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# Forces in Fluids

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• The stronger the attraction, the less freely the particles flow past one another and the more viscous the fluid is.
• Example: if a force of 1 Newton was applied by the thumb to the nail, what would be the pressure of the two ends of the nail? Let’s say the nail head is 10 meters sqaure and the point is 1 square meter. P head = 1/10 Pascals and P point = 1/1 Pascals
• The pressure at a certain depth in the liquid is the same at all points at that depth. Pressure at A = Pressure at B = Pressure at C = Pressure at D = Pressure at E Notice that it is only the depth of the liquid that matters; not the angle of the tube, the cross sectional area of the liquid column or the area of the liquid surface.
• ### Forces in Fluids

1. 1. Forces in Fluids
2. 2. Review Newton’s Laws Explanation Summary First The velocity of an object will not change unless it is acted on by an unbalanced force. inertia Second When an object is acted on by an unbalanced force, it will accelerate in the direction of that force. F=ma Third For every action there is an equal and opposite reaction. action-reaction
3. 3. Fluids <ul><li>A fluid is any substance that lacks a definite shape and has the ability to flow. </li></ul><ul><li>Fluids = liquids and gasses </li></ul><ul><li>Viscosity describes how “sticky” or strongly attracted fluid molecules are to each other </li></ul>
4. 4. <ul><li>Pressure: the amount of force acting on a unit of area. P = F/A </li></ul><ul><li>Measured in units of pascals (N/m 2 ). </li></ul><ul><ul><li>One pascal (Pa) = 1 N of force per square meter </li></ul></ul>Pressure
5. 5. Calculating Pressure <ul><li>Pressure = force or P = F </li></ul><ul><li> area A </li></ul><ul><li>Weight = 100 N </li></ul><ul><li>A foot = 5 m 2 </li></ul><ul><li>A snowshoe = 50 m 2 </li></ul><ul><li>P foot = 100 N = 20 Pa </li></ul><ul><li>5 m 2 </li></ul><ul><li>P snowshoe = 100 N = 2 Pa </li></ul><ul><li>50 m 2 </li></ul>
6. 6. Pascal’s Principle <ul><li>When a force is applied to a fluid in a closed container, an increase in pressure is transmitted equally to all parts of the fluid. </li></ul><ul><li>That pressure (force per area) is undiminished throughout the fluid. </li></ul><ul><li>The force is usually a push or squeeze. </li></ul>
7. 7. Force Pumps <ul><li>Fluids are moved from one place to another by increasing the pressure with a push or squeeze </li></ul><ul><ul><li>Ex.: bicycle pumps, tubes of toothpaste, your heart </li></ul></ul><ul><ul><li>How it works </li></ul></ul>
8. 8. Hydraulic Devices
9. 9. P = Force/Area (Area = length x width) <ul><li>10N ÷ 1cm 2 </li></ul><ul><ul><li>First change cm 2 to m 2 </li></ul></ul><ul><ul><li>1 cm = .01 m </li></ul></ul><ul><ul><li>.01 m x .01 m = .0001 m 2 </li></ul></ul><ul><li>10 N ÷ .0001 m 2 = 100,000 N/ m 2 (Pascals) </li></ul>1cm 1cm
10. 10. <ul><li>Pressure = force or P = F and F= PA </li></ul><ul><li> area A </li></ul><ul><li>F 1 = 10 N </li></ul><ul><li>A 1 = 1 m 2 </li></ul><ul><li>P = 10 N </li></ul><ul><li>1 m 2 </li></ul><ul><li>Pressure = 10 Pa </li></ul><ul><li>If A 2 = 50 m 2 then what is the force on the bigger piston? </li></ul><ul><li>F = P x A, so F 2 = 10 Pa x 50 m 2 </li></ul><ul><li>F 2 = 500 N </li></ul>
11. 11. <ul><li>Video </li></ul><ul><li>Explanation </li></ul>
12. 12. Pressure & Depth <ul><li>The weight of a fluid can exert a pressure on anything underneath it. </li></ul><ul><li>Pressure increases with depth due to the weight of the fluid above pressing down. </li></ul><ul><li>Atmospheric pressure differs with elevation. </li></ul><ul><ul><li>100 kPa at sea level </li></ul></ul><ul><ul><ul><li>(1008 milibars or 15 lbs/in 2 ) </li></ul></ul></ul><ul><ul><li>The higher up in the atmosphere, the less air there is pushing down, so the lower the pressure will be. </li></ul></ul>
13. 13. Pressure & Depth <ul><li>Water pressure also increases with depth. </li></ul><ul><li>The water pressure at the bottom of a lake is greater than at the surface because of all the water (and atmospheric gas) pressing down. </li></ul>
14. 14. <ul><li>At which point is water pressure the greatest? </li></ul><ul><li>They are all equal. </li></ul><ul><ul><li>Only depth affects pressure. </li></ul></ul>