Materials

Question 1

What’s the equation used in the viscosity experiment, and what are the conditions of it being valid?

Answer 1

η = 2r²g(ρb - ρf) / 9v

• Must be at terminal velocity
• Stoke’s law must apply:
• Fluid flow must be laminar (far from edges, not moving too fast)
• Ball must be a small, smooth sphere

Question 2

What happens to the viscosity of liquids and gases as their temperature increases?

Answer 2

Temperature increases:
- Liquids: viscosity decreases
- Gases: viscosity increases

Question 3

What is important to remember when commenting on the validity of a calculation involving viscosity?

Answer 3

Always name-drop Mr Stoke

E.g. turbulent flow, therefore Stoke’s Law doesn’t apply.

Question 4

What should you remember when comparing the flow of a fluid at different temperatures?

Answer 4

Refer to the SPEED of the flow, rather than the ‘amount’.
However, this was from a PPQ and a different context may require a different explanation.
Always consider the control when an experiment is described to you to inform how you interpret the results.

Question 5

What is stiffness?

Answer 5

Force/extension in Nm⁻¹

Question 6

What should you remember when evaluating multiple claims?

Answer 6

Remember that one claim could be true ASSUMING that the other claims are true (even if they aren’t). Do point out this assumption though. For example, if a claim required extrapolation, it could still be true if it was valid under the other claims being valid (even if they weren’t).

Question 7

What should you always remember to do when analysing any force/extension graph?

Answer 7

• Always state whether and when it obeys Hooke’s law.
• Always state whether the material is elastic (whether it returns to its original shape)

Question 7

What does the area between loading and unloading lines represent on a force/extension graph?

Answer 7

The area between the lines is the energy transferred to the internal energy of the material.

Question 8

What are the benefits of building with a material that has a higher Young’s modulus or stiffness?

Answer 8

• Can hold heavier weights
• Can withstand greater forces
• Can hold its own structural weight better, so can be taller
• Can be built with smaller dimensions

Question 9

What is k in F = kx?

Answer 9

Stiffness

Question 10

What happens when you stack springs in series vs in parallel?

Answer 10

Series: k = k0 / n
Parallel: k = nk0