Energy "is an abstract concept invented by scientists in the nineteenth century to describe quantitatively a wide variety of natural phenomena.

Transcription

1 Energy Energy "is an abstract concept invented by scientists in the nineteenth century to describe quantitatively a wide variety of natural phenomena." David Rose What is energy? Energy makes changes; it does things for us. It moves cars along the road and boats over the water. It bakes a cake in an oven and keeps ice frozen in the freezer. It plays our favorite songs on the radio and lights our homes. Energy makes our bodies grow and allows our minds to think. Energy is a non material property capable of causing changes in matter. In fact, whenever something moves, heats, cools, grows, changes or produces light or sound, energy is involved. 1

2 Energy comes in different forms: electrical energy (battery) chemical energy (food) thermal energy (water heater) kinetic energy (moving train) nuclear energy (bomb) solar energy (sun) potential energy (slingshot) 2

3 Work and Energy The effect of doing work on an object is that you give it energy. Example When you lift a box of books onto a shelf, you give the box energy. If the box falls, it might exert a force on another object causing the object to be displaced. Energy: The ability to do work. Energy is a non material property capable of causing changes in matter. Energy and work have the same units, joules (J). Energy has no direction, and is therefore considered to be a scalar quantity. 3

4 The Story of Kinetic and Potential Energy Conservation of Energy 4

5 Forms of Energy Energy is found in different forms, such as light, heat, sound and motion. They can all be put into two categories: kinetic and potential. Kinetic Energy energy due to motion of waves, electrons, atoms, molecules, substances and objects Potential Energy energy of an object due to its state or position (stored energy) 5

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7 Kinetic Energy In order to change the energy of an object, work must be done Work is a transfer of energy Energy can be considered the ability to do work Kinetic Energy is energy of motion We can derive an equation to calculate the kinetic energy of an object Consider the case where a car with an initial velocity is being pushed horizontally along a table with a net force. Since there is a net force in the direction of motion, there is net work being done. 7

8 From our Kinematic Equations solve for d Substitute d into work net equation Simplify We can now define kinetic energy to be 8

9 Kinetic Energy The amount of kinetic energy an object has is dependent on the object's mass and its velocity. Equation KE Kinetic Energy (J) m mass (kg) v velocity (m/s) Note KE α m and KE α v 2 9

10 Change in Kinetic Energy Doing work on an object will change the object's velocity. If positive work is done, the object's velocity will increase force and displacement are in the same direction. If negative work is done, the object's velocity will decrease force and displacement are in opposite directions. If an object's velocity changes, its kinetic energy will also change. 10

11 Work-Energy Theorem The work done on an object is equal to the object's change in kinetic energy. NOTE Kinetic energy is sometimes defined as the amount of work needed to accelerate an object to a given velocity. 11

12 Example 1 A kg rugby ball is thrown with a horizontal speed of 15.0 m/s. a)what is its kinetic energy? b)what was the work done by the person throwing the ball? KE = 1/2 mv 2 12

13 Example 2 How much work is required to accelerate a 1250 kg car from 50.0 km/h to 95.0 km/h? 13

14 Check Your understanding A kg car is travelling at a speed of 50.0 km/h when the driver applies the brakes. The car comes to a stop after travelling 15 m. a) How much work was done by friction in stopping the car? b) What was the magnitude of the average frictional force applied to the car? c) If the same car were travelling at 100. km/h when the driver applied the brakes and the car experienced the same frictional force, how far would the car travel before coming to a stop? 14

15 Gravitational Potential Energy Potential energy is stored energy elastic, chemical, gravitational An object with gravitational potential energy has the ability to do work if it is dropped suppose that we are lifting an object vertically at a constant speed; we must exert an upward force equal to the force of gravity 15

