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Week 10 Classroom Assignment 2 Heat and Heat Transfer Name: Grade: Must show all work for full credit Answers must include units if required and highlighted or in bold (if these rules are not met, they will be considered wrong) 1. How much heat transfer (in kilocalories) is required to thaw a 0.450-kg package of frozen vegetables originally at 0oC if their heat of fusion is the same as that of water? 2. A bag containing 0C ice is much more effective in absorbing energy than one containing the same amount of 0C water. (a) How much heat transfer is necessary to raise the temperature of 0.800 kg of water from 0C to 30.0C ? (b) How much heat transfer is required to first melt 0.800 kg of 0C ice and then raise its temperature? (c) Explain how your answer supports the contention that the ice is more effective. 3. How much heat transfer is required to raise the temperature of a 0.750-kg aluminum pot containing 2.50 kg of water from 30.0C to the boiling point and then boil away 0.750 kg of water? (b) How long does this take if the rate of heat transfer is 500 W [ 1 watt = 1 joule/seco nd (1 W = 1 J/s) ]? 4. On a trip, you notice that a 3.50-kg bag of ice lasts an average of one day in your cooler. What is the average power in watts entering the ice if it starts at and completely melts to water in exactly one day [ 1 watt = 1 joule/seco nd (1 W = 1 J/s) ]? Week 10 Classroom Assignment 2 Heat and Heat Transfer Week 10 Classroom Assignment 2 Heat and Heat Transfer Week 8 Homework Fluid Statics Name: Grade: Must show all work for full credit Answers must include units if required and highlighted or in bold (if these rules are not met, they will be considered wrong) 1) Gold is sold by the troy ounce (31.103 g). What is the volume of 1 troy ounce of pure gold? 2) Mercury is commonly supplied in flasks containing 34.5 kg (about 76 lb). What is the volume in liters of this much mercury? 3) What is the mass of a deep breath of air having a volume of 2.00 L? (b) Discuss the effect taking such a breath has on your body’s volume and density. 4) A straightforward method of finding the density of an object is to measure its mass and then measure its volume by submerging it in a graduated cylinder. What is the density of a 240-g rock that displaces 29.0𝑐𝑚3 of water? (Note that the accuracy and practical applications of this technique are more limited than a variety of others that are based on Archimedes’ principle.) 5) (a) A rectangular gasoline tank can hold 50.0 kg of gasoline when full. What is the depth of the tank if it is 0.500-m wide by 0.900-m long? (b) Discuss whether this gas tank has a reasonable volume for a passenger car. 6) As a woman walks, her entire weight is momentarily placed on one heel of her highheeled shoes. Calculate the pressure exerted on the floor by the heel if it has an area of 1.50 cm2 and the woman’s mass is 55.0 kg. Express the pressure in Pa. (In the early days of commercial flight, women were not allowed to wear high-heeled shoes because aircraft floors were too thin to withstand such large pressures.) 7) The greatest ocean depths on the Earth are found in the Marianas Trench near the Philippines. Calculate the pressure due to the ocean at the bottom of this trench, given its depth is 11.0 km and assuming the density of seawater is constant all the way down. 8) What force must be exerted on the master cylinder of a hydraulic lift to support the weight of a 2000-kg car (a large car) resting on the slave cylinder? The master cylinder has a 2.00-cm diameter and the slave has a 24.0-cm diameter. Week 8 Homework Fluid Statics 9) A certain hydraulic system is designed to exert a force 100 times as large as the one put into it. (a) What must be the ratio of the area of the slave cylinder to the area of the master cylinder? (b) What must be the ratio of their diameters? (c) By what factor is the distance through which the output force moves reduced relative to the distance through which the input force moves? Assume no losses to friction. Week 10 Homework Heat and Heat Transfer Name: Grade: 1. a. b. c. d. e. A microwave heats up a cold piece of pizza. What type of heat transfer is this? Convection Radiation Induction There is no heat transfer Conduction 2. a. b. c. d. e. A pot of water boils. What type of heat transfer occurs within the pot? Convection Radiation Induction There is no heat transfer Conduction 3. A piece of ice is placed on a hot surface and begins to melt, but its temperature does not increase. What type of heat transfer is this? a. Convection b. Radiation c. Induction d. There is no heat transfer e. Conduction 4. A piece of tin sitting on a stove top becomes very hot. What type of heat transfer was involved? a. Convection b. Radiation c. Induction d. There is no heat transfer e. Conduction 5. On a cold day, the temperature often feels colder due to a wind chill. This is an example of what type of phenomenon? a. Convection b. Radiation c. Induction Week 10 Homework Heat and Heat Transfer d. There is no heat