11:04Introduction to power: definition, examples, power vs. time graphs, constant velocity power formula.Zak's Lab733views
Multiple ChoiceHow much power must an elevator motor supply in order to lift a 1000 kg elevator at constant speed a height of 100m in 50 seconds?1158views16rank8comments
Textbook QuestionA 20.0-kg rock is sliding on a rough, horizontal surface at 8.00 m/s and eventually stops due to friction. The coefficient of kinetic friction between the rock and the surface is 0.200. What average power is produced by friction as the rock stops?2794views
Textbook Question(II) An outboard motor for a boat is rated at 35 hp. If it can move a particular boat at a steady speed of 35 km/h, what is the total force resisting the motion of the boat?164views
Textbook Question(II) A driver notices that her 950-kg car, when in neutral, slows down from 95 km/h to 65 km/h in about 7.0 s on a flat horizontal road. Approximately what power (watts and hp) is needed to keep the car traveling at a constant 80 km/h?169views
Textbook Question(II) During a workout, football players ran up the stadium stairs in 75 s. The distance along the stairs is 83 m and they are inclined at a 33° angle. If a player has a mass of 88 kg, estimate his average power output on the way up. Ignore friction and air resistance.206views
Textbook QuestionA gardener pushes a 12 kg lawnmower whose handle is tilted up 37° above horizontal. The lawnmower's coefficient of rolling friction is 0.15. How much power does the gardener have to supply to push the lawnmower at a constant speed of 1.2 m/s? Assume his push is parallel to the handle.664views1rank
Textbook Question(a) How much work must you do to push a 10 kg block of steel across a steel table at a steady speed of 1.0m/s for 3.0 s? (b) What is your power output while doing so?410views
Textbook QuestionA hydroelectric power plant uses spinning turbines to transform the kinetic energy of moving water into electric energy with 80% efficiency. That is, 80% of the kinetic energy becomes electric energy. A small hydroelectric plant at the base of a dam generates 50 MW of electric power when the falling water has a speed of 18 m/s . What is the water flow rate—kilograms of water per second—through the turbines?717views
Textbook QuestionAstronomers using a 2.0-m-diameter telescope observe a distant supernova—an exploding star. The telescope's detector records 9.1 x 10¯¹¹ J of light energy during the first 10 s. It's known that this type of supernova has a visible-light power output of 5.0 x 10³⁷ W for the first 10 s of the explosion. How distant is the supernova? Give your answer in light years, where one light year is the distance light travels in one year. The speed of light is 3.0 x 10⁸ m/s .378views
Textbook QuestionWhen you ride a bicycle at constant speed, nearly all the energy you expend goes into the work you do against the drag force of the air. Model a cyclist as having cross-section area 0.45 m² and, because the human body is not aerodynamically shaped, a drag coefficient of 0.90 . Use 1.2 kg/m³ as the density of air at room temperature. (c) The food calorie is equivalent to 4190 J. How many calories does the cyclist burn if he rides over level ground at 7.3 m/s for 1 h?367views
Textbook Question(a) How much work does an elevator motor do to lift a 1000 kg elevator a height of 100 m? (b) How much power must the motor supply to do this in 50 s at constant speed?401views
Textbook QuestionA farmer uses a tractor to pull a 150 kg bale of hay up a 15° incline to the barn at a steady 5.0 km/h . The coefficient of kinetic friction between the bale and the ramp is 0.45. What is the tractor's power output?511views
Textbook QuestionA 90 kg firefighter needs to climb the stairs of a 20-m-tall building while carrying a 40 kg backpack filled with gear. How much power does he need to reach the top in 55 s?277views
Textbook QuestionA 70 kg human sprinter can accelerate from rest to 10 m/s in 3.0 s . During the same time interval, a 30 kg greyhound can go from rest to 20 m/s . What is the average power output of each? Average power over a time interval ∆t is ∆E/∆t .577views
Textbook QuestionIn a hydroelectric dam, water falls 25 m and then spins a turbine to generate electricity. (b) Suppose the dam is 80% efficient at converting the water's potential energy to electrical energy. How many kilograms of water must pass through the turbines each second to generate 50 MW of electricity? This is a typical value for a small hydroelectric dam.428views
Textbook Question(I) An 85-kg football player traveling 5.0 m/s is stopped in 1.0 s by a tackler.(b) What average power is required to stop him?202views
Textbook QuestionSome electric power companies use placement of water to store energy. Water is pumped from a low reservoir to a high reservoir. To store the energy produced in 1.0 hour by a 180-MW electric power plant, how many cubic meters of water will have to be pumped from the lower to the upper reservoir? Assume the upper reservoir is 380 m above the lower one. Water has a mass of 1.00 x 10³ kg for every 1.0m³ ( 1MW = 10⁶ W).192views
Textbook Question(II) Energy may be stored by pumping water to a high reservoir when demand is low and then releasing it to drive turbines (Fig. 20–15) during peak demand. Suppose water is pumped to a lake 105 m above the turbines at a rate of 1.00 x 10⁵ kg/s for 10.0 h at night.(a) How much energy (kWh) is needed to do this each night?<IMAGE>161views
Textbook Question(II) Energy may be stored by pumping water to a high reservoir when demand is low and then releasing it to drive turbines (Fig. 20–15) during peak demand. Suppose water is pumped to a lake 105 m above the turbines at a rate of 1.00 x 10⁵ kg/s for 10.0 h at night.(b) If all this energy is released during a 14-h day, at 75% efficiency, what is the average power output?<IMAGE>187views
Textbook QuestionA bicylist of mass 75 kg (including the bicycle) can coast down a 5.0° hill at a steady speed of 12 km/h. Pumping hard, the cyclist can descend the hill at a speed of 32 km/h. Using the same power, at what speed can the cyclist climb the same hill? Assume the force of friction is proportional to the square of the speed v; that is, F𝒻ᵣ = bv² , where b is a constant.204views
Textbook QuestionIII) The position of a 280-g object is given (in meters) by x = 4.0t³ - 8.0t² - 44t, where t is in seconds.(c) What is the average net power input during the interval from t = 0s to t = 2.0 s , and in the interval from t = 2.0 s to 4.0 s?149views
Textbook QuestionWater flows slowly over a dam at the rate of 320 kg/s and falls vertically 88 m before striking the turbine blades. Calculate (b) the rate at which mechanical energy is transferred to the turbine blades, assuming 55% efficiency.152views
Textbook Question(II) Water is stored in an artificial lake created by a dam (Fig. 20–22). The water depth is 48 m at the dam, and a steady flow rate of 35m³/s is maintained through hydroelectric turbines installed near the base of the dam.(a) How much electrical power can be produced?<IMAGE>118views
Textbook QuestionSuppose it takes 75 kW of power for your car to travel at a constant speed on the highway.(a) What is this in horsepower?27views
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