91影视

(a) Why do you push down harder on the pedals of a bicycle when first starting out than when moving at a constant speed? (b) Why do you need to pedal at all when the cycle is at a constant speed?

A fisherman in a boat is using a 鈥�10 lb test鈥� fishing line. This means that the line can exert a force of 45 N without breaking$\left(\mathbf{1}\mathbf{lb}\mathbf{=}\mathbf{4}\mathbf{.}\mathbf{45}\mathbf{N}\right)$. (a) How heavy a fish can the fisherman land if he pulls the fish upward vertically at a constant speed? (b) If he accelerates the fish upward at$\mathbf{2}\mathbf{.}\mathbf{0}\mathbf{m}{\mathbf{/}\mathbf{s}}^{\mathbf{2}}$, what heaviest fish can he land? (c) Is it possible to land a 15 lb trout on a 10 lb test line? Why or why not?

A 鈥榙oomsday鈥� asteroid with a mass of$1.0脳{10}^{10}\mathrm{kg}$is hurling through space. Unless the asteroid鈥檚 speed is changed by about$0.20\mathrm{cm}/\mathrm{s}$, it will collide with the earth and cause tremendous damage. Researchers suggest that a small 鈥榮pace tug鈥� sent to the asteroid鈥檚 surface could exert a gentle, constant force of 2.5 N. For how long must this force act?

Three mountain climbers, who are roped together in a line, are ascending on an ice field inclined at 31.0掳 to the horizontal (Fig. 4鈥�69). The last climber slips, pulling the second climber off his feet. The first climber is able to hold them both. If each climber has a mass of 75 kg, calculate the tension in each of the two sections of the rope between the three climbers. Ignore the friction between the ice and the fallen climbers.

FIGURE 4-69 Problem 83

Question: Two rock climbers, Jim and Karen, use safety ropes of similar length. Karen鈥檚 rope is more elastic, called a 鈥榙ynamic rope鈥� by climbers. Jim has a static rope not recommended for safety purposes in pro-climbing. (a) Karen (Fig. 4鈥�71) falls freely about 2.0 m, and then the rope stops her over a distance of 1.0 m. Estimate how large a force (assume constant) she will feel from the rope. (Express the result in multiples of her weight.) (b) In a similar fall, Jim鈥檚 rope stretches by only 30 cm. How many times his weight will the rope pull on him? Which climber is more likely to get hurt?

FIGURE 4-71 Problem 85

A coffee cup on the horizontal dashboard of a car slides forward when the driver decelerates from 45 km/h to rest in 3.5 s or less, but not if she decelerates in a longer time. What is the coefficient of static friction between the cup and the dashboard? Assume that the road and the dashboard are level (horizontal).

A roller coaster reaches the top of the steepest hill with a speed of 6.0 km/h. It then descends the hill, which is at an average angle of 45掳 and is 45.0 m long. What will its speed be when it reaches the bottom? Assume that${渭}_k=0.12$.

A motorcyclist is coasting with the engine off at a steady speed of 20.0 m/s but enters a sandy stretch where the coefficient of kinetic friction is 0.70. Will the cyclist emerge from the sandy stretch without having to start the engine if the sand lasts for 15 m? If so, what will be the speed upon emerging?

The 70.0-kg climber in Fig. 4鈥�72 is supported in the 鈥渃himney鈥� by the friction forces exerted on his shoes and back. The static coefficients of friction between his shoes and the wall, and between his back and the wall, are 0.80 and 0.60, respectively. What is the minimum normal force he must exert? Assume the walls are vertical and that the static friction forces are both at their maximum. Ignore his grip on the rope.

A 0.140-kg baseball traveling at 35.0 m/s strikes the catcher鈥檚 mitt, which, in bringing the ball to rest, recoils backward 11.0 can. What was the average force applied by the ball on the glove?