16 Gravitational Potential Energy The gravitational potentital energy of an object depends on its vertical height and the object's mass. Equation PE = mgh where m represents the mass of an object (kg) g represents the magnitude of the acceleration of gravity (9.8 m/s 2 ) h represents the height of an object above a certain reference level (m), since there is no such thing as an absolute zero position Determine the gravitational potential energy of the ball at the positions shown in the diagrams. 16

17 The reference level is arbitrary. hole in floor Reference Level floor basement If an object is located above the designated reference level, it has positive gravitational energy. If an object is located at the designated reference level, it has zero gravitational potential energy. If the object is below the designated reference level, it has negative gravitational energy. 17

18 Example A 1.0 kg teddy rests on top of your 0.80 m tall bed. reference level a) What is the teddy's gravitational potential energy relative to the top of the bed? m = 1.0 kg h = 0 m PE = mgh PE = (1.0)(9.80)(0) = 0 J Teddy's gravitational potential energy relative to the top of the bed is 0 J b) If the teddy falls off the bed and lands on the floor, what is his change in gravitational potential energy relative to the top of the bed? m = 1.0 kg h = 0.80 m PE = mgh PE = (1.0)(9.80)( 0.80) = 7.84 J Teddy's change in gravitational potential energy relative to the top of the bed is 7.84 J 18

19 Example A cat is loaded into a cart and pulled at constant speed along an inclined plane. a) A force of 14.4 N is used to drag the loaded cart along the inclined plane for a distance of 0.90 m. How much work is done on the cart? F= 14.4 N d = 0.90m W= F d = (14.4)(0.90) = 13.0 J 13 J of work is done on the cart. b) If the mass of the loaded cart is 3.0 kg and it is pulled to a height of 0.45 m, what is the gravitational potential energy of the cart? m = 3.0kg h = 0.45 m PE = mgh = (3.0)(9.80)(0.45) = 1.4 J The gravitational potential energy of the cart is 13 J 19

20 NOTE The work done to move the cart up the inclined plane at constant speed is equal to the cart's gravitational potential energy at height h. W = ΔPE 20

21 Example 1 A 225 g book is sitting on a table that is 1.2 m above the floor. It must be lifted to a shelf that is 2.1 m above the floor. a) What is the book's final potential energy relative to the floor? b) What is the book's final potential energy relative to the table? c) How much work was required to lift the book to the shelf? 21

22 Roller Coasters 22

23 Mechanical Energy Mechanical energy is the energy possessed by an object due to its motion and/or position. Examples A moving camel has mechanical energy due to its motion. A barbell held above a weightlifter's head has mechanical energy due to its position above the ground. A moving cannonball has mechanical energy due its velocity and its vertical position above the ground. 23

24 Mechanical Energy The amount of mechanical energy, ME, is the sum of an object's potential energy and its kinetic energy. ME = PE + KE The diagram below shows the motion of a polar bear as he skis down a hill and makes one of his record setting jumps. NOTE The sum of PE and KE is the same at each position. 24

25 The key to understanding and using the constancy of energy is in selecting the system, the collection of objects we want to study. We need a closed, isolated system. If the system is closed, no objects can enter or leave it. If the system is isolated, no external forces can act on any object in the system. No work can be done on or by objects outside the system. 25

26 When the woman leaps from the burning building, the sum of her PE and KE remains constant at each successive position all the way down to the ground. 26

27 The Law of Conservation of Energy " Energy cannot be created or destroyed. It can be transformed from one form into another, but the total amount of energy never changes." The equation describing the conservation of energy is: 27

28 Example 2 An 1125 kg elevator is being lifted to a height of 12.5 m from rest. If the cable on the elevator exerts an upward force of 1.2 x 10 4 N, what is the final speed of the elevator? 28

29 Check Your understanding A 34.0 kg child cycles up a hill to a point that is 6.45 m above her starting point. Find a)the change in the child's gravitational potential energy. b) the amount of work done by the child against gravity. 29

30 Module Summary 30

31 Attachments Man Lifting Box man_lifting_cardboard_box_right_lg_clr.gif