1. 2.1 Fluid Flow Upthrust and flotation

2. Starter: test your knowledge Basic: find the density in kg m -3 Medium: state the percentage uncertainty in each quantity used in the calculation. High: find the percentage error in the calculated density and give the answer to an appropriate number of significant figures.

3. Length = 23 cm ± 0.5 cm = ± 2.2% Width = 10 cm ± 0.5 cm = ± 5.0% Height = 7cm ± 0.5 cm = ± 7.1% Mass = 3.38 kg ± 0.005 kg = ± 0.1% Uncertainty in volume = 2.2 + 5.0 + 7.1 % = ± 14.3% Uncertainty in density = 14.3 + 0.1 = ± 14.4 % ± 14.4% of 2099.38 = ± 302, so we can only justify an answer to 2 sig figs. ρ = 2100 kg m -3

4. Text book page 53

5. What determines if an object will float or sink? As an object is lowered into a fluid it progressively displaces more and more of that fluid. It therefore experiences a gradually increasing upthrust equal to the weight of the fluid it has so far displaced. The resultant force on the object will be the difference between the upthrust, U and the objects own weight, W In the diagram the weight exceeds the upthrust. The resultant force is downwards The object will continue to sink. If, before it is totally submerged, upthrust = weight, Then the resultant force will be zero. The object will float, supported by the fluid. But if when totally submerged, upthrust < weight, The object will sink. W U

6. For an object to sink.. Upthrust < weight of the object Remember it is displacing it own volume of the fluid in which it is immersed. So the object’s weight must be more than that of the fluid it has displaced W object > W fluid for the same volume. So,m object g > m fluid g Dividing both sides of the equation by the same volume, V ρ object > ρ fluid W U VV

7. HYDROMETERS Air Test solution Density Scale Density reading Basically a float (like those used in fishing) The less dense the fluid the more of it that has to be submerged to achieve an upthrust equal to its weight. Therefore the lower the density the lower it floats in the fluid. The scale is calibrated so that the level of the water on the scale indicates the density of the fluid. Common uses: Density of battery acid (car batteries) Alcohol content of beers and wines. Concentration of solutions.