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Potential and Kinetic Energy Section 2.2 and 2.3 Review What types of energy did we read about last class? What is gravitational potential energy? What is kinetic energy? Objectives distinguish between types of potential energy relate gravitational potential energy to work done calculate gravitational and elastic potential energy calculate kinetic energy Potential Energy What is potential energy? Energy based on position Has potential to do work Gravitational potential energy Energy stored based on position above the Earth Mass versus Weight What is the difference between mass and weight? Mass = scalar quantity Weight = vector quantity Measured in kilograms (kg) Measure of gravitational attraction on an object – a force Measured in Newtons (N) Fg = mg g = acceleration due to gravity (9.81 m/s2) Which one does not change? If I was on the moon (given moon’s g is 1/6 that of earth), would my mass change or my weight? Gravitational Potential Energy and Work Gravitational potential energy = work done to lift object to vertical height Ep(grav) = W = Fgd What are the units of gravitational potential energy? Ep(grav) = mgh m= mass (kg) g= acceleration due to gravity (9.81 m/s2) h= height above ground (m) Example A 3.00 kg box is lifted by an upward force 1.50 m above the surface of Earth to the top of a table. What is the potential energy stored in the box at its new position? Answer: 44.1 J Practice A child with a mass of 25.0 kg is at the top of a slide in an amusement park. If the vertical height of the slide is 4.00 m, calculate the gravitational potential energy of the child relative to the ground The Tower of Terror in Disneyland drops its “victims” 13 stories (3 m/story). What would be the potential energy of a elevator car of 5 people who have a total mass of 116 kg? Practice A 25.0 kg trick-or-treater has run away from a group of older kids looking to steal her candy. She climbs up a tree to escape and is measured to have 2.6 kJ of gravitational potential energy. How high is she above ground? Peter Pan is hovering 3.00 m above the ground and has 1.47 x 103 J of gravitational potential energy. Calculate his mass. Elastic Potential Energy Type of energy stored in a stretched or compressed spring Energy stored in elastic when force used to stretch elastic (i.e. work done on the elastic or spring) Ex. Elastics, trampoline Stored as potential energy How do we calculate? Do we use gravity? What other relationship can we use? (hint we saw it earlier this class) Practice Robin Hood pulls back his bow to shoot an arrow at King John’s men. He uses a force of 45 N to pull the string back 64 cm. Calculate the elastic potential energy in the bow string. Chemical Potential Energy Store energy in chemicals as potential energy Stored in bonds Energy is released through chemical reactions Ex. Potential energy of fossil fuels allow us to drive our cars! Question When is potential energy useful to us? Only useful when it is converted to something else! Ex. Potential energy in a battery useful when converted to electrical energy Ex. Gravitational potential energy of a diver useful when start to dive Kinetic Energy What is kinetic energy? Energy of movement Ek = ½ mv2 Ek = kinetic energy m = mass (kg) v = velocity (m/s) Rearranging the formula… In pairs, to be marked, please solve for the following quantities: Mass velocity Practice A purple-people eater is running after a group of Science 10 students. If its mass is 65.0 kg and it is running at a velocity of 8.40 m/s, what is its kinetic energy? Dracula is flying through the night to his castle in Transylvania at a uniform speed of 5.6 m/s and has a kinetic energy of 25 J. Calculate his mass in bat form. Calculate the velocity of Shrek as he runs away from villagers with pitchforks if his mass is 75.0 kg and his kinetic energy is 2.78 kJ. Practice Homework P. 178 #2, 6, 8 P. 182 #4, 6, 7