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Chapter 7: Q7.4-12CQ (page 258)
What is the relationship of potential energy to conservative force?
The conservative force equals to the negative gradient of the potential energy.
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A subway train is brought to a stop from a speed ofinby a large spring bumper at the end of its track. What is the force constant k of the spring?
Using values from Table 7.1, how many DNA molecules could be broken by the energy carried by a single electron in the beam of an old-fashioned TV tube? (These electrons were not dangerous in themselves, but they did create dangerous x rays. Later model tube TVs had shielding that absorbed x rays before they escaped and exposed viewers.)
(a) What is the efficiency of an out-of-condition professor who does \(2.10 \times {10^5}{\rm{ J}}\) of useful work while metabolizing \(500{\rm{ kcal}}\) of food energy?
(b) How many food calories would a well-conditioned athlete metabolize in doing the same work with an efficiency of \(20\% \)?
Work done on a system puts energy into it. Work done by a system removes energy from it. Give an example for each statement.
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 \(1.1{\rm{ m}}/{\rm{s}}\).
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