tailieunhanh - economic english 9

Tham khảo tài liệu 'economic english 9', ngoại ngữ, anh ngữ phổ thông phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | PHYSICAL SCIENCE Contact force Contact forces are forces that exist as a result of an interaction 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 potential 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 converted from potential to .