Physics 504 Chapter 16 Energy
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The document discusses different types of energy including kinetic, potential, elastic, chemical, and nuclear energy. It provides formulas for calculating kinetic energy as 1/2mv^2 and potential energy as mgh. The document also explains how the total energy of a system is conserved as energy is transferred between potential and kinetic forms.
Physics 504 Chapter 16 Energy
- 1. Mechanical Energy Chapter 16/17
- 2. Kinds of Energy • Gravitational Potential Energy: due to position • Kinetic Energy: due to motion • Heat Energy: due to movement of heat energy from regions of high energy to areas of low energy. • Radiant Energy: due to light • Chemical Potential Energy: due to bonds • Elastic Energy: stressed objects that return to their original shape. • Electrical Energy: due to movement of electrons • Nuclear Energy: due to atomic fission/fusion
- 3. Kinetic Energy • Kinetic Energy is the energy an object has due to its motion. • Kinetic energy is the energy of a moving object. • The KE depends on the mass and the speed. • Ek, KE= ½ mv2, unit is Joule, J • m is mass in kg, v is velocity in m/s
- 4. Activity • What is the KE of a 6 kg curling stone moving at 4 m/s? • KE = ½ mv2 • = ½ x 6kg x (4 m/s)2 • = ½ x 6 x 16 • = 48 J • Do page 341, Q. 1- 4
- 5. Exam Question The kinetic energy of an object depends on several factors. Which one of the following graphs represents the change in kinetic energy of an object as a function of its speed? A) C) Kinetic Kinetic energy energy 0 Speed 0 Speed B) D) Kinetic Kinetic energy energy 0 Speed 0 Speed
- 6. Exam Question An car, travelling along a horizontal road, has kinetic energy of 1.6 × 106J. The driver slows the car to half of its original speed. What is the new kinetic energy of the car? A) 1.6 × 106 J B) 8.0 × 105 J C) 4.0 × 105 J D) 1.6 × 105 J
- 7. Activity • Page 341, Q 1 - 6
- 8. Potential Energy • If we put work into an object (W=FΔd) by lifting it up against gravity, it now has the ability to move; it has the potential to fall down and use up the energy we put into it. • W = FΔd = mgd • Ep , PE = mgh, unit is Joules, J • m is the mass in kg • g is the acceleration due to gravity • h is the height above the Earth’s surface, m.
- 9. Activity • What is the PE of a 10 kg weight, 8 m above the ground? • PE = mgh = 10 kg x 9.81m/s2 x 8m • = 784.8 J • Page 349, Q. 1-4
- 10. Exam Question A weather balloon with a mass of 4.0 kg, including the weather instruments, rises vertically in the air. It passes an altitude of 200 metres at a velocity of 2.0 m/s. 2.0 m/s 200 m At this point what is its potential energy with respect to the ground? A) 8.0 × 103 J B) 8.0 × 102 J C) 8.0 × 101 J D) 8.0 J
- 11. Total Energy • The energy of a system transfers between Potential Energy and Kinetic Energy. • Total Energy = PE + KE • The PE of an object gets transferred to KE as it speeds up. • As the PE decreases, the KE increases.
- 12. Total Energy • What is the speed of a 500g rock that drops from a height of 78.4 m, just before it hits the ground? • ET = KE + PE, at first, v = 0 m/s • = ½ mv2 + mgh, since v = 0, KE = 0 • = 0.5kgx9.81m/s2x78.4m, ET = PE only • = 384.6 J • As the rock approaches the ground all its PE is transferred to KE, so PE = 0. So…
- 13. Total Energy, Part Deux • ET = PE + KE • 384.6 J = KE • 384.6 = ½ mv2 • 384.6 = 1/2x 0.5kg x v2 • 1538.4 J = v2 • v = 39.2 m/s • So just before it hits the ground, the rock has a speed of 39.2 m/s
- 14. Another way to solve it • Same situation, just look at the speed and distance. • We could use v2 = u2 + 2as • = 02 + 2(9.81)(78.4) • = 1538 • So, v = 39.2 m/s • Both methods work, depending on the information given. • Page 353, Q. 2-5 • Page 356, Q. 6
- 15. Exam Question A small airplane with a mass of 1000 kg, is flying at 60 m/s at an altitude of 250 m. 60 m/s 250 m What is the total mechanical energy of this airplane with respect to the ground? A) 1.8 × 106 J B) 2.5 × 106 J C) 4.3 × 106 J D) 6.1 × 106 J
- 16. Exam Question A golf ball is dropped out of a window which is 10 m above the ground. The ball has a mass of 50 g. Disregard the effects of air resistance. 10 m What is the kinetic energy of the ball just before it hits the ground? A) 10 J B) 7.5 J C) 5.0 J D) 2.5 J
- 17. Exam Question A stone with a mass of 100 g is thrown horizontally from the top of a cliff overlooking the ocean with a velocity of 20 m/s. Disregard the effects of air resistance. 20 m/s 15 m What is the kinetic energy of the stone just before it hits the water? A) 15 J B) 20 J C) 30 J D) 35 J
- 18. Exam Question A 100 g ball is thrown vertically upward from the ground with a velocity of 20 m/s. Disregard the effects of air resistance. What is the kinetic energy of this ball after it has risen 5.0 metres? A) 20 J B) 15 J C) 10 J D) 5.0 J
- 19. Summary • Energy is the ability to do work. • Work is the transfer of energy. (W=ΔE) • Friction often does negative work on an object because it removes energy from it. • Gravitational Potential Energy is the energy of an object due to its height above the Earth’s surface. PE = mgh • Kinetic Energy is the energy of a moving object. KE = ½ mv2
- 20. Summary • The Law of Conservation of Energy states that in any transfer or transformation of energy, the total amount of energy remains the same. • The form of the energy may be changed, e.g. noise, heat, vibration, friction. • In situations where friction and air resistance are small enough to be ignored, and where no other energy is added to the system, the total mechanical energy is conserved.
- 21. Summary • E total = KE + PE (before) = KE + PE (after) • Heat is the measure of the amount of thermal energy that flows from one body to another because of a difference in temperature. • Work done on an object can cause an increase in the temperature of an object.
- 22. Exam Question The starter motor of a car is not working. One person stays in the car while the other pushes it from behind to get it up to a certain speed so that it can be started. The car and the person inside it have a total mass of 1.00 × 103 kg. Disregard the effects of friction. The car is at a complete stop. The person who pushes it exerts a force of 2.00 × 102 newtons for 15.0 seconds. How much kinetic energy does the car gain as a result of the push? A) 1.80 × 104 J B) 1.35 × 104 J C) 9.00 × 103 J D) 4.50 × 103 J
- 23. Activity • Page 361, Q. 1-7
- 24. Elastic Potential Energy • We know Hooke’s Law – F = kx – k – spring constant, N/m – x – elongation of the spring, m • The energy stored in a spring. • E PE = ½ k (x)2
- 25. Activity • The length of a compressed spring, unextended, is compressed by 5 cm, using a force of 20 N. Calculate the energy stored in the spring. • Using F = kx • 20 = k (0.05 m) • k = 400 N/m
- 26. Continued • E PE = ½ k (x)2 • = ½ (400) (0.05) 2 • = 0.5 J • Do Page 373, Q 1-3.