Conceptual Physics Chapter 7 Energy PDF

Chapter 7 Energy Work and Energy 1. How much work (energy) is needed to lift an object that weighs 200 N to a height of 4 meters? 2. How much power is needed to lift the 200-N object to a height of 4 m in 4 seconds?. 3. What is the power output of an engine that does 60,000 J of work in 10 seco...

Chapter 7 Energy Work and Energy 1. How much work (energy) is needed to lift an object that weighs 200 N to a height of 4 meters? 2. How much power is needed to lift the 200-N object to a height of 4 m in 4 seconds?. 3. What is the power output of an engine that does 60,000 J of work in 10 seconds? 4. The block of ice weighs 500 newtons. (Neglect friction.) a. How much force parallel to the incline is needed to push it to the top? b. How much work is required to push it to the top of the incline? c. What is the potential energy of the block relative to ground level? d. What would be the potential energy if the block were simply lifted vertically 3 m? 5. All the ramps below are 5 m high. We know that the KE of the block when reaching the bottom of each ramp will be equal to the loss of PE (conservation of energy). Find the speed of the block at ground level in each case. [Hint: Do you recall from earlier chapters how much time it takes something to fall a vertical distance of 5 m from a position of rest (assume g = 10 m/s2)? And how much speed a falling object acquires in this time? This gives you the answer to Case 1. Discuss with your classmates how energy conservation provides the answers to Cases 2 and 3.] Case 1 Case 2 Case 3 Speed m/s Speed m/s Speed m/s 41 Chapter 7 Energy Work and Energy—continued 6. Which block gets to the bottom of the incline first? Assume no friction. (Be careful!) Explain. 7. The KE and PE of a block freely sliding down a ramp are shown in only one place in the sketch. Fill in the missing values. 8. A big metal bead slides due to gravity along an upright friction-free wire. It starts from rest at the top of the wire, Point A, as shown in the sketch. How fast is it traveling as it passes Point B?. Point D?. Point E?. Maximum speed occurs at Point. 9. Rows of wind-powered generators are used in various windy locations to generate electric power. Does the power generated affect the speed of the wind? That is, would locations behind the “windmills” be windier if windmills weren’t there? Discuss this in terms of energy conservation with your classmates. 42 Chapter 7 Energy Conservation of Energy 1. Fill the blanks for the six systems. 43 Chapter 7 Energy Conservation of Energy—continued 2. The woman supports a 100-N load with the friction-free pulley systems shown below. Fill in the spring-scale readings that show how much force she must exert. 3. A 600-N block is lifted by the friction-free pulley system shown. a. How many strands of rope support the 600-N weight? b. What is the tension in each strand? c. What is the tension in the end held by the man? d. If the man pulls his end down 60 cm, how many cm will the weight rise? e. If the man does 60 J of work, what will be the increase of PE of the 600-N weight? 4. Why don’t balls bounce as high during the second bounce as they do in the first bounce? 44 Chapter 7 Energy Momentum and Energy Bronco Brown wants to put Ft = mv to the test and try bungee jumping. Bronco leaps from a high cliff and experiences free fall for 3 seconds. Then the bungee cord begins to stretch, reducing his speed to zero in 2 s. Fortunately, the cord stretches to its maximum length just short of the ground below. Fill in the blanks. Bronco’s mass is 100 kg. Acceleration of free fall is 10 m/s2. Express values in SI units (distance in m, velocity in m/s, momentum in kgm/s, impulse in Ns, and deceleration in m/s2). The 3-second free-fall distance of Bronco just before the bungee cord begins to stretch = mv during the 3-s interval of free fall = mv during the 2-s interval of slowing down = Impulse during the 2-s interval of slowing down = Average force exerted by the cord during the 2-s interval of slowing down = How about work and energy? How much KE 45  does Bronco have 3 seconds after his jump? = How much does gravitational PE decrease during this 3 seconds? = What two kinds of PE are changing during the slowing-down interval? 46 Chapter 7 Energy Energy and Momentum A compact car and a full-size sedan are initially at rest on a horizontal parking lot at the edge of a steep cliff. For simplicity, we assume that the sedan has twice as much mass as the compact car. Equal constant forces are applied to each car and they accelerate across equal distances (we ignore the effects of friction). When they reach the far end of the lot, the force is suddenly removed, whereupon they sail through the air and crash to the ground below. (The cars are wrecks to begin with, and this is a scientific experiment!) 1. Which car has the greater acceleration? (Think a = F/m.) 2. Which car spends more time along the surface of the lot? (The faster or slower one?) 3. Which car has the larger impulse imparted to it by the applied force? (Think Impulse = Ft.) Defend your answer. 4. Which car has the greater momentum at the edge of the cliff? (Think Ft = mv.) Defend your answer. 5. Which car has the greater work done on it by the applied force? (Think W = Fd.) Defend your answer in terms of the distance traveled. 6. Which car has the greater kinetic energy at the edge of the cliff? (Think W = KE.) Does your answer follow from your explanation of Question 5? Does it contradict your answer to Question 3? Why or why not? 7. Which car spends more time in the air, from the edge of the cliff onto the ground below? 8. Which car lands farther horizontally from the edge of the cliff onto the ground below? Challenge: Suppose the slower car crashes a horizontal distance of 10 m from the ledge. Then at what horizontal distance does the faster car hit? 47