CONTACT FORCE Contact forces are forces that exist as a result of an inter- action between objects that are physically in contact with one another. They include frictional forces, tensional forces, and normal forces. The friction force opposes the motion of an object across a surface. For example, if a glass moves across the surface of the dinner table, there exists a friction force in the direction opposite to the motion of the glass. Friction is the result of attractive intermolecular forces between the molecules of the surface of the glass and the surface of the table. Friction depends on the nature of the two surfaces. For example, there would be less friction between the table and the glass if the table was moistened or lubricated with water. The glass would glide across the table more easily. Friction also depends on the degree to which the glass and the table are pressed together. Air resistance is a type of frictional force. Tension is the force that is transmitted through a rope or wire when it is pulled tight by forces acting at each end. The tensional force is directed along the rope or wire and pulls on the objects on either end of the wire. The normal force is exerted on an object in contact with another stable object. For example, the dinner table exerts an upward force on a glass at rest on the surface of the table. NUCLEAR FORCE Nuclear forces are very strong forces that hold the nucleus of an atom together. If nuclei of different atoms come close enough together, they can interact with one another and reactions between the nuclei can occur. Forms of Energy Energy is defined as the ability to do work. We have already stated that energy can’t be created or destroyed; it can only change form. Forms of energy include poten- tial energy and kinetic energy. Potential energy is stored energy. Kinetic energy is the energy associated with motion. Look at the following illustration. As the pendulum swings, the energy is con- verted from potential to kinetic, and back to potential. When the hanging weight is at one of the high points, the gravitational potential energy is at the maximum, and kinetic energy is at the minimum. At the low point, the kinetic energy is maximized, and gravitational potential energy is minimized. The change of potential energy into kinetic energy, and kinetic energy into potential energy, in a pendulum Examples of potential energy include nuclear energy and chemical energy—energy is stored in the bonds that hold atoms and molecules together. Heat, hydrodynamic energy, and electromagnetic waves are examples of kinetic energy—energy associated with the movement of molecules, water, and electrons or photons (increments of light). Interactions of Energy and Matter Energy in all its forms can interact with matter. For example, when heat energy interacts with molecules of water, it makes them move faster and boil. Waves— including sound and seismic waves, waves on water, and light waves—have energy and can transfer that energy when they interact with matter. Consider what happens if you are standing by the ocean and a big wave rolls in. Sometimes, the energy carried by the wave is large enough to knock you down. Waves Energy is also carried by electromagnetic waves or light waves. The energy of electromagnetic waves is related to their wavelengths. Electromagnetic waves include radio waves (the longest wavelength), microwaves, infrared radiation (radiant heat), visible light, ultraviolet radia- tion, X-rays, and gamma rays. The wavelength depends on the amount of energy the wave is carrying. Shorter wavelengths carry more energy. When a wave hits a smooth surface, such as a mirror, it is reflected. Sound waves are reflected as echoes. Mat- ter can also refract or bend waves. This is what happens when a ray of light traveling through air hits a water sur- face. A part of the wave is reflected, and a part is refracted into the water. Maximum Potential Energy Maximum Potential Energy Maximum Kinetic Energy – PHYSICAL SCIENCE– 230 Each kind of atom or molecule can gain or lose energy only in particular discrete amounts. When an atom gains energy, light at the wavelength associated with that energy is absorbed. When an atom loses energy, light at the wavelength associated with that energy is emitted. These wavelengths can be used to identify elements. Nuclear Reactions In a nuclear reaction, energy can be converted to matter and matter can be converted to energy. In such processes, energy and matter are conserved, according to Einstein’s formula E = mc 2 ,where E is the energy, m is the mass (matter), and c is the speed of light. A nuclear reaction is different from a chemical reaction because in a nuclear reaction, the particles in nuclei (protons and neutrons) interact, whereas in a chemical reaction, electrons are lost or gained by an atom. Two types of nuclear reactions are fusion and fission. Fusion is a nuclear process in which two light nuclei combine to form one heavier nucleus. A fusion reaction releases an amount of energy more than a million times greater than the energy released in a typical chemical reaction. This gain in energy is accompanied by a loss of mass. The sum of the masses of the two light nuclei is lower than the mass of the heavier nucleus produced. Nuclear fusion reactions are responsible for the energy output of the sun. Fission is a nuclear process in which a heavy nucleus splits into two lighter nuclei. Fission reaction was used in the first atomic bomb and is still used in nuclear power plants. Fission, like fusion, liberates a great amount of energy. The price for this energy is a loss in mass. A heavy nucleus that splits is heavier than the sum of the masses of the lighter nuclei that result. Key Concepts This chapter gave you a crash course in the basics of physical science. Here are the most important concepts to remember: ➧ All matter is composed of tiny particles called atoms. ➧ Atoms combine with other atoms to form molecules. ➧ In a chemical reaction, atoms in molecules rearrange to form other molecules. ➧ The three common states of matter are solid, liquid, and gas. ➧ The disorder in the universe is always increasing. ➧ Mass and energy can’t be created or destroyed. ➧ Energy can change form and can be trans- ferred in interactions with matter. – PHYSICAL SCIENCE– 231 . basic principles of heredity were developed by Gregor Mendel, who experimented with pea plants in the 19th century. He mathematically analyzed the inherited traits (such as color and size) of a large