91影视

If a bicyclist of mass 65 kg (including the bicycle) can coast down a $6.5^{\mathrm{o}}$hill at a steady speed of $6.0\mathrm{km}/\mathrm{h}$because of air resistance, how much force must be applied to climb the hill at the same speed (and the same air resistance)?

Francesca dangles her watch from a thin piece of string while the jetliner she is in accelerates for takeoff, which takes about 16 s. Estimate the takeoff speed of the aircraft if the string makes an angle of${25}^{\mathrm{o}}$with respect to the vertical, Fig. 4鈥�64.

Bob traverses a chasm by stringing a rope between a tree on one side of the chasm and a tree on the opposite side, 25 m away, Fig. 4-65. Assume the rope can provide a tension force up to 29 KN before breaking, and use a 鈥渟afety factor鈥� of 10 (that is, the rope should only be required to undergo a tension force of 2.9 kN). (a) If Bob鈥檚 mass is 72.0 kg, determine the distance x that the rope must sag at a point halfway across if it is to be within its recommended safety range. (b) If the rope sags by only one-fourth the distance found in (a), determine the tension force in the rope. Will the rope break?

Figure 4-65 Problem 74

Piles of snow on slippery roofs can become dangerous projectiles as they melt. Consider a chunk of snow at the ridge of a roof with a slope of${\mathbf{34}}^{\mathbf{o}}$. (a) What is the minimum value of the coefficient of static friction that will keep the snow from sliding down? (b) As the snow begins to melt, the coefficient of static friction decreases and the snow finally snips. Assuming that the distance from the chunk to the edge of the roof is 4.0 m and the coefficient of the kinetic friction is 0.10, calculate the speed of the snow chunk when it slides off the roof. (c) If the roof edge is 10.0 m above ground, estimate the speed of the snow when it hits the ground.

What minimum force F is needed to lift the piano (mass M) using the pulley apparatus shown in Fig. 4-66? (b) Determine the tension in each section of rope: $F_{\mathbf{T}\mathbf{1}}$, $F_{\mathbf{T}2}$, $F_{\mathbf{T}3}$and $F_{\mathbf{T}4}$. Assume pulleys are massless and frictionless, and that ropes are massless.

Figure 4-66Problem 76

In the design of a supermarket, there are to be several ramps connecting different parts of the store. Customers will have to push grocery carts up the ramps and it is desirable that this not be too difficult. The engineer has done a survey and found that almost no one complaints if the force required is no more than 18 N. Ignoring friction, at what maximum angle$胃$should the ramps be built, assuming a full 25-kg cart?

A jet aircraft is accelerating at $\mathbf{3}\mathbf{.}\mathbf{8}\mathbf{m}\mathbf{/}{\mathbf{s}}^{\mathbf{2}}$as it climbs at an angle of${18}^o$above the horizontal (Fig. 4-67). What is the total force that the cockpit seat exerts on the 75-kg pilot?

Figure 4-67Problem 78

A 7180-kg helicopter accelerates upward at$\mathbf{0}\mathbf{.}\mathbf{80}\mathbf{m}\mathbf{/}{\mathbf{s}}^{\mathbf{2}}$While lifting a 1080-kg frame at a construction site, Fig. 4-68. (a) What is the lift force exerted by the air on the helicopter rotors? (b) What is the tension in the cable (ignore its mass) which connects the frame to the helicopter? (c) What force does the cable exert on the helicopter?

Figure 4-68Problem 79

A golf ball is hit with a golf club. Which force(s) act(s) on the ball when the ball flies through the air? Neglect the air resistance.

(a) The force of the golf club acting on the ball

(b) The force of gravity acting on the ball

(c) The force of the ball moving forward through the air

(d) All the above

(e) Both (a) and (c)

Estimate the average force exerted by a short-putter on a 7.0 kg shot if the shot is moved through a distance of 2.8 m and is released with a speed of 13 m/s.