FIG. 3-1 Work is done by a force when the object it acts on moves while the force is applied. No work is done by pushing against a stationary wall. Work is done when throwing a ball because the ball moves while being pushed during the throw.

FIG. 3-2 When a force and the distance through which it acts are parallel, the work done is equal to the product of F and d. When they are not in the same direction, the work done is equal to the product of d and the projection of F in the direction of d.

FIG. 3-3 Kinetic energy is proportional to the square of the speed. A car traveling at 30 m/s has 9 times the KE of the same care traveling at 10 m/s.

FIG. 3-4 When a hammer strikes this nail, the hammer's kinetic energy is converted into the work done to push the nail into the wooden board.

FIG. 3-5 A raised stone has potential energy because it can do work on the ground when dropped.

FIG. 3-6 Two examples of potential energy.

FIG. 3-7 The increase in the potential energy of a raised object is equal to the work used to lift it.

FIG. 3-8 In the absence of friction, a car can coast from the top of one hill into a valley and then up to the top of another hill of the same height as the first. During the trip the initial potential energy of the care is converted into kinetic energy as the car goes downhill, and this kinetic energy then turns into potential energy as the care climbs the next hill. The total amount of energy (KE + PE) remains unchanged.

FIG. 3-9 Energy transformations in planetary motion. The total energy (KE + PE) of the planet is the same at all points in its orbit. (Planetary orbits are much more nearly circular than shown here.)

FIG. 3-10 Energy transformations in pendulum motion. The total energy of the ball stays the same but is continuously exchanged between kinetic and potential forms.

FIG. 3-11 Joule's experimental demonstration that heat is a form of energy. As the weight falls, it turns the paddle wheel, which heats the water by friction. The potential energy of the weight is converted first into the kinetic energy of the paddle wheel and then into heat.

FIG. 3-12 The linear momentum mv of a moving object is a measure of its tendency to continue in motion at constant velocity. The symbol > means "greater than."

FIG. 3-13 When a running girl jumps on a stationary sled, the combination moves off more slowly than the girl's original speed. The total momentum of girl + sled is the same before and after she jumps on it.

FIG. 3-14 The momentum m_Cv_C to the right of the thrown camera is equal in magnitude to the momentum m_Av_A to the left of the astronaut who threw it away.

FIG. 3-15 How the effects of a head-on collision with a stationary target object depend on the relative masses of the two objects.

FIG. 3-16 Rocket propulsion is based upon conservation of momentum. If gravity is absent, the downward momentum of the exhaust gases is equal in magnitude and opposite in direction to the upward momentum of the rocket at all times.

FIG. 3-17 Conservation of angular momentum. Angular momentum depends upon both the speed of turning and the distribution of mass. When the skater pulls in her arms and extended leg, she spins faster to compensate for the change in the way her mass is distributed.

FIG. 3-18 The faster a top spins, the more stable it is. When all its angular momentum has been lost through friction, the top falls over.

FIG. 3-19 The relativity of mass. The greater the speed of an object relative to an observer, the greater the object's mass appears to the observer. This effect is conspicuous only at speeds near the speed of light c, which is 3 x 10⁸ m/s, about 186,000 mi/s.

FIG. 3-20 The irregular path of a microscopic particle bombarded by molecules. The line joins the positions of a single particle observed at 10-s intervals. This phenomenon is called brownian movement and is direct evidence of the reality of molecules and their random motions. It was discovered in 1827 by the British botanist Robert Brown.

FIG. 3-21 General relativity pictures gravity as a warping of the structure of space and time due to the presence of a body of matter. An object nearby experiences an attractive force as a result of this distortion in space-time, much as a marble rolls toward the bottom of a saucer-shaped hole in the ground.

FIG. 3-22 Starlight that passes near the sun is deflected by its strong gravitational pull. The deflection, which is very small, can be measured during a solar eclipse when the sun's disk is obscured by the moon.

FIG. 3-23 A total of about 330 EJ (1 EJ = exajoule = 10¹⁸ J) of energy was produced commercially in the world in 1997 from the sources shown. The percentages have been rounded off and so do not add up to 100 percent. Fossil fuels are responsible for 90 percent of the world's energy consumption (apart from firewood, still widely used, which is not included here). The percentages for energy sources in the United States are not very different from those of the world as a whole.