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 • Measured in kilograms (kg) • Weight = vector quantity • 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 • What are the units of gravitational potential energy? Ep(grav) = W = Fgd 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 • Ex. Elastics, trampoline • Energy stored in elastic when force used to stretch elastic (i.e. work done on the elastic or spring) • 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