(and Conservation of Energy)

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Philip Hill
7 years ago
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1 (and Conservation of Energy)

2 Energy What does the word energy mean to you? (Talk amongst yourselves) Just like always, the physicsdefinition is a little bit different, but we re going to hold off on it for a bit As it turns out, energy is another made-up quantity Energy is something that objects possess (just like momentum) Objects possess energy like people possess money! just as there are different types of money (credit card, bank account, cash, etc.), there are different types of energy as well.

off on it for a bit As it turns out, energy is another made-up quantity Energy is something that objects possess (just

3 Energy Symbol: [various it depends on the type!] Units: J (Joules) We ll discuss a bit later what these units are made of. Types of energy: Mechanical Chemical Thermal Electrical Elastic Nuclear and more! James Prescott Joule

4 Mechanical Energy The most important type of energy The only type that is split into two subtypes Kinetic energy Potential energy An object s mechanical energy is the sum of its potential energy and its kinetic energy Mechanical energy = KE + PE What do you think these subtypes mean? Think about the words kinetic and potential

is the sum of its potential energy and its kinetic energy Mechanical energy = KE

5 Kinetic Energy Energy of motion In order to have kinetic energy, an object must be moving! Symbol: KE The faster an object is moving, the more KE it has! Equation: KE = ½mv 2 Ex: Your little brother (m=34 kg) runs down the street at 3 m/s. How much KE does he have? KE= ½mv 2 = ½(34 kg)(3 m/s)² =153 J

Symbol: KE The faster an object is moving, the more KE it has!

6 Potential Energy Object has the potential to GET kinetic energy by falling In order to have potential energy, an object must be above the ground! Symbol: PE The higher up an object is, the more PE it has! Equation: PE = mgh Ex: Your little brother (m=34 kg) is dangling from the monkey bars, 1.5 m off the ground. How much PE does he have? PE= mgh= (34 kg)(9.81 m/s²)(1.5 m) =500 J 2

Symbol: PE The higher up an object is, the more PE it has!

7 Unit check KE=½mv 2 PE=mgh Based on these two equations, how is our unit for energy (J) built from the fundamental units kg, m, and s? J = kg m²/s² As compared to N = kg m/s² Momentum unit: kg m/s 3

8 How is this useful to us??? Two ways, right off the bat! As it turns out, as long as we ignore things like friction and air resistance (outside forces that pickpocket some energy away from an object), we can say that energy is conserved. MANY problems have ONLY the two types of mechanical energy potential and kinetic. Later, we ll also use energy and conservation of energy to solve elastic collision problems. 7

pickpocket some energy away from an object), we can say that energy is conserved.

9 Conservation of Energy When things happen within a system (internal forces), energy can transform from one form to another. All that s important is that the TOTAL amount of energy stays the same. Example: A block on a frictionless surface is held so that it compresses a spring. What s our system? The block and the spring Initially, all of the energy in the system is stored in the spring (elastic energy). When the spring is released, all the energy is converted into kinetic energy the block moves!

Example: A block on a frictionless surface is held so that it compresses a spring. What s our system?

10 LOL Diagrams

11 LOL Diagrams LOL Diagrams (AKA Energy Flow Diagrams ) show how energy is transformed within a system. Blank LOL diagram

12 LOL Diagrams Let s use the same example block spring Step 1: Define your system in the O

13 LOL Diagrams Let s use the same example block spring E el. KE Step 2: Graph ALL of the energy on the initialside.

14 LOL Diagrams Let s use the same example block spring E el. KE E el. KE Step 3: Graph ALL of the energy on the finalside.

15 Pendulum problem Conservation of mechanical energy also GUARANTEED that I wouldn t lose any teeth today!

16 Pendulum problem Let s say that position b represents h=0 How much potential energy does the bob have at position b? PE=mgh PE=0 J

17 Pendulum problem Imagine I m standing near position a. When I hold the pendulum bob at a, it is at its maximum height. How much kinetic energy does the bob have at position a? KE= ½mv 2 KE= 0 J

18 Pendulum problem pendulum earth PE KE PE KE

19 Pendulum problem I m guaranteed to be OK because when the bob swings back to position a, it CANNOT POSSIBLY swing higherthan it started! (In other words, it can t swing PAST position a, because that would be higherthan position a, and it would then have MORE potential energy than it started with Impossible!

(In other words, it can t swing PAST position a, because that would be

20 More LOL practice A car is driving and suddenly slams on the brakes, coming to a complete stop. car ground KE car Q Q KE car

21 More LOL practice Charges build up in a cloud; then, a bolt of lightning strikes. cloud, air, ground E electrical Q E light E sound E electrical Q E light E sound

22 More LOL practice An atom of Uranium-235 is hit by a neutron, resulting in a nuclear explosion. neutron U-235 atom air E nuclear KE n KE products Q E light E sound

Gravitational Potential Energy

Gravitational Potential Energy Consider a ball falling from a height of y 0 =h to the floor at height y=0. A net force of gravity has been acting on the ball as it drops. So the total work done on the