Transcript File
Chapter 3 466 Kinetic Energy Kinetic energy is the energy of motion. By definition, kinetic energy is given by: The equation shows that . . . • the more mass a body has • or the faster it’s moving . . . the more kinetic energy it’s got. Kinetic Energy Example A 55 kg toy sailboat is cruising at 3 m/s. What is its kinetic energy? Gravitational Potential Energy Objects high above the ground have energy by virtue of their height. This is potential energy. If allowed to fall, the energy of such an object can be converted into other forms like kinetic energy, heat, and sound. Gravitational potential energy is given by: The equation shows that . . . • the more mass (m)a body has • or the stronger the gravitational field (g) it’s in • or the higher (h) up it is . . . the more gravitational potential energy it’s got. Conservation of Energy One of the most important principles in all of science is conservation of energy. It is also known as the first law of thermodynamics. It states that energy can change forms, but it cannot be created or destroyed. This means that all the energy in a system before some before event must be accounted for afterwards. For example, suppose a mass is dropped from some m height. The gravitational potential energy it had originally is not destroyed. Rather it is converted into kinetic energy and some heat. heat The initial energy is given by Ep = mgh. The final energy is given by Ek = ½ mv 2 . after Therefore, the total mechanical energy is m v Work p. 90 Work is done when a force is applied to an object causing it to move in the same direction as the force. The simplest definition for the amount of work a force does on an object is magnitude of the force times the distance over which it’s applied: F D Work Example A 50 N horizontal force is applied to a 15 kg crate of granola bars over a distance of 10 m. The amount of work this force does is: Granola Bars 50 N 10 m When the force is at an angle When a force acts in a direction that is not in line with the displacement, only part of the force does work. The component of F that is parallel to the displacement does work, but the perpendicular component of F does zero work. So, a more general formula for work is F Granola Bars F cos x Force A force is an action that can change the motion of an object or deform the object by pushing or pulling on it. Unit of force: Types of Forces p.81 Gravitational, Electromagnetic, Nuclear Forces Earth’s Gravitational force Earth’s gravitational force is 9.8 2 m/s What are mass and weight? Mass and weight are not the same! Mass is the amount of matter in an object and is measured in kilograms. Mass is not a force and has the same value anywhere in the Universe, including outer space. Weight is a force and is caused by the pull of gravity acting on a mass. Like other forces, weight is measured in Newtons. Weight has different values depending on where the object is in the Universe. How do we calculate an objects weight? The weight of an object depends on its mass and the gravitational field strength: weight = mass x gravitational field strength The units for these quantities are as follows: The gravitational field strength depends on the force of gravity. On Earth it is . Calculating the weight of a car A car has a mass of 10,000 kg. What is the weight of the car? (Use 9.8 N/kg as the value of the gravitational field strength.) Calculating the weight of an astronaut An astronaut and his equipment have a mass of 150 kg. What is the weight when he is standing on the Moon?