91影视

(I) At room temperature, an oxygen molecule, with mass of \({\bf{5}}{\bf{.31 \times 1}}{{\bf{0}}^{{\bf{ - 26}}}}\;{\bf{kg}}\) typically has a kinetic energy of about \({\bf{6}}{\bf{.21 \times 1}}{{\bf{0}}^{{\bf{ - 21}}}}\;{\bf{J}}\). How fast is it moving?

The energy transformations in pole vaulting and archery are discussed in this chapter, in a similar fashion, discuss the energy transformations related to: (a) hitting a golf ball; (b) serving a tennis ball; and (c) shooting a basket in basketball.

(I) (a) If the kinetic energy of a particle is tripled, by what factor has its speed increased? (b) If the speed of a particle is halved, by what factor does its kinetic energy change?

Describe precisely what is 鈥渨rong鈥� physically in the famous Escher drawing shown in Fig. 6鈥�32.

Two identical arrows, one with twice the speed of the other, are fired into a bale of hay. Assuming the hay exerts a constant 鈥渇rictional鈥� force on the arrows, the faster arrow will penetrate how much farther than the slower arrow? Explain.

A heavy ball is hung from the ceiling by a steel wire. The instructor pulls the ball back and stands against the wall with the ball against his chin. To avoid injury, the instructor is supposed to release the ball without pushing it (Fig. 6鈥�33). Why?

Describe the energy transformations when a child hops around on a pogo stick (there is a spring inside).

You push very hard on a heavy desk, trying to move it. You do work on the desk:

(a) whether or not it moves, as long as you are exerting a force.

(b) only if it starts moving.

(c) only if it doesn鈥檛 move.

(d) never鈥攊t does work on you.

(e) None of the above.

(II) An outboard motor for a boat is rated at 35 hp. If it can move a particular boat at a steady speed of 35 km/h,what is the total force resisting the motion of the boat?

(I) A 75.0-kg firefighter climbs a flight of stairs 28.0 m high. How much work does he do?