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At the end of this unit you should:

At the end of this unit you should: 1. Understand that density is the measure of mass in a solid/liquid per unit volume. 2. Be able to compare densities of different solids/liquids. 3. Be able to determine the flotation for a variety of solids and liquids in water and other liquids.

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At the end of this unit you should:

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  1. At the end of this unit you should: 1. Understand that density is the measure of mass in a solid/liquid per unit volume. 2. Be able to compare densities of different solids/liquids. 3. Be able to determine the flotation for a variety of solids and liquids in water and other liquids.

  2. buoyant force compound unit density flotation mass volume

  3. LIGHTBULB QUESTION It all depends on what we are comparing. If we are trying to find out which of the two blocks is heavier, then yes, this is a fair comparison. However, if we are trying to figure out which of the metals is heavier (pound for pound), then this is not a fair comparison. Consider it like boxing: we wouldn’t compare a featherweight with a heavyweight. They need to be in the same division. So it is similar with comparing density.

  4. Density: The mass per unit volume of a substance.

  5. (a) What is the density of a block of wood which has a mass of 100 g and a volume of 200 cm3? Density = mass/volume. 100 g/200 cm3 = 0.5 g/cm3

  6. (b) What is the density of a bar of gold which has a mass of 386 g and a volume of 20 cm3? 19.3 g/cm3

  7. (c) What mass of gold would be contained in 12 cm3? Density = mass/volume Mass = density x volume Mass = 19.3 g/cm3 (from previous question) x 12 cm3 = 231.6 g

  8. (d) Limestone has a density of 1.96 kg/m3. Calculate the volume of a rock of limestone which has a mass of 2.25 kg. 1.15 m3

  9. Investigation 13.01.01: Finding the density of an object Equipment: Graduated cylinder, overflow can, a stone, a block of wood, newton weights, mass balance.

  10. Investigation 13.01.01: Finding the density of an object Instruction: 1. Weight a rock on a balance. 2. Fill an overflow can until the water is a drop away from spilling out. (It might be best to overfill it and let the water spill out into a sink as you will then know it is one drop from overflowing.) 3. Place the graduated cylinder below the spout of the overflow can. 4. Gently place the rock in the overflow can. As an extra safeguard, it is advised you tie a string around the rock so you can lower it in. 5. Measure the amount of displaced water in the graduated cylinder. This is the volume of the object. 6. Repeat steps for wood and newton weights.

  11. 1. How did you ensure this was a fair test? • The object was slowly placed in the water. No extra objects were placed in the water (such as hand or finger tips). The water was one drop from overflowing each time a new object was measured. All the water that overflowed from the graduated cylinder was removed so the next object could be accurately measured.

  12. 2. Can you describe any pattern or relationship from your observations? • Did you notice that the amount of displaced liquid from the overflow can is the volume of the object?

  13. Copy and complete Table 13.01.01, ticking the variables that should stay the same, and putting an x where the variables can be different, with Object 1 being the gold and Object 2 the aluminium as depicted in Fig. 13.01.02.

  14. Investigation 13.01.02: Finding the density of a liquid Equipment: Beaker, water, mass balance. Instructions: 1. Get the mass of a graduated cylinder. 2. Find the mass of the liquid. This can be done by measuring out a specific volume of the liquid in a graduated cylinder. 3. Subtract the masses to get the mass of the liquid. 4. Calculate the density.

  15. 1. How did you ensure this was a fair test? • By using clean, dry equipment, ensuring the balance was at zero and subtracting the masses.

  16. 2. How did you compare the densities of each liquid? • By keeping a set volume to be measured in each instance.

  17. Flotation: The action of floating in a liquid or gas.

  18. Investigation 13.01.03: Sink or float? Equipment: Mass balance, graduated cylinder, salt and overflow can, oil, full cans of regular and diet fizzy drink, empty can of fizzy drink, ice.

  19. Investigation 13.01.03: Sink or float? Equipment: Mass balance, graduated cylinder, salt and overflow can, oil, full cans of regular and diet fizzy drink, empty can of fizzy drink, ice. Instructions: 1. Fill a large container with water. 2. Place each can in the water and observe what happens. 3. Measure the mass of the cans as well as the volume so you can calculate the actual density. 4. You can also get a bottle and use varying amounts of salt to see what effect it has on the density of the bottle of salt water.

