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FOR EVERYTHING THAT MOVES, THE MECHANICAL ENERGY (PE + KE) IS CONVERTED TO HEAT BY FRICTION. AFTER SOME TIME, ALL OF THE MECHANICAL ENERGY IS CONVERTED INTO THERMAL ENERGY AND ALL MOTION STOPS NUCLEAR FUSION – TWO HYDROGEN MOLEUCLES FUSE TOGETHER TO MAKE ONE HELIUM ATOM THE CALORIE IS USED TO MEASURE HOW MUCH ENERGY IS IN THE FOOD WE EAT. DIFFERENT TYPES OF FOOD HAVE DIFFERENT AMOUNTS OF CALORIES. EVERY GRAM OF FAT HAS 9 CALORIES. EVERY GRAM OF CARBOHYDRATES AND PROTIEN HAS ONLY 4 CALORIES. NO WONDER EATING A LOT OF FATTY FOODS WILL CAUSE US TO BE OVERWEIGHT. the Work-Energy Theorem If I do work on an object, the energy of the object will change. The change in the object’s energy is equal to the amount of work I did on it. the Work-Energy Theorem If I do work on an object, the energy of the object will change. If I lift a ball up and put it on a shelf, now the ball’s GPE is the same as the work I did as I lifted it up. the Work-Energy Theorem If I lift a ball up and put it on a shelf, now the ball’s GPE is the same as the work I did as I lifted it up. The change in the object’s energy is equal to the amount of work I did on it. KINETIC ENERGY IS THE ENERGY OF THE MOTION OF AN OBJECT.. KINETIC ENERGY = (MASS) x (VELOCITY)2 / 2 KE = m x 2 v /2 KINETIC ENERGY IS THE ENERGY OF THE MOTION OF AN OBJECT.. KINETIC ENERGY = (MASS) x (VELOCITY)2 / 2 WORK = (FORCE) (DISTANCE) IF I DO WORK ON AN OBJECT, THE CHANGE IN THE OBJECT’S KINETIC ENERGY WILL EQUAL THE AMOUNT OF WORK I DID. IF I DO WORK ON AN OBJECT, THE CHANGE IN THE OBJECT’S KINETIC ENERGY WILL EQUAL THE AMOUNT OF WORK I DID. For example – If I push on a book and increase it’s velocity from zero to 2 m/s, I increased the Kinetic Energy of the book. The work I did on the book is equal to the increase in it’s Kinetic Energy. IF AN OBJECT INITIALLY HAS 20 JOULES OF KINETIC ENERGY AND, AFTER YOU DO WORK ON THE OBJECT, IT THEN HAS 60 JOULES OF KINETIC ENERGY, THEN THE AMOUNT OF WORK YOU DID IS… 60 JOULES - 20 JOULES = 40 JOULES OF WORK W = KE2 – KE1 W = KE2 – KE1 Work = Force x Distance 2 KE = mass x (velocity) / 2 So F x d = (m x 2 v / 2)2 - (m x 2 v / 2)1 Gravitational Potential Energy An object that has been lifted up against the force of gravity has stored energy. This stored energy is called Gravitational Potential Energy GPE = (MASS) (9.8) (HEIGHT) Ug = m x g x h Choosing a reference level The choice of a reference level, at which the potential energy is defined to be zero, is arbitrary. The reference level may be taken as any position that is convenient for solving a given problem. Conservation of Energy The total amount of energy is constant. In other words, energy can be neither created nor destroyed. Energy is conserved. The total amount of energy remains the same. Conservation of Energy The total amount of energy remains the same. As an object falls, it loses height and thus loses Potential Energy. As an object falls, it goes faster and faster and thus gains Kinetic Energy. As it falls, the Potential Energy is changed into Kinetic energy. Conservation of Energy The total amount of energy remains the same. When an object rises, just the reverse is true… As an object rises, the Kinetic Energy is changed into Potential energy. The total amount of energy remains the same. As it falls, the Potential Energy is changed into Kinetic energy. An any time during the fall, if you add together the PE + KE you will get the same amount of Total Energy. We call this total amount of energy – Mechanical Energy. The total amount of energy remains the same. We call this total amount of energy – Mechanical Energy. ME = KE + PE And ME will always be the same (constant) at any point in the fall or rise of an object Conservation of mechanical energy We will be concerned only with kinetic and gravitational potential energy while investigating motion in this course. The sum of these forms of energy is referred to as mechanical energy. In any given system, if no other forms of energy are present… Conservation of Energy As long as the system under investigation is closed so that objects do not move in and out, and as long as the system is isolated from external forces, then energy can only change form. The total amount of energy is constant. In other words, energy can be neither created nor destroyed. In a closed, isolated system, energy is conserved. The total amount of energy remains the same. The path an object follows in reaching the ground does not affect the final kinetic energy of the object. Ug= mgh Ug =(10.0 N)(2.00m)= 20.0 J Total Energy = 20J Half way down... Ug= (10.0 N)(1.00 m)= 10.0 J E= K+ Ug, so K= E- Ug K= 20.0 J- 10.0 J =10.0 J