Back(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?
Some 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).
(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?
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(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?
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A 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.
III) 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?
Water 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.