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Chapter 7: Q15 CQ (page 259)
Do devices with efficiencies of less than one violate the law of conservation of energy? Explain
No, devices with an efficiency of less than one do not violate the law of energy conservation.
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A car鈥檚 bumper is designed to withstand a \(4.0 - {\rm{km}}/{\rm{h}}\) \(\left( {1.1 - {\rm{m}}/{\rm{s}}} \right)\) collision with an immovable object without damage to the body of the car. The bumper cushions the shock by absorbing the force over a distance. Calculate the magnitude of the average force on a bumper that collapses \(0.200{\rm{ m}}\) while bringing a \(900 - {\rm{kg}}\) car to rest from an initial speed of \(1.1{\rm{ m}}/{\rm{s}}\).
Using energy considerations, calculate the average force a 60.0-kg sprinter exerts backward on the track to accelerate from 2.00 to 8.00 m/s in a distance of 25.0 m, if he encounters a headwind that exerts an average force of 30.0 N against him.
(a) Calculate the force needed to bring a 950-kg car to rest from a speed of 90.0 km/h in a distance of 120 m (a fairly typical distance for a non-panic stop).
(b) Suppose instead the car hits a concrete abutment at full speed and is brought to a stop in 2.00 m. Calculate the force exerted on the car and compare it with the force found in part (a).
Suppose the ski patrol lowers a rescue sled and victim, having a total mass of 90.0 kg, down a 60.0潞 slope at constant speed, as shown in Figure 7.37. The coefficient of friction between the sled and the snow is 0.100.
(a) How much work is done by friction as the sled moves 30.0 m along the hill?
(b) How much work is done by the rope on the sled in this distance?
(c) What is the work done by the gravitational force on the sled?
(d) What is the total work done?
Figure 7.37 A rescue sled and victim are lowered down a steep slope.
(a) How long can you rapidly climb stairs\(\left( {116/{\rm{min}}} \right)\)on the\(93.0{\rm{ kcal}}\)of energy in a\(10.0 - {\rm{g}}\)pat of butter?
(b) How many flights is this if each flight has\(16\)stairs?
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