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DISCUSSION 2 – NEWTON’S LAWS
Now let’s consider forces and Newton’s Laws. Discuss the following. A box with a mass of 100 kg is on a frozen lake (no friction). A force of 200 N to the left, and another force of 200 N to the right act on the box. Describe the motion of the box.
Also, if a car with a constant speed is turning a level corner, is there a net force acting on the car?
DISCUSSION 3 – CONSERVATION OF ENERGY
You have been studying ideas of potential energy and kinetic energy (KE). Potential energy due to position in the earth’s gravitational field or the potential energy of a stretched or compressed spring are examples of mechanical potential energy (PE). Energy stored in a lump of coal that is released when it is burned is chemical potential energy and not mechanical potential energy. This is the subject of chemistry.
The sum of kinetic and mechanical potential energy is called total mechanical energy. If there is no friction. The total mechanical energy of a system is conserved or constant if forces such as gravity or spring forces are the only forces acting on the object. These are called conservative forces. When a rock is thrown up, down, horizontally, or at a weird angle and air resistance is ignored, the total mechanical energy of the rock stays constant, or KE + PE = constant. This also means KE can become PE and vice versa. If there is friction, the work done by friction “gobbles up” total mechanical energy. For example, if you jump off of a high diving board into a swimming pool, as you fall toward the water, your initial potential energy becomes kinetic energy, but your total mechanical energy is conserved until you hit the water.
Discussion Question: What happens to your total mechanical energy when you hit the water? Where does the lost mechanical energy go? Why do you fill a swimming pool with water before diving in? Can you think of another important example of non-conservation of total mechanical energy?
Now I want to make a very important point. Total mechanical energy is conserved if there is no friction or other non-conservative forces doing work. If non-conservative forces are doing work, then total mechanical energy is not conserved. Total energy is all energy, not just mechanical energy. Now here is the big lesson, total energy is always conserved!!! This is a fundamental law of nature.
DISCUSSION 4 – CARS
I have a couple of car questions for you.
1. Does your gas-guzzling car conserve energy? What does it mean to say your car wastes energy?
2. Your car battery is a 12 volt battery. You can’t start your car with a 12 volt flashlight battery. Why not?
DISCUSSION 5 – NUCLEAR PHYSICS
The final discussion topic has three parts and is about nuclear physics. Some background information first. By the way, there will be a few questions on the final exam about this background information.
The modern theory of the atom is that the atom consists of a nucleus of protons and neutrons. Electrons surround the nucleus not as particles orbiting the nucleus, but as standing matter waves like the standing waves on a guitar string that has been plucked. As such, there location is not precisely known. Most of the atom is empty space. The total energy of an electron in an atom is quantized meaning it has certain discrete energy values such as 1 or 2 or 3 energy units, but nothing in between like 1,23 or 2.76 energy units. When an electron in an atom has a transition (i.e. falls) from a higher energy state to a lower energy state, the atom can give off a particle of light called a photon.
With regards to the nucleus, if the ratio of the number of neutrons to the number of protons is greater than about 1. The nucleus is unstable. First, the three main decay products of an unstable or radioactive nucleus are alpha particles, beta particles, and gamma rays. Alpha particles are just the nucleus of a helium atom (2 protons and 2 neutrons). They do not penetrate very far. Dead skin or a sheet of paper can stop them. Beta particles are electrons and can penetrate a thin layer of steel. A 1-cm thick piece of plastic can stop them. Gamma rays are high-energy photons and are very penetrating.
If a radioactive isotope that emits alpha particles is ingested, the living tissue stops them and absorbs their energy. Smoke detectors contain a radioactive isotope that emits alpha particles.
Part 1: Are smoke detectors safe?
Part 2: Radon gas is an inert gas that is not toxic or poisonous, but is radioactive and emits alpha particles. Why is radon dangerous?
Part 3: What are the advantages and problems with nuclear energy?