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TAKS Objective 5 Energy and Energy Changes Physics is the study of energy and provides answers to our energy needs: • Mechanical energy to do work such as grind wheat or churn milk into cheese saves fossil fuels and electricity and can be produced by wind. • Using solar energy to produce heat or hot water also saves electricity. Energy Is defined as the ability to do work Energy has two types: Kinetic (Energy of Motion) and Potential (Stored Energy) Law of Conservation of Energy • Energy can change forms, but is never created nor destroyed • Loss in one form = gain in an another form 6 Simple Machines make work easier Wheel & Axle Wedge Lever Screw Inclined Pulley Plane Potential Energy 2 possibilities Gravitational PE - Object lifted to some height Elastic PE - A stretched or compressed object (spring or rubber band) Elastic Potential Energy 5 According to the data in the table, about how far can a spring be expected to stretch when a force of 3.92 N is applied? A 10.5 cm B 13.6 cm C 14.0 cm Although this question is about reading the D 17.3 cm table, it does show that a stretched spring can do work (force x distance). Look at the numbers, as Force increases by 0.98, the distance doubles, so the answer would be: Gravitational Potential Energy PE = mgh 41 What is the potential energy of the rock? m = 95 kg A 59,900 joules m/s2 g = 9.8joules B 64,600 h = 100 m C 93,100 joules 95 kg x 9.8 m/s2 x 100 D 121,600 joules =93,100 joules C Kinetic Energy KE = m v 2 2 Ex: The man is doing work, using kinetic energy. Weight= Mass (m) X gravity (g) • Weight Unit of mass = kg • gravity = 9.8 m/s/s • Unit of weight = Newton • 1 Newton= about ¼ pound On Earth gravity = 9.8 m/s/s For every second that an object falls it accelerates 9.8 m/s each second. A maximum speed is reached when the air resistance (friction) slowing the object increases to prevent further acceleration. This is called terminal velocity. Universal Law of Gravitation All objects in the universe attract each other by the force of gravity Gravity varies depending on two factors: 1) the mass of the object doing the pulling, and 2) the distance from the center of that 46 Which of these would cause the gravitational force between Earth and the sun to decrease? F An increase in the length of a day on Earth G An increase in the distance between Earth and the sun H An increase in the number of planets orbiting the sun J An increase in the masses of Earth and the sun What are the 2 factors that effect gravity? Distance and Mass; decrease distance, increases gravity. increase mass, increase gravity. . . . Energy Conversion • Changing from one type of energy to another is the basis of all machines, and life processes on earth. • Our main energy use, electricity is converted from mechanical energy, nuclear energy or chemical energy. • During conversions, some of the energy is converted to heat by friction so it is not a 100% efficient process. Motion can be described as • a change in an object’s position. • Average speed is the change of position (distance) of an object over time • If speed also has a direction it is called velocity. Definition of a Force • A Force is a push or a pull Velocity Graphs V = distance Pink, time • Velocity (v) is steeper the slope (rise slope! Velocity over run) of a 60 Distance (m) distance (d) vs. 40 Series1 time (t) graph Series2 20 Which line, pink or 0 blue has the greatest 1 3 5 7 9 11 13 15 velocity? Time (sec) 40 The diagram represents the total travel of a teacher on a Saturday. Which part of the trip is made at the greatest average speed? FQ How do we work this one? GR HS Calculate v = d/t for each segment. J T 4 The picture shows the position of a ball every 0.25 second on a photogram. Using a ruler, determine the velocity of the ball. Use the ruler on the side of the chart to measure from the front of ball 1 to the front of ball 2 and multiply by 4 (since the pictures are at ¼ seconds). OR front of ball 1 to the front of ball 3, and multiply by 2 since two pictures would be ½ a second. V = d/t Acceleration is a change in an object’s velocity over time • When an object’s speed changes over time it is accelerating (or decelerating) • A = vfinal – vinitial time • Units for acceleration m/s/s or m/s2 Acceleration Graphs • Acceleration (a) is the slope of a Acceleration velocity (v) vs. time Velocity ((m/s)(m) 60 (t) graph 40 • When plotted on a 20 0 distance vs. time 1 3 5 7 9 11 13 15 Time (sec) graph, acceleration is an exponential curve (J curve) Read the graph first, what is each segment telling you? In the first second? 41 According to this graph, what 4 During the nextwas the bicycle’s acceleration seconds, from 1 to 5 or 6 seconds? 6 and 10 between seconds? What about between 6- A seconds? 10 0.0 m/s2 Between 102and 12.5 B 0.65 m/s seconds? 2 C 1.6 m/s What about 12.5 to 15 D 6.5 m/s2 seconds? Balanced Forces • A force that produces no change in an object’s motion is balanced by an equal, opposite force. F=ma, the mass is 40kg and the acceleration is Remember to read diagrams first! What happened in the pictures? Now read 40kg 3.0m/s – 0.0m/s divided by 1.2s. So it is the x 2.5m/s/s or 100N question . . . 