiny cracks get bigger

and bugger until the material breaks

completely. This is called brittle

fracture.



Rubber and Polythene are Polymeric Materials

Str

ess (

Nm

-2)

Strain

Str

ess (

Nm

-2)

Strain

Loading

Unloading

Rubber

Loading

Unloading

Polythene

Rubber returns to its original length

when the load is removed- it behaves

elastically.

Polythene behaves plastically- it has

been stretched to a new shape. It is a

ductile material.

DEFINITIONS

Acceleration (a)- the rate of change of velocity, measured in metres per second

squared (m s-2); a vector quantity

Sample- definition

Physics Unit 1 Mechanics

Documents

Transcript of Physics Unit 1 Mechanics