Materials

Question 1

Define density, giving units

Answer 1

mass of the object per volume it takes up
ρ = m/v
kgm^-3

Question 2

Explain density qualitatively

Answer 2

Measure of how compact a substance is. Therefore, doesn’t vary with size or shape, just depends on what the object is made of
(kind of like resistivity)

Question 3

What determines whether an object sinks or floats

Answer 3

The average density of the object
A solid object will float on a fluid if it has a lower density than the fluid

Question 4

What is the density of water

Answer 4

ρ = 1 gcm^-3

so 1 cm^3 of water has a mass of 1g

Question 5

Why do objects deform ie stretch, twist, bend etc

Answer 5

Opposite forces acting on the object

e.g. force down on a spring and reaction force up from the support

Question 6

State Hooke’s law qualitatively

Answer 6

The extension of a stretched object is proportional to the load or force

Question 7

What is the equation for Hooke’s law

Answer 7

F = k∆L
where k is a constant, being the stiffness, which depends on the object being stretched

Question 8

What is the difference between tensile and compressive forces

Answer 8

Tensile forces stretch the object

Compressive forces squash the object

Question 9

Why are springs a special case of Hooke’s law

Answer 9

The value of k, known as the spring constant for springs, is the same for both tensile and compressive forces

Question 10

How is Hooke’s law shown on a graph

Answer 10

Plot a graph of force against extension and will be a straight line

Question 11

Why does Hooke’s law stop applying

Answer 11

Hooke’s law assumes reversibility from the deformation so will not apply if the deformation begins to be permanent due to too much force being applied

Question 12

What is the point on the force/extension graph where Hooke’s law no longer applies

Answer 12

Up to the limit of proportionality, where the graph starts becoming curved instead of a straight line

Question 13

What is the elastic limit

Answer 13

The point on the graph (after the limit pf proportionality) where any more force would cause permanent deformation

Question 14

Define elastic deformation

Answer 14

Deformation in which the material returns back to its original shape and size once all forces are removed

Question 15

Explain why elastic deformation occurs

Answer 15

Tension on a material pulls atoms apart. Atoms have an equilibrium point so they can move small distances and then return back to the equilibrium point after the force is removed

Question 16

Define plastic deformation

Answer 16

Deformation in which the material is permanently stretched

Question 17

Explain why plastic deformation occurs

Answer 17

When a material is stretched past its elastic limit, the atoms move too far from their equilibrium points so they do not return once the force is removed

Question 18

Explain the energy transfer during elastic deformation, giving example

Answer 18

Work is done when stretching the material, which is stored as elastic strain energy
When the force is removed, this energy is transferred to other forms

e.g. elastic band flies across room when released from stretch

Question 19

Explain the energy transfer during plastic deformation

Answer 19

Work is done to separate atoms and this energy is not stored as strain energy, it is mainly just dissipated as heat

Question 20

Explain how the energy transfers during plastic deformation are helpful in the real world

Answer 20

Crumple zones in cars are designed to deform plastically in a crash
This means energy goes in to changing the shape of the car and then dissipated as heat instead of being stored and transferred to passengers.