You are Here: Home >< Physics

# Viscosity Exam Q Watch

1. I am terrible at viscosity questions and i just don't get how you rearrange Hookes law to find out certain values!

Heres an exam question i can't figure out

Q: A bubble has a diameter of 0.070m

(i) Show that the upthrust acting on it is about 2x10^-3 N
( Density of air =1.3Kgm^-3)

(ii)The bubble reaches a terminal velocity of 0.71ms^-1. Calculate its weight
(Viscosity of air =1.8x10^-5 Nsm^-2

I've got my AS exam in 2 weeks time , and i just don't get viscosity!
2. Can anyone help?
3. (Original post by 16ARTER)
I am terrible at viscosity questions and i just don't get how you rearrange Hookes law to find out certain values!

Heres an exam question i can't figure out

Q: A bubble has a diameter of 0.070m

(i) Show that the upthrust acting on it is about 2x10^-3 N
( Density of air =1.3Kgm^-3)

(ii)The bubble reaches a terminal velocity of 0.71ms^-1. Calculate its weight
(Viscosity of air =1.8x10^-5 Nsm^-2

I've got my AS exam in 2 weeks time , and i just don't get viscosity!
Viscosity is just a measure of how much a fluid (liquid or gas) resists "flow".
Something like honey or treacle has a high viscosity.
First, I don't think you mean "Hooke's" Law, as that is for springs. You must be referring to Stoke's Law, which deals with viscosity. In particular, there is a formula which gives you the size of the viscous (resistive or drag) force on an object that is moving through a fluid.
Because liquids and gasses have viscosity, then any object that moves through them will experience resistance to its motion.

The first question is not about viscosity at all. It's about Archimedes' Principle.
This is about the upthrust (buoyancy) force that all objects experience when placed in a fluid. (Liquid or gas)
The rule says that the upward force is equal to the weight of the amount (volume) of fluid displaced by the object.
So in the 1st question, you need to find the weight of the air displaced by the bubble.
Weight = mg
The volume of air displaced is the volume of the bubble.
Find this from the bubble's radius. (Diameter is given)
The mass of the air is volume times density. (density given)
The weight of the air is mg.

The 2nd question is about Stoke's Law.
When the bubble reaches its terminal velocity, you know that the forces on it (upwards and downwards) are balanced. This is because there is no acceleration at the terminal velocity. If a=0 then F=0 from F=ma.

The forces on the bubble are
1. Its weight downwards
2. The upthrust upwards (calculated in part 1)
3. The viscous drag (resistance) due to the air its falling through. (Acts upwards to oppose the downwards motion)

Stokes Law says the viscous drag force is given by

v is the velocity of the bubble (given)
r is the radius of the bubble (diameter given)
is the viscosity of air (given)
So you can calculate the drag force which acts upwards against the motion of the bubble downwards.
So you know upthrust (upwards) and the viscous drag (upwards)
These two are balanced by the weight of the bubble downwards.
This means you can find the weight of the bubble.
4. Thanks

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: December 29, 2010
Today on TSR

### Degrees to get rich!

... and the ones that won't

### Women equal with Men?

Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• Poll
Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.