36 terms

Topic 4: Materials (Quiz)

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Which two properties should be defined to make selecting the right materials easier?
1. Tensile force per unit area
2. Extension per unit length
What type of force is applied to a spring being stretched?
Only tensile force
What does a greater Young modulus mean in regards to the stiffness of a material?
The stiffer a material, the greater its Young modulus
What are the three main times of stress?
1. Tensile
2. Compressive
3. Shear
How can one measure how 'hard' a material is?
Measures the size of a dent produced by pressing a diamond into the surface with a certain force
What did Friedrich Mohs measure and compile into a scale?
MIneral hardness -by scratching one over the other and seeing which was left with a mark
What is the most malleable material?
Gold - can be rolled, pressed or hammered with the greatest ease
What is an example of a solid fluid?
Flow of sand through an hourglass
What is the size of upthrust equivalent to?
The weight of fluid that is displaced
When does an object float?
When upthrust= weight (balanced forces)
What is another name for laminar flow?
Streamline flow
When does laminar flow become turbulent flow?
When the fluid velocity increases past a certain point/value. It depends on the fluid and the shape of the area through which it is flowing
What is the effect of streamline flow?
Produces much lower air resistance than turbulent flow
What is the frictional force in fluids caused by?
Viscosity
What does low viscosity mean?
When the frictional force caused by movement through a fluid is small
What is terminal velocity near Earth's surface?
Acceleration due to gravity

g = 9.81 ms-2
What is Hooke's Law?
The force F exerted by a spring is proportional to its extension ∆x
What is an example of a material that does not obey Hooke's Law?
Plasticine
Which materials do not obey Hooke's Law?
Those with a very low elastic limit
What happens beyond the elastic limit?
The extension of a spring increases rapidly and the spring is permanently deformed when the load is removed
What is the energy stored in the stretched spring equal to?
The work done on the spring - as a result of the average force used to stretch it to extension ∆x
What happens to a spring when work is done by the spring (in terms of energy)?
The spring gains energy (since work done is negative)
What are the units of fluid density?
Mass per unit volume
Why does a brick sink in water?
the upthrust of the brick is less than its weight and so it sinks
When fluids move at low speeds, what type of flow is it?
Laminar (streamline) flow
When fluids move at high speeds, what type of flow is it?
Turbulent flow
What are the two types of deformation?
Deformations can be either elastic or plastic
How does the viscosity of a liquid change as the temperature increases?
Liquids get less viscous as temperature increases
How does the viscosity of a gas change as the temperature increases?
Gases get more viscous as temperature increases
When does Stokes' law apply?
Stokes' law only applies to small spherical objects moving at low speeds with laminar flow (or in the absence of turbulent flow)
How are drag forces related to the radius/velocity of the object moving through a fluid?
Drag force is directly proportional to the sphere's radius and velocity
When stretching a spring, what type of force is applied?
When stretching a spring, only tensile force is applied
How is rubber able to dissipate energy?
Rubber is able to dissipate energy through a process called hysteresis
What does a high Young's modulus mean?
A stiff material has a high Young's modulus and changes its shape only slightly under elastic loads (e.g. diamond)
What does a small Young's modulus mean?
A flexible material has a low Young's modulus and changes its shape considerably (e.g. rubbers)
What are the conditions for terminal velocity?
constant velocity
no acceleration
no resultant force = balanced forces = forces in equilibrium

For objects in fluids: upthrust + drag = weight