![]() ![]() Since speeds are given, we can use 1 2 m v 2 1 2 m v 2 to calculate the person’s kinetic energy. (a) What is the person’s kinetic energy relative to the car? (b) What is the person’s kinetic energy relative to the tracks? (c) What is the person’s kinetic energy relative to a frame moving with the person? Kinetic Energy Relative to Different FramesĪ 75.0-kg person walks down the central aisle of a subway car at a speed of 1.50 m/s relative to the car, whereas the train is moving at 15.0 m/s relative to the tracks. The thermal neutron in part (c) has a kinetic energy of about one fortieth of an electron-volt. At the other extreme, the energy of subatomic particle is expressed in electron-volts, 1 eV = 1.6 × 10 −19 J. ![]() The Chicxulub asteroid’s kinetic energy was about a hundred million megatons. The energy of the impactor in part (b) can be compared to the explosive yield of TNT and nuclear explosions, 1 megaton = 4.18 × 10 15 J. Different units are commonly used for such very large and very small values. In this example, we used the way mass and speed are related to kinetic energy, and we encountered a very wide range of values for the kinetic energies. You also have to look up the mass of a neutron.ĭon’t forget to convert km into m to do these calculations, although, to save space, we omitted showing these conversions. To answer these questions, you can use the definition of kinetic energy in Equation 7.6. What is the kinetic energy of such a particle? What was its mass? (c) In nuclear reactors, thermal neutrons, traveling at about 2.2 km/s, play an important role. (a) What is the kinetic energy of an 80-kg athlete, running at 10 m/s? (b) The Chicxulub crater in Yucatan, one of the largest existing impact craters on Earth, is thought to have been created by an asteroid, traveling atĢ2 km/s and releasing 4.2 × 10 23 J 4.2 × 10 23 J of kinetic energy upon impact. ![]() Since objects (or systems) of interest vary in complexity, we first define the kinetic energy of a particle with mass m. At speeds comparable to the speed of light, the special theory of relativity requires a different expression for the kinetic energy of a particle, as discussed in Relativity. Note that when we say “classical,” we mean non-relativistic, that is, at speeds much less that the speed of light. With this history in mind, we can now state the classical definition of kinetic energy. (If you have ever played billiards or croquet, or seen a model of Newton’s Cradle, you have observed this type of collision.) The idea behind this quantity was related to the forces acting on a body and was referred to as “the energy of motion.” Later on, during the eighteenth century, the name kinetic energy was given to energy of motion. The first body stops, and the second body moves off with the initial velocity of the first body. ![]() At the end of the seventeenth century, a quantity was introduced into mechanics to explain collisions between two perfectly elastic bodies, in which one body makes a head-on collision with an identical body at rest. This does not depend on the direction of the velocity, only its magnitude. It’s plausible to suppose that the greater the velocity of a body, the greater effect it could have on other bodies. Evaluate the kinetic energy of a body, relative to different frames of reference.Calculate the kinetic energy of a particle given its mass and its velocity or momentum.By the end of this section, you will be able to: ![]()
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