  20. 1. How did you ensure this was a fair test? • By keeping the volumes in each can the same.

  21. 2. Construct a table which clearly presents your data and findings. • Similar to this example.

  22. 3. Describe all differences between the full can of fizzy drink and the empty can of fizzy drink. • Did you suggest differences such as the lack of liquid and the lack of gas?

  23. 4. Describe all differences between the regular can and diet can of fizzy drink. • The sugar content is the main difference.

  24. 5. Suggest a reason why ice did/didn’t float in water. • It has a lower density than the water.

  25. Buoyant Force: The upward force in a fluid that opposes the weight of an object immersed in the fluid.

  26. Copy and Complete In this unit I learned that density is the mass per unit volume of a substance. The unit for density is the kg/m3or the g/cm3. All physical substances have a density. What causes the density of an object is the packing of molecules/atoms in the substance. A denser object has more molecules/atoms packed tightly, whereas a less dense object has less atoms/molecules packed loosely. To calculate density we need to find the object’s massand volume. Once we have these we can use the formula density = mass / volume. For an object to float in a liquid the density of the object needs to be lessthan that of the liquid. For example ice/oil/etc. floats in water whereas rocks/etc. does not.

  27. 1. Describe how you would measure the density of the following: a) A block of wood with a mass of 2000 g If regular, measure the volume using L x W x H. If irregular, find the volume using an overflow can. The mass of the wood was measured using a balance. The density was calculated by dividing the mass by the volume.

  28. 1. Describe how you would measure the density of the following: b) Paraffin oil. Extract a set volume, i.e. 25 ml, 50 ml, 1 l etc. Zero a graduated cylinder on a balance and add the liquid to the cylinder, then record the mass. Calculate density.

  29. 2. Find the density of a block of wood with a volume of 400 cm3 and a mass of 360 g. D = m/v D = 360g/400cm3 D = 0.9 g/cm3

  30. 3. Find the volume of a mass of lead of 90 g. Density of lead = 11.342 g/cm3 D = m/v 11.342 g/cm3 = 90/v V = 90/11.342 V = 7.94 cm3

  31. 4. What volume does 67 g of mercury have? 13.534 g/cm3 = 67g/V V = 4.95 cm3

  32. 5. Which of the following will float in liquid mercury (density 13.6 g/cm3): (a) Gold (density 19.3 g/cm3) No. (b) Aluminium (density 2.7 g/cm3) Yes. (c) Iron (density 7.9 g/cm3) Yes. (d) Silver (density 9.32 g/cm3) Yes. (e) Copper (density 8.94 g/cm3) Yes. (f) Tungsten (density 19.3 g/cm3). No.

  33. 6. Fig. 13.01.06 shows the cargo hold in a large ship filled in three different ways. Ship captains are careful how they load their ships as the weight distribution can contribute to how easy or difficult it is to manoeuvre the ship, or indeed may contribute to the ship sinking in a heavy storm. In such a storm, which arrangement of the cargo is more likely to ‘turn turtle’ (turn upside-down) and sink? Which cargo arrangement makes it easier to steer the ship? Justify your answer.

  34. 6. Cargo arrangement A is most likely to ‘turn turtle’ in a storm because the weight is all to one side. Cargo C is easier to steer because the weight is distributed forward and aft (a nautical term meaning towards the stern [rear] of the ship). Cargo B will sit lower in the water at the back so it is easier for waves to rock the front part of the ship from side to side.

  35. 7. Submarines dive and submerge by changing their density, which changes their buoyancy. They do this by filling or emptying ballast tanks in their hull. In Fig. 13.01.07, diagram A shows a submarine on the surface, while diagram B shows a submarine submerged at depth. What is happening to the submarine in diagram C? Give reasons for your answer. In diagram C, the submarine is not fully submerged as its ballast tanks are not full. It is either sitting low in the water or is not finished diving so its ballast tanks will fill eventually.

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