3.4.2.1 Bulk properties of solids

Question 1

What is density?

Answer 1

The compactness of a substance, mass it has per unit volume it takes up

Question 2

What does density depend on?

Answer 2

The material an object is made from (not its shape or size)

Question 3

How do you know whether an object will sink or float?

Answer 3

Depends on the average density of the substance - if the object is more dense than the fluid, it will sink, if it is less dense, it will float.

Question 4

What is Hooke’s Law?

Answer 4

The extension of a stretched wire is proportional to the load (force applied). F=kdeltaL

Question 5

What is k?

Answer 5

The stiffness of the object being stretched, also called the spring constant.

Question 6

How can a force be applied to a spring?

Answer 6

Tensile force - stretch the spring
Compressive force - squash the spring
- Hooke’s law applies to both

Question 7

What is the limit of proportionality/ Hooke’s law limit?

Answer 7

The point at which a material no longer obeys Hooke’s law when a force is applied

Question 8

What is the elastic limit?

Answer 8

The point at which a material no longer returns to its original shape when the force is removed

Question 9

What is elastic deformation?

Answer 9

If a material is elastically deformed, it returns back to its original shape when the force is removed

Question 10

How does elastic deformation work?

Answer 10

When a material is put under tension, the atoms of the material are pulled apart from each other. They can move small distances relative to their equilibrium positions, without actually changing positions in the material. When the load is removed, atoms return to their equilibrium distance apart.

Question 11

What is plastic deformation

Answer 11

The material no longer returns to its original shape when the force is removed - atoms don’t return to their original positions. A metal stretched past its elastic limit show plastic deformation

Question 12

What is tensile stress?

Answer 12

The force applied per cross-sectional area (F/A)

Question 13

What is tensile strain?

Answer 13

The change in length, i.e. deltaL/l (extension/original length)

Question 14

What type of deformation forces cause strain/ stress?

Answer 14

Can be tensile or compressive

Question 15

What is breaking stress?

Answer 15

When more and more stress is applied to a material it will cause atoms to move farther and farther apart. Eventually, the stress becomes so great that the atoms separate completely and the material breaks

Question 16

What is UTS?

Answer 16

Ultimate Tensile Stress - the maximum stress that the material can withstand

Question 17

What is elastic strain energy?

Answer 17

When a material is stretched, work has to be done in stretching the material. Before the elastic limit, all work done is stored as potential energy in the material, called elastic strain energy.

Question 18

What is the formula for energy stored?

Answer 18

0.5FdeltaL
or 0.5k(deltaL)^2

Question 19

What happens to energy when a spring is streched?

Answer 19

Energy is always conserved when stretching

Question 20

Energy transfers in elastic deformation:

Answer 20

Work is done to stretch the material, if elastic deformation, all work done is stored as elastic strain energy in the material. When the stretching force is removed, the energy store is transferred to other forms

Question 21

Energy transfers in plastic deformation:

Answer 21

Work is done to separate atoms, energy not stored as strain energy an mostly dissipated as heat

Question 22

What are the energy changes in an oscillating spring?

Answer 22

Change in kinetic energy = change in potential energy
- potential energy includes both gravitational potential energy and elastic strain energy

Question 23

How is energy conservation used in transport design?

Answer 23

Modern cars have crumple zones that deform plastically in a crash, meaning that some of the car’s kinetic energy goes into changing the chape of the vehicle’s body, so less transferred to the people inside. People more likely to survive but damage to the car is greater.

Question 24

What is the stiffness of a material exactly?

Answer 24

A material’s ability to return back to its original shape after being subjected to a force