Topic 4 Materials definitons and questions

Archimedes’ Principle

When a body is fully or partially submerged in a fluid, it experiences an upthrust equal to the weight of the fluid it has displaced.

Breaking Stress

The maximum stress a material can withstand without fracturing.

Brittle

A material that fractures without plastic deformation first.

Density

Mass per unit volume, with units kgm-3 .

Ductile

A material that produces a large plastic deformation, under tension, this allows these materials to be stretched into thin wires.

Elastic Deformation

When a material can return to its original shape, after an applied stress, without a permanent change to its shape. Any work done in elastically deforming an object is stored as elastic strain energy.

Elastic Limit

The maximum stress that can be applied to an object without plastic deformation.

Hooke’s Law

The extension of an object is directly proportional to the force being applied to the object.

Hard

resistant to indentation

Laminar Flow

A state of flow where layers of fluid move together in parallel with no mixing between layers.

Limit of Proportionality

The point at which the stress on an object is so great that Hooke’s law no longer applies to an object.

Malleable

A material that is able to undergo plastic deformation under compression. And form thin sheets

Plastic Deformation

When a material is permanently deformed after an applied stress due to the atoms moving relative to one another in the material. Work is done in plastically deforming the material and is dissipated as heat.

Stoke’s Law

The magnitude of the force of viscosity acting on a spherical body as it moves through a fluid is proportional to its radius, its velocity and the fluid’s viscosity. This only applies to spherical bodies travelling in laminar, non-turbulent, flow.

Stiff

high Young’s Modulus

Tensile Strain

The extension of an object divided by its original length.

Tensile Stress

The internal resistance of an object against a force that acts to deform it. It is the force applied per unit cross-sectional area.

High Tensile strength

a material which can withstand a large stress before breaking

Turbulent Flow

A state of flow where layers of fluid mix together unpredictably causing a chaotic state.

Viscosity

A quantity measuring the internal friction of a fluid, it acts to reduce the flow of a fluid. It is temperature dependent.

Upthrust

The force felt against the weight of an object when fully or partially submerged in a fluid. It is equal to the weight of the fluid displaced by the object.

Yield Point

The point on a force-extension graph at which a material begins to rapidly extend without any additional stress.

Young’s Modulus

The ratio of stress to strain of an object. It is a measure of how stiff a material is.

what does k represent in ∆F = k∆x

the stiffness or spring constant of an object

the graph represents a force extension graph of a polymer, suggest 2 differences between that of a graph of a metal

larger and steeper gradient

clear yield point

suggest one property of a material with a high youngs modulus

very stiff

how do you find the elastic strain energy in a force extension graph?

area under force extension graph where the line is straight

What does the area of a stress against strain graph represent?

work done or elastic strain energy per unit volume

area=stress x strain

A second wire of the same material and cross-sectional area is stretched by the same force. If it has twice the length of the first wire its extension will be

• young modulus and stress is constant

• so the extension must double if the length doubled

A material which can be drawn into a wire is described as being

ductile

What does each point represent?

maximum tensile stress

area of a force extension graph represents

elastic strain energy or work done

area of a stress strain graph

elastic strain energy or work done per unit volume

Which material has the greatest strength?

A- highest point for ultimate tensile strength

Which material has the greatest value for the Young modulus?

B-steepest gradient

Suggest a reason why some people prefer to use this type of line that have been pre-stretched by loading and unloading.

Initially the extension of the graph increased linearly, State what is meant by increased linearly and what can be concluded about the wire

• mass is directly proportional to extension

• wire obeys Hooke’s Law

How do you find the maximum energy the wire can store while behaving linearly?

area underneath the straight line

then multiply by gravity (9.81)

to find the elastic potential energy

Describe the behaviour of the wire when a mass greater than 2.9kg is added (2 marks)

there is a large increase in extension for a small increase in applied mass

the wire is permanently deformed

Describe 4 modifications that would produce a greater extension for a given mass of a wire

• Use a wire with a smaller cross sectional area

• Use longer wire

• Use wire with a lower spring constant

• Or use a wire with a lower young’s modulus

If a ball is moving upwards through a fluid.

Upthrust=

Upthrust= weight + drag

explain 3 resaons why the ball does not obey Stoke’s Law (3 marks)

1. there is turbulent flow

2. it is a large sphere

3. it is moving quickly

add flow lines to indicate laminar and turbulent flow

How does velocity of a fluid vary with it’s viscosity (1 mark)

• if viscosity of the fluid increases, the velocity of the fluid decreases

If the temperature of a fluid increases. How would it affect it’s velocity? (2 marks)

increase viscosity of the fluid, which therefore decreases the velocity of the fluid

Viscosity is sometimes given the units kg m^-1 s^-1 and Pa s^-1

Show how these are equivalent

explain why the new trailer uses less fuel than the old one (4 marks)

1. New design is more streamlined

2. reducing turbulent flow

3. less air resistance

4. less energy transferred to the air from the trailer

(3 marks)

1. there is a resultant upward force

2. there is a greater volume of fluid displaced

3. So upthrust increases

Initially Upthrust = weight+ drag

Upthrust - (weight + drag)= 0

temperature decreases

so the density of the drop decreases

so the upthrust reduces