Presentation on theme: "Law of Conservation of Energy"— Presentation transcript:
1 Law of Conservation of Energy The total amount of energy in the system remains constantIt can only be converted from one form to anotherBUTNo new energy can be createdIt can never be destroyed
2 Potential and Kinetic Energy Energy: is the ability to do work
3 Potential Energy The energy of position The amount of energy contained in an object at rest
4 Determining Potential Energy By its position and its weight (mass X gravity)PE = (mass)(gravity)(height) = mghwhere m is mass in kgg is the force of gravity = 9.8 m/s2h is the heightThe SI unit that represents potential energy is the Joule (J) (kg m2/s2).
5 Examine an example of potential energy A flower pot with a mass of 15 kg is sitting on a window sill 15 meters above the ground. Howmuch potential energy does the flower pot contain?PE = (mass)(gravity)(height)= (15 kg)(9.8 m/s2)(15 m)= 2205 kg m2/s2= 2205 J= 2.2 x 103J
6 Kinetic EnergySO….Once force is applied to an object, the object is set into motion.A moving object is said to contain kinetic energy or energy of motion.The amount is related to the mass of the object in motion and it’s velocity.
7 Calculating kinetic energy If we know the mass of an object and its velocity we can determine the amount of kinetic energy possessed by using the following formula:kinetic energy = 1/2 (mass of object)(velocity of object)2or KE = 1/2 mv2or KE = 0.5mv2The SI unit for kinetic energy is the Joule (J).A Joule is kg m2/s2
8 A bicycle with a mass of 14 kg traveling at a velocity of 3 A bicycle with a mass of 14 kg traveling at a velocity of 3.0 m/s east has how much kinetic energy?KE = 0.5mv2= 0.5(14 kg)(3.0 m/s)2= 0.5(14 kg)(9.0 m2/s2)= 63 kg m2/s2 = 63 J
12 Conversion of Potential to Kinetic Energy In this picture both kinds of energy are evident. Can you point them out?
13 The water at the top has potential energy When water falls to a lower level, the potential energy is converted to kinetic energy.
14 FORCESThe term force refers to the interaction of objects and their environment.All forces are exerted on one object by another object.Forces have both size and direction and are normally classified as “pushes or pulls”.All forces have both size and direction
15 Gravity – most familiar force Gravity is the basic force of attraction that is spread throughout the universe. Gravity pulls objects towards each other.Gravity on earth pulls you and all objects towards the earth.You must overcome gravity each time you lift something.Gravitational force on earth is 9.8m/s2Other forces –Buoyancy -FrictionElectricity -Pressure
16 Calculating ForceThe relationship between an object's mass m, its acceleration a, and the applied force FForce =(mass)(acceleration) or F = maThe SI units for force is the Newton (N)A Newton is equivalent to the units:N = kg x ms2
17 ExampleAn object with a mass of 15.0 kg is moving with an acceleration of 25.0 m/s2. What is the force acting on that object?F = ma= (15.0 kg) x 25.0m/s2) = 375 kg• m/s2= 375 N
18 Another Force - WeightWeight is a force applied to an object as a result of gravity.Weight = mass x (gravitational force)Fw = (m) (g)On earth, the force of gravity is nearly constant = 9.8 m/s2
20 Weight = (mass) (gravity) It is different depending on where the object is located and the amount of gravity acting on it.Weight is expressed in Newtons (N)Weight of an object can be determined by the following formulaWeight = (mass) (gravity)ORFw = (m)(g)
21 ExampleIf an object has a mass of 75 kg on earth, what is it’s weight?Fw = (m)(g) = (75 kg) x (9.8 m/s2)= 735 kg • m/s2= 735 N= 740 N
22 Re-Arrange the Formula Solve for weightFw = (m)(g)Solve for massm= Fw ÷ (g)Solve for gravityg= Fw ÷ (m)
23 How Energy Relates to Work Energy - the ability to do workWork - a measure of how productive an applied force is
24 WorkWork is the product of the force applied to an object time the distance through which the force actsEXAMPLES OF WORKLifting a bookPulling a cartPushing a door openSometimes there are easy ways and hard ways to do the same amount of work.
25 Work The formula for work is: Work = (force) (distance) or W = Fd The unit for work is the JouleJ = N * m = kg *m2s2It is important that you understand that all units used in the equation are in Kg, m and seconds. The problem will not be accurate (or correct) if the units are not in this form.
