Topic 4

Question 1

What is the density of a material?

Answer 1

The density of a material is its mass per unit volume

Question 2

State the equation used to calculate density

Answer 2

Density = Mass / Volume

Question 3

What is the unit of density?

Answer 3

kg/m^3

Question 4

When an object is submerged in a fluid, what determines the upthrust experienced?

Answer 4

The upthrust of a submerged object is equal to the weight of the fluid that it displaces

Question 5

What determines whether an object floats or sinks?

Answer 5

The balance between the weight and upthrust of the object. If the weight exceeds the upthrust, the object will sink

Question 6

Why will a uniform object with a density greater than that of the liquid it is submerged in, always sink?

Answer 6

The upthrust of the object will be equal to the weight of the liquid displaced.
Therefore, the maximum upthrust will be equal to the denstiy of the fluid x volume of object x gravitational field stength
The weight of the object is equal to the density of the object x volume of object x gravitational field strength
If the object’s density is greater, the weight will always be greater than the upthrust and the object will sink

Question 7

What shaped objects does Stokes’ Law apply to?

Answer 7

Stokes’s law only applies to small spherical objects

Question 8

What type of flow is required for Stokes’ Law to apply?

Answer 8

Laminar Flow

Question 9

What does Stokes’ Law allow you to calculate?

Answer 9

The viscous drag force that a small spherical object experiences when falling at low speeds through a viscous fluid with laminar flow

Question 10

State the Stokes’ Law equation

Answer 10

F=6πηrv

η is the viscosity of the fluid
r is the radius of the sphere
v is the speed of the sphere

Question 11

What is elastic deformation?

Answer 11

An object has undergone elastic deformation if it returns to its original shape once the deforming forces are removed

Question 12

What is plastic deformation?

Answer 12

An object has undergone plastic deformation if it no longer returns to its original shape once the deforming forces are removed. It will have permanent deformation

Question 13

Express Hooke’s Law in words

Answer 13

The extension of an elastic object is directly proportional to the force that is applied to it, up to its limit of proportionality

Question 14

What is the limit of proportionality?

Answer 14

The point beyond which the force and extension will no longer be directly proportional to each other - Hooke’s law is no longer obeyed

Question 15

What is the elastic limit?

Answer 15

The point beyond which the object will no longer elastically deform, and will instead deform plastically

Question 16

State the defining equation of Hooke’s Law

Answer 16

F=kΔx
Force applied (N) = Stiffness constant x extension (m)

Question 17

What is mechanical stress?

Answer 17

The force experienced by an object per unit area

Question 18

State the equation for stress

Answer 18

σ = F/A
Stress = Force / Area

Question 19

What is the unit of stress?

Answer 19

N/m^2 or Pascals

Question 20

State the equation for strain

Answer 20

ε = ΔL/L
Strain = extension / original length

Question 21

What is the unit of strain?

Answer 21

Strain is a unitless quantity because it is the ratio of two lengths

Question 22

What does the Young Modulus of a material tell you?

Answer 22

A material’s Young Modulus is a measure of how much force is required for a given extension, regardless of the object’s dimensions

Question 23

What equation is used to calculate a material’s Young Modulus?

Answer 23

Young Modulus = Stress / Strain

Question 24

What unit is used for a material’s Young Modulus?

Answer 24

N/m^2

Question 25

What is breaking stress?

Answer 25

Breaking stress is the maximum stress that an object can withstand before fracturing

Question 26

What is the yield point?

Answer 26

The point beyond which the object will experience a large extension without substantial increase in the force applied

Question 27

What type of energy is stored in an object that has been stretched?

Answer 27

Elastic Potential Energy

Question 28

State two equations used to calculate the energy stored in a spring

Answer 28

E = 1/2 FΔx
E = 1/2 KΔx^2

Question 29

What does the gradient and area represent on a force-extension graph?

Answer 29

The gradient of the linear region represents the elastic constant (k)
The area is equal to the elastic potential energy stored in the spring