Answers to Idea 3 Study Questions -- 7 Ideas Sec. 005



(1) Before the collision, they had kinetic energy. After the collision the kinetic energy was converted to heat. If the gliders had not stuck to each other but had bounced off, they would still have had kinetic energy.

(2) Before I dropped it, the ball had gravitational potential energy. As it was falling, this was converted into kinetic energy. At impact with the clay, most of this went into heat while some became sound. Of course, everything also had a certain amount of heat energy anyway at room temperature.

(3) Simple: Potential energy due to gravity = mass x gravitational acceleration x height.

(4) It would have much less potential energy on Mars than the Earth since the acceleration of gravity is lower on Mars than on Earth (because Mars is much less massive than the earth).

(5) For example, the energy stored in a stretched spring and the energy stored in chemicals like alcohol and oxygen that can be released by burning.

(6) A car engine, for example, or the "Sterling Engine" I showed in class.

(7) One cannot convert heat into useful work with 100% efficiency. Crudely speaking this is due to the fact that the kinetic energy inherent in heat is random (in all directions) which cannot be converted to work along one direction with perfect efficiency.

(8) Work is defined as force x distance. For an example, see #13.

(9) This is a tricky question. The short answer is no - the object is not moving so force x distance = 0 (it goes no distance). However, work is being done at a microscopic level in you muscle cells…

(10) A calorie is a unit of energy, just as a Joule is. A "dietary calorie" (the kind used on food labels and stuff that equals 1000 physicist calories) is over 4000 Joules. Recall that a joule is about the kinetic energy of a one pound weight after falling 8 inches.

(11) We did this in class - we converted light to heat and lit a match with it and we converted light to electrical energy with a "solar cell".

(12) No - this is "conservation of energy".

(13) A bit complicated - the engine applies a force to the wheels using the heat energy of the burning fuel. The wheels apply a force to the road, which pushes back on the wheels, applying a force to the car which does work (moves the car over a distance).

(14) How much heat energy required to raise the temperature of an object one degree.

(15) The sparks are hot but have very little heat capacity and thus carry too little energy to hurt me.

(16) A measure of the ability of an object to carry heat.

(17) The wood conducts the heat out of your hand poorly due to a low thermal conductance.

(18) The random motion of molecules (heat) causes many of them to hit a larger object (like a milk fat glob) and move it randomly around.

(19) Heat is the random motion of the molecules that make up an object.

(20) The more molecules there are in an object, the more kinetic energy there is at a given temperature (since the average speed of a molecule is simply determined by the temperature).

(21) The molecules at the "hot" end of the object jiggle those towards the "cold" end, transferring some of their kinetic energy and thus the heat to the cold side.

(22) The basic constituents of the material - the molecules.

(23) Atoms are objects that are about 10-8 inches in size that are the smallest objects that posses distinct chemical properties. Molecules are atoms that are stuck together by (mainly electrical) forces. An element is a substance made out of one kind of atom, like pure copper or iron. There are about 100 kinds of elements and a huge number of kinds of molecules.

(24) In a solid, the molecules are constrained - they cannot just fly around (that's why they are "solid"). The energy of heat is simply the energy of the molecules vibrating in place. In gases the molecules are free to move (though the generally collide and bounce off of each other a lot). In liquids they are also free to move but collide much more, are generally closer, and interact a lot more, making the material at least a little cohesive.

(25) The motion due to heat is random - that of a ball flying through the air certainly is not.

(26) Gas pressure is the force of gas molecules bouncing off of a wall. The higher the temperature, the faster they move, and the higher the pressure.

(27) It gets bigger - no atmospheric pressure pushing back.

(28) About 1000 mph.

(29) The hotter most objects are the bigger they are. Naively, you might expect this - the molecules jiggle more and so sort of "stretch" the object. However, the reality is somewhat more complicated.

(30) Electrons zip around in a "cloud" perhaps 10-8 inches in size. Inside (perhaps 100000 times smaller) is the nucleus with neutrons and protons. The protons have positive charge and "hang onto" the negative electrons. There are always the same number of electrons and protons so that the atom is electrically neutral.

(31) Radiation is anything emitted by an atom that has too much energy - it can be gamma rays (high energy light), beta rays (electrons), neutrons, whatever. An isotope of an element is simply an atom of that element with a particular number of neutrons - remember, which element it is depends solely on the number of electrons…

(32) Two things: neutrons hitting nuclei of many elements can make them radioactive and/or change into another element and neutrons hitting a few kind of nuclei (like U235) can cause fission.

(33) One neutron causes fission which emits more neutrons, each of which cause fissions, which emit even more, etc. etc. If each fission emits two neutrons that go on to cause fissions the number of fissions is 1,2,4,8,16,32,… . Pretty soon you're talkin' lots o' fissions and … bang.

(34) Several ways, the most important being that almost all reactors depend on slow neutrons causing fissions while bombs use fast neutrons - thus reactors use moderators.

(35) Kiloton - equivalent explosive energy of 1000 tons (2 million pounds) of the common explosive TNT. Megaton - 1 million tons (2 billion pounds).

(36) The random chunks left over after a fission - these can be any element lighter than Uranium and are usually radioactive, making nuclear waste.

(37) Well,

(a) It has 3 fewer neutrons and

(b) It is very easy to fission with fast or slow neutrons. U238 will eat neutrons (especially fast ones) but will not fission well.