1. Unit 1 Physics on the go Topic 2 Materials: Viscosity

2. Viscous Drag When a solid object moves through a fluid, the laminar flow layers surrounding the object experience friction. These frictional forces not only act between the solid and the fluid, but also between the layers themselves. This is called viscous drag If the flow is turbulent the viscous drag is greater

3. Viscosity The amount of viscous drag depends on the type of fluid – the ‘stickiness’, or its resistance to flow It is easier to move through water than golden syrup. Viscosity may be measured in terms of rate of flow. A low rate of flow means the fluid is viscous.

4. Investigate factors that affect viscosity http://www.planetseed.com/flash/science/lab/liquids/visco_exp/en/viscosity.htm

5. Factors that affect viscous drag Temperature Viscosity of the fluid Speed of movement through fluid Whether the flow is laminar or turbulent

6. Stoke’s Law

7. Question 1 A marble of diameter 30mm and mass 15g is dragged through a bath of water which has a viscosity of 0.001 Pa s at 20⁰C at a speed of 2 ms -1 Calculate the viscous drag force and comment on your result. If we compare this force to the weight of the ball, then it is tiny. This suggests that the conditions for Stoke’s Law are not met

8. Terminal Velocity Free body diagram for a sphere falling through a fluid Three forces are acting: Weight (downwards) Upthrust (upwards) Viscous drag (upwards) At first the sphere will accelerate downwards. The resultant force ΣF = W - (U + F drag ) U + F drag W

9. Using Stoke’s law to find the viscosity When the terminal velocity is reached we know that U + F drag = W

10. Using Stoke’s Law to find the terminal velocity of a sphere At terminal velocity, v term the weight is balanced by the upthrust and drag force (sometimes called Stoke’s Force) Weight = upthrust + drag force Using the expressions for the weight (equal to volume displaced), upthrust and drag force gives us:

11. Using Stoke’s Law to find the terminal velocity of a sphere cont’d Terminal velocity increases as the sphere gets larger (r 2 )

12. Question 2 A helium balloon of radius 30cm is released. The weight of the balloon is 0.17N, the upthrust is 0.18N and the viscosity of air is 1.8 x 10 -5 Pa S a)Draw a free body diagram of the forces acting b)Use this to help you find the terminal velocity using Stoke’s Law In this example the viscous drag acts downwards – it always opposes the motion Weight + drag force Upthrust Upthrust = weight + drag so drag = 0.18-0.17 =0.01N V term = 98.2 ms -1