transfer e. Conduction 6. A piece of bread is burned on one side and uncooked on the other after being heated in a toaster. What was the most likely form of heat transfer to cause the burn? a. Convection b. Radiation c. Induction d. We need to know more information to reach a conclusion e. Conduction 7. Which of the following methods of heat transfer requires the two objects to be touching? a. Convection b. Radiation c. Induction d. Thermodynamic discharge e. Conduction 8. A marshmallow on a stick is placed above a fire, but not touching the fire. The marshmallow heats up and softens. How is the marshmallow being heated? I. Conduction II. Convection III. Radiation a. b. c. d. e. I, II, and III II and III III Only II Only I and II Week 10 Classroom Assignment 1 Heat and Heat Transfer Name: Grade: Must show all work for full credit Answers must include units if required and highlighted or in bold (if these rules are not met, they will be considered wrong) 1. On a hot day, the temperature of an 80,000-L swimming pool increases by 1.500C. What is the net heat transfer during this heating? Ignore any complications, such as loss of water by evaporation. 2. To sterilize a 50.0-g glass baby bottle, we must raise its temperature from 22oC to 95oC. How much heat transfer is required? 3. The same heat transfer into identical masses of different substances produces different temperature changes. Calculate the final temperature when 1.00 kcal of heat transfers into 1.00 kg of the following, originally at 20oC: (a) water; (b) concrete; (c) steel; and (d) mercury. 4. Rubbing your hands together warms them by converting work into thermal energy. If a woman rubs her hands back and forth for a total of 20 rubs, at a distance of 7.50 cm per rub, and with an average frictional force of 40.0 N, what is the temperature increase? The mass of tissues warmed is only 0.100 kg, mostly in the palms and fingers. 5. A 0.250-kg block of a pure material is heated from 20oC to 65oC by the addition of 4.35 kJ of energy. Calculate its specific heat and identify the substance of which it is most likely composed, from the list below. Week 10 Classroom Assignment 1 Heat and Heat Transfer Week 3 Classroom Assignment 2 Part 2 Kinematics (Two-Dimensional Kinematics) Name: Grade: Must show all work for full credit Answers must include units if required and highlighted or in bold (if these rules are not met, they will be considered wrong) 1. Kilauea in Hawaii is the world’s most continuously active volcano. Very active volcanoes characteristically eject red-hot rocks and lava rather than smoke and ash. Suppose a large rock is ejected from the volcano with a speed of 25.0 m/s and at an angle 35o above the horizontal, as shown in Figure 3.39. The rock strikes the side of the volcano at an altitude 20.0 m lower than its starting point. (a) Calculate the time it takes the rock to follow this path. (b) What are the magnitude and direction of the rock’s velocity at impact? (a) Let the time taken is t sec Initial velocity in y direction (𝑢𝑦 ) = 25𝑠𝑖𝑛35° = 14.34𝑚/𝑠 1 Using 𝑠 = 𝑢𝑡 + 2 𝑎𝑡 2 in y-direction: 1 −20 = 14.34𝑡 − (9.8)𝑡 2 2 ⇒ 4.9𝑡 2 − 14.34𝑡 − 20 = 0 14.34 ± ඥ(−14.34)2 − 4(4.9)(−20) ⇒𝑡= 2 ∗ 4.9 ⇒ 𝑡 = 3.96𝑠𝑒𝑐, −1.03 𝑠𝑒𝑐 As, time t can’t be negative, so 𝑡 = 3.96𝑠𝑒𝑐 So, it takes the rock 𝟑. 𝟗𝟔𝒔𝒆𝒄 to follow this path. (b) Initial velocity in x direction (𝑢𝑥 ) = 25𝑐𝑜𝑠35° = 20.48𝑚/𝑠 As, there is no force in x-direction so the velocity at impact in x direction (𝑣𝑥 ) = 𝑢𝑥 = 20.48 𝑚/𝑠 Using 𝑣 = 𝑢 + 𝑎𝑡in y-direction: 𝑣𝑦 = 14.34 − 9.8 ∗ 3.96 ⇒ 𝑣𝑦 = −24.47𝑚/𝑠 Week 3 Classroom Assignment 2 Part 2 Kinematics (Two-Dimensional Kinematics) So, magnitude of the rock’s velocity at impact = ඥ𝑣𝑥2 + 𝑣𝑦2 = ඥ20.482 + (−24.47)2 = 31.91𝑚/𝑠 𝑣𝑦 24.47 Angle 𝜃 in the diagram = arctan ቀቚ𝑣 ቚቁ = arctan ቀ20.48ቁ = 50.1° 𝑥 So, the magnitude of the rock’s velocity at impact is 𝟑𝟏. 𝟗𝟏𝒎/𝒔 and it makes 𝟓𝟎. 𝟏° with x-axis in clockwise direction. 2. An airline passenger drops a coin while the plane is moving at 260 m/s. What is the velocity of the coin when it strikes the floor 1.50 m below its point of release: (a) Measured relative to the plane? (b) Measured relative to the Earth? (a) Relative to plane, the coin has only the y-component of velocity. Using 𝑣 2 = 𝑢2 + 2𝑎𝑠 in y-direction: 𝑣𝑦2 = 02 + 2(−9.8)(−1.50) 𝑚 ⇒ 𝑣𝑦 = −ξ2 ∗ 9.8 ∗ 1.50 = −5.42 𝑠 So, velocity of the coin when it strikes the floor, measured relative to the plane is 5.42 m/s in downward direction (b) Relative to the Earth, the coin has the x-component of velocity as well, which is same as the velocity of the airplane So, 𝑣𝑥 = 260𝑚/𝑠 So, magnitude of the velocity of the coin Measured relative to the Earth = ඥ𝑣𝑥2 + 𝑣𝑦2 = ඥ2602 + (−5.42)2 = 260.06𝑚/𝑠 Direction of the velocity of the coin (clockwise from positive x-axis) Measured relative to the Earth 𝑣𝑦 5.42 = arctan ቀቚ𝑣 ቚቁ = arctan ቀ 260 ቁ = 1.19° 𝑥 So, velocity of the coin when it strikes the floor, measured relative to the Earth is 𝟐𝟔𝟎. 𝟎𝟔 m/s and it makes 𝟏. 𝟏𝟗° with x-axis in clockwise direction.

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