Topic 4

Question 1

Equation: Density *

Answer 1

ρ = m / v

ρ = density (kgm⁻³)

m = mass (kg)

v = volume (m^-3)

Question 2

Definition: Archimedes’ Principle

Answer 2

Upthrust is equal to the weight of the water displaced by the object.

Question 3

Equation: Stoke’s Law *

Answer 3

F = 6πηrv

F = force opposing motion of object moving through fluid (N)

η = viscosity of fluid (Pa s)

r = radius of sphere (m)

v = velocity of object (ms^-1)

Question 4

What are the conditions needed to apply Stoke’s Law?

Answer 4

• Small, spherical object moving slowly.
• Laminar flow.

Question 5

How does temperature affect viscosity?

Answer 5

As temperature increases, the viscosity decreases.

Question 6

What is the difference between low and high viscosity?

Answer 6

Low viscosity = runny

High viscosity = sticky

Question 7

Why does an object released in water accelerate downwards first before reaching a constant velocity?

Answer 7

Initially, the weight of the object is larger than the viscous drag + upthrust. As the object’s velocity increases, so does the viscous drag acting on the object. This occurs until the weight is equal to the viscous drag + upthrust, meaning there is no acceleration.

Question 8

What is the relationship between forces of an object travelling with constant velocity in a liquid?

Answer 8

W = U + D

W = weight

U = upthrust

D = viscous drag

Question 9

What are the forces acting on an object moving through a liquid?

Answer 9

Weight

Upthrust

Viscous drag

Question 10

Why does upthrust decrease as a balloon rises?

Answer 10

Because the density of air decreases meaning the upthrust decreases.

Question 11

Definition: Hooke’s Law

Answer 11

The extension of an object is proportional to the force applied, provided elastic deformation doesn’t occur.

Question 12

Equation: Hooke’s Law *

Answer 12

ΔF = kΔx

ΔF = force applied (N)

k = stiffness (Nm⁻¹)

Δx = extension (m)

Question 13

Definition: Limit of proportionality

Answer 13

The point at which an object stops obeying Hooke’s Law.

Question 14

Definition: Elastic limit

Answer 14

The point at which the object will no longer return to its original length if stretched beyond.

Question 15

Definition: Yield point

Answer 15

When plastic deformation begins and the extension quickly increases as force is increased.

Question 16

Definition: Breaking force

Answer 16

The point at which the object will break, even if the force is reduced after this point has been reached.

Question 17

What are the features of a force-extension graph?

Answer 17

PL = limit of proportionality

EL = elastic limit

Y = yield point

BF = breaking force

Question 18

Definition: Elastic strain energy

Answer 18

The work done to stretch or compress an object.

Question 19

How do you stretch an object?

Answer 19

Apply a tensile force.

Question 20

How do you compress an object?

Answer 20

Apply a compressive force.

Question 21

Equation: Elastic strain energy *

Answer 21

ΔE_el = ½FΔx

ΔE_el = elastic strain energy (J)

F = force applied (N)

Δx = extension (m)

Question 22

How do you work out elastic strain energy from a graph?

Answer 22

The area below a force-extension graph.

Question 23

Definition: Stress

Answer 23

The force per unit cross-sectional area acting on an object. (Nm⁻² or Pa)

Question 24

Equation: Stress *

Answer 24

σ = F / A

σ = stress (Pa or Nm^-2)

F = force (N)

A = cross-sectional area (m²)

Question 25

Definition: Breaking stress

Answer 25

The limit of force per unit cross-sectional area that may be applied without the unit failing.

Question 26

Definition: Strain

Answer 26

The extension per unit length of a material.

(no unit)

Question 27

Equation: Strain *

Answer 27

ε = Δx / x

ε = strain

Δx = extension (m)

x = original length (m)

Question 28

Definition: Young modulus

Answer 28

The ratio of stress to strain for a material that is behaving elastically which is a measure of stiffness for the object.

Question 29

Equation: Young modulus *

Answer 29

E = σ / ε

E = young modulus (Pa)

σ = stress (Pa)

ε = strain

Question 30

What are the features of a stress-strain graph?

Answer 30

Question 31

Definition: Viscosity

Answer 31

A measure of a fluid’s resistance to flow.

Question 32

What is elastic deformation?

Answer 32

When a small load is removed from a material and the material returns to its natural length.

Question 33

What is plastic deformation?

Answer 33

When a large load is removed from a material resulting in a permanent change in the length/shape of a material.