91影视

You lift a heavy book from a table to a high shelf. List the forces on the book during this process, and state whether each is conservative or nonconservative.

^{(II) Figure 9鈥�50 shows a pair of forceps used to hold a thin plastic rod firmly. If the thumb and finger each squeeze with a force\({F_{\rm{T}}} = {F_{\rm{F}}} = 11.0\;{\rm{N}}\), what force do the forceps jaws exert on the plastic rod?}

Question: Suppose a power plant delivers energy at 880 MW using steam turbines. The steam goes into the turbines superheated at 625 K and deposits its unused heat in river water at 285 K. Assume that the turbine operates as an ideal Carnot engine. (a) If the river flow rate is \({\bf{37}}\;{{{{\bf{m}}^{\bf{3}}}} \mathord{\left/{\vphantom {{{{\bf{m}}^{\bf{3}}}} {\bf{s}}}} \right.} {\bf{s}}}\), estimate the average temperature increase of the river water immediately downstream from the power plant. (b) What is the entropy increase per kilogram of the downstream river water in \({{\bf{J}} \mathord{\left/{\vphantom {{\bf{J}} {{\bf{kg}} \cdot {\bf{k}}}}} \right.} {{\bf{kg}} \cdot {\bf{k}}}}\)?

(II) What minimum horsepower must a motor have to be able to drag a 370-kg box along a level floor at a speed of 1.20 m/s if the coefficient of friction is 0.45?

Question: A car engine whose output power is 135 hp operates at about 15% efficiency. Assume the engine鈥檚 water temperature of 85掳C is its cold-temperature (exhaust) reservoir and 495掳C is its thermal 鈥渋ntake鈥� temperature (the temperature of the exploding gas鈥揳ir mixture). (a) What is the ratio of its efficiency relative to its maximum possible (Carnot) efficiency? (b) Estimate how much power (in watts) goes into moving the car, and how much heat, in joules and in kcal, is exhausted to the air in 1.0 h.

Question: (II) Suppose our Sun eventually collapses into a white dwarf, losing about half its mass in the process, and winding up with a radius 1.0% of its existing radius. Assuming the lost mass carries away no angular momentum, (a) what would the Sun鈥檚 new rotation rate be? Take the Sun鈥檚 current period to be about 30 days. (b) What would be its final kinetic energy in terms of its initial kinetic energy of today?

(III) A bicyclist coasts down a 6.0掳 hill at a steady speed of . Assuming a total mass of 75 kg (bicycle plus rider), what must be the cyclist鈥檚 power output to climb the same hill at the same speed?

Spiderman uses his spider webs to save a runaway train moving about \(60\;{{km} \mathord{\left/{\vphantom {{km} h}} \right.} h}\)Fig. 6鈥�44. His web stretches a few city blocks (500 m) before the \({\bf{1}}{{\bf{0}}^{\bf{4}}}\;{\bf{kg}}\) train comes to a stop. Assuming the web acts like a spring, estimate the effective spring constant.

FIGURE 6鈥�44Problem 72.

Question: (II) A uniform disk turns at \({\bf{3}}{\bf{.3}}\;{{{\bf{rev}}} \mathord{\left/{\vphantom {{{\bf{rev}}} {\bf{s}}}} \right.} {\bf{s}}}\) around a frictionless central axis. A non rotating rod, of the same mass as the disk and length equal to the disk鈥檚 diameter, is dropped onto the freely spinning disk, Fig. 8鈥�56. They then turn together around the axis with their centers superposed. What is the angular frequency in \({{{\bf{rev}}} \mathord{\left/{\vphantom {{{\bf{rev}}} {\bf{s}}}} \right.} {\bf{s}}}\) of the combination?

A 36.0-kg crate, starting from rest, is pulled across a floor with a constant horizontal force of 225 N. For the first 11.0 m the floor is frictionless, and for the next 10.0 m the coefficient of friction is 0.20. What is the final speed of the crate after being pulled these 21.0 m?