32 Starting from rest at the center of a skating rink, two skaters push off from each other over a time period of 1.2 s. What is the force of the push by the smaller skater? ANSWER F 16 N G 32 N H 88 N J 100 N Friction A force that acts to oppose motion. It occurs between any two surfaces in contact with each other, or as air resistance. Friction Friction causes an object to slow down and stop. Since the amount of energy stays constant, the energy becomes heat. To reduce friction oil or other lubricants are used. 44 A wet bar of soap slides 1 meter across a wet tile floor without appearing to slow down. Which of these statements explains why the bar of soap fails to slow down? F A constant force on an This is an example object produces a constant of how a lubricant positive acceleration. works, reducing G An object in motion tends friction so the to remain in motion in the inertia keeps the absence of an external soap moving. force. H A moving object having constant velocity contains kinetic energy. J An object’s weight is proportional to its mass. Momentum = Mass x Velocity • The larger the mass OR the faster an object moves, the more momentum it has. That means it will hit harder, or do more damage. • If a penny is thrown it probably won’t hurt very much, but if it is shot from a sling shot with a higher velocity, it will hurt a lot! 27 A ball moving at 30 m/s has a momentum of 15 kg·m/s. The mass of the ball is — A 45 kg B 15 kg C 2.0 kg Formula Page says that Momentum = Mass x Velocity D 0.5 kg So 15 kg.m/s = M x 30 m/s solving for M it is: Newton’s 1st Law of Motion • Objects in motion stay in motion • And objects at rest stay at rest • Until they are acted upon by an unbalanced force. Newton’s 2nd Law of Motion The greater the force applied to an object, the greater it will accelerate. Force = Mass X Acceleration or F=ma 20 How many Newtons of force does a 70.0 kg deer exert on the ground because of gravity? Record and bubble in your answer on the answer document. • This is an example of a weight problem since the weight is the force is due to gravity. • F = mg or = 70kg x 9.8m/s/s = 686 Newtons Example 2nd Law Problem 18 What is the net force exerted on a 90.0 kg race-car driver while the race car is accelerating from 0 to 44.7 m/s in 4.50 s? F 9.8 N F = ma, where G 20 N m= 90.0kg H 201 N a = (44.7 – 0)/ 4.5s J 894 N So, the answer is: Newton’s 3rd Law of Motion • For every action force there is an equal and opposite reaction force. 52 When the air is released from a balloon, the air moves in one direction, and the balloon moves in another direction. Which statement does this situation best illustrate? F What goes up must come down. G For every action there is an equal and opposite reaction. H The shape and size of an object affect air resistance. J The acceleration due to Earth’s gravity is 9.8 m/s2. Your turn!! 1. The typical automobile’s kinetic energy is converted to heat energy when the brakes are applied. A braking system that converts kinetic energy to electrical energy instead of heat energy has been designed. The electrical energy can be used later to move the car again. How does a system like this benefit the environment? F Greater use of solar energy G Ability to harness wind energy H Decreased use of fossil fuels J Use of renewable hydroelectricity 2. Assuming the chart contains all energy transformations in the Earth system, how much solar radiation goes toward evaporating water? F 133,410 terajoules G121,410 terajoules H 92,410 terajoules J 40,000 terajoules 3. Why is the sum of the products’ energy in this reaction less than the sum of the reactants’ energy? A Energy is trapped in the reactants. B The products absorb available energy. C Energy is given off as heat. D The reactants’ energy is less than the melting point of glucose 4. What is the approximate difference in gravitational potential energy of the two shaded boxes? 5.0 m A 79 J B 59 J 2.0 m C 39 J D 19 J 5. A catapult was designed to project a small metal ball at a target. The resulting data are shown in the table. Which of these might explain the difference between the calculated and actual distances? A The ball landed short of the calculated distance because of an increase in momentum. B Air resistance caused the ball to land short of the calculated distance. C Initial mass of the ball changed with each trial. D The metal ball was too small for accurate measurements to be made. 6. A 1-kilogram ball has a kinetic energy of 50 joules. The velocity of the ball is — F 50 m/s G 25 m/s H 10 m/s J 5 m/s 7. A 0.50 kg ball with a speed of 4.0 m/s strikes a stationary 1.0 kg target. If momentum is conserved, what is the total momentum of the ball and target after the collision? A 0.0 kgm/s B 0.5 kgm/s C 1.0 kgm/s D 2.0 kgm/s 8. The table contains data for two wrecking balls being used to demolish a building. What is the difference in momentum between the two wrecking balls? F 300 kgm/s G 200 kgm/s H 150 kgm/s J 0 kgm/s Car velocity = 5.5m/s Car velocity = 0m/s Car velocity = 0m/s Driver velocity = 5.5m/s Driver velocity = 5.5m/s Driver velocity = 0m/s Driver mass = 100kg Driver mass = 100 kg Driver mass = 100 kg 9. The pictures show how an air bag functions in a collision. How much momentum in kg m/s does the air bag absorb from the crash-test dummy if all the crash-test dummy’s momentum is absorbed by the air bag? Record and bubble in your answer to the nearest whole number on the answer document. (Just write it down) 10. The picture above shows the directions in which water leaves this scallop’s shell. Which picture below shows the direction the scallop will move? Check your answers. 1. H: Cars usually run on gasoline, so any heat energy saved, reduces use of fossil fuels. 2. J: Subtract each of the lines from the total energy, as Energy is Conserved. 3. C: This is the equation for cellular respiration and the “lost energy” is often used as body heat. 4. B: Calculate PE for each and subtract. 5. B: Air resistance is a form of friction which is where the rest of the energy dissipates as heat. 6. H: KE=½(mv2) so 50= ½(1 x v2) or 100 = v2. 7. D: use the formula page, mass x velocity 8. J: again, mass x velocity, they are both 300 kg m/s. 9. Momentum = mass x velocity or 5.5 x 100 or 550 kg m/s. 10. D: For each action an equal opposite reaction, since the force is out and down, the shell will move up.
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