26 Example A book weighing 3.0N is lifted 5m. How much work is done? W = FdW = (3.0N) (5m)W = 15J
27 Rearrange for distance d = w ÷ F You need to rearrange the equation to get force.F = W ÷ dRearrange for distanced = w ÷ F
28 Using Simple Machines to do Work More Easy Devices that allow us to perform the same amount of work more easily.
29 Simple Machines Work in One of 3 Ways Can take the force exerted by the individual and redirect itCan turn a small effort or force into a larger force (mechanical advantage)Can magnify the distance that a force acts onMachines do not reduce the amount of work needed to perform a task, they reduce the effort needed from the user.
30 3 Kinds We Will ConsiderLeverInclined PlanePulley
31 The LeverIs a narrow beam that rotates around a single point called the fulcrumBy placing an object to be moved, called the load, at one point on the beam and by applying an effort at another point the object can be moved more easily
32 1st Class Lever Load Effort Fulcrum 1st class – where the fulcrum lies between the load and the effortFulcrumLoadEffort
38 Review of Levers 1st Class Lever Load – Fulcrum – Effort 2nd Class LeverFulcrum – Load – Effort3rd Class LeverFulcrum – Effort - Load
39 Formula for Levers Effort X distance from the fulcrum = weight X distance from the fulcrumThe ability of the lever to help perform work is dependent on the length of the lever and on the mass applied to the lever.Too heavy of a mass or too long of lever the lever will break.
40 How much mass can a lever handle? Apparatus for lab looks like this:
41 Inclined PlaneDevice designed to reduce the force needed to raise an object.For example, pushing a load up a ramp onto a platform requires less force than lifting the load onto the platform.Ramps and steps are forms of inclined planes.
42 ScrewScrew is an inclined plane wrapped in a spiral around a shaft.
43 Wedge Wedge is actually 2 inclined planes joined back-to-back The planes exert lateral forces to split the piece of wood
44 Remember!An inclined plane does not reduce the amount of work being done –It simple reduces the force necessary to complete that work by creating a mechanical advantage.
45 Pulleysis a wheel over which a rope or belt is passed for the purpose of transmitting energy and doing work.
46 PulleysReduce the effort to raise an object or it redirects the applied force, depending on the type of pulley.
48 Defining Velocity Kinetic energy was KE=1/2 (mass) (velocity)2Describes both the rate and direction of the motionIf an object speeds up or slows down in the given direction we say there is a change in velocity
49 VELOCITY AND SPEED Velocity is a measure of how fast an object is traveling in a certain direction.Example: A plane moving at 600mph to the north has a velocity.Important to realize that for you to use velocity, you must have a direction!Speed is a measure of how fast something is moving, but there is not a directional element to it.
50 VELOCITY AND SPEEDSpeed is a measure of how fast something is moving, but there is not a directional element to itIs the distance on object moves per timeSpeed = Distance X Time (S=D x T)If speed changes, so does the velocity
51 VELOCITY Velocity = distance ÷ time The units we use are m/s and d is distance.Rearranging the formulas for all possibilities:V= d/td = vtt = d/v
52 VELOCITY What is the velocity of a car that travels 100m in 2 hours? V = d/t m/2h = 50.0m/hA car travels 65.0m/h for 3.00 hours how far did it go?d = vt (65.0m/h) (3.00h) = 195m =How long would it take a car to travel 200 miles at a velocity of 70m/h?t = d/v t = 200m/70m/h t = 2.9h = 3hrMake sure you work your problems so that units cancel out.
53 ACCELERATION Acceleration is the change in velocity per unit of time. An example of this is when you travel in your car.Your velocity is not constant throughout the entire trip as you slow down and speed up as necessary.A positive acceleration means that you are speeding up and a negative acceleration means that you are slowing down.
54 ACCELERATION Acceleration has the formula: Acceleration = (Final Velocity) – (initial velocity)(Final time) – (Initial time)OR(time it takes to change velocity)A = vf – vi = ∆v ∆ means “change in”tf – ti ∆ tAcceleration has the units of (distance unit)/(time unit)Ex: m/s2 or mi/h2
55 ACCELERATION Example acceleration problems Calculate the acceleration of an object with:Initial Velocity : 0.0m/sFinal Velocity: 14m/sTime 4sA = 14m/s – 0m/s4sA = 3.5m/s2
56 ACCELERATIONA car stops from a velocity of 55m/s in 15 seconds. What is the cars acceleration? Is the car speeding up or slowing down?A = 0 – 55m/s m/s15 s sA = -3.7m/s2Car is slowing down