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Energy Chapter 8 – Hewitt Review Questions 1. A force sets an object in motion. When the force is multiplied by the time of its application, we call the quantity impulse, which changes the momentum of the object. What do we call the quantity force x distance, and what quantity can it change? (8.1) Work It changes the object’s energy. 2. Work is required to lift a barbell. How many times more work is required to lift the barbell three times as high? (8.1) Three Work is equal to force multiplied by distance SO three times the distance (three times as high) requires three times as much work. 3. Which requires more work, lifting a 10-kg load a vertical distance of 2 m or lifting a 5-kg load a vertical distance of 4 m? (8.1) BOTH the SAME. F x d = 10kg(9.8-m/s2) x 2m = 196 J F x d = 5kg(9.8-m/s2) x 4m = 196 J 4. How many joules of work are done on an object when a force of 10-N pushes it a distance of 10-m? Given Find F= 10 N W d = 10 m Basic Equation(s) W= F x d Working Equation W= F x d Key Idea Solution Answer 10 N x 10 m= 100 J 5.(a) How much power is required to do 100-J of work on an object in a time of 0.5-s? Given W Find P Key Idea = 100 J t = 0.5 s Basic Equation(s) Solution Answer P = W/t 100 J / 0.5 s = 200 W Working Equation 5.(b) How much power is required to do 100-J of work on an object in a time of 1.0-s? (8.2) Given Find Key Idea W = 100 J P t = 1.0 s Basic Equation(s) Solution Answer P = W/t 100 J / 1.0 s = 100 W Working Equation 6. What are the two main forms of mechanical energy? (8.3) Potential Energy and Kinetic Energy. Potential Energy is the energy of position Kinetic Energy is the energy of motion 7.a. If you do 100-J of work to elevate a bucket of water, what is its gravitational potential energy relative to its starting position? (8.4) 100-J because the work you do goes into increasing the gravitational potential energy. Work-Energy Theorem W = Δ E, work done equals the change in 7.b. What would the gravitational potential energy be if the bucket were raised twice as high? (8.4) PE h Or d = mg , so twice the height means twice the energy. W = F , so twice the distance means twice the work means twice the energy. 200-J 8. A boulder is raised above the ground so that its potential energy relative to the ground is 200-J. Then it is dropped. What is its kinetic energy just before it hits the ground? (8.5) Conservation of energy say the total energy must remain the same. Since the boulder will lose 200-J of gravitational potential energy as it falls it will then gain 200-J of kinetic energy. 9. Suppose an automobile has 2000-J of kinetic energy. When it moves at twice the speed, what will be its kinetic energy? (8.5) = ½ mv2 If v = 2 v, then (2 v)2 = 4v Then KEfinal = 4 KEinitial What if the velocity is 3 times the original? KE 10. What will be the kinetic energy of an arrow having a potential energy of 50-J after it is shot from a bow? (8.6) 50-J Conservation of energy, energy is transferred not lost or gained. SO, the 50 J of potential energy stored in the arrow as it is pulled back (position), is immediately transferred to kinetic (motion) when the arrow is released. 11. What does it mean to say that in any system the “total energy score” stays the same? (8.6) It means Energy is Conserved. 12. In what sense is energy from coal actually solar energy? (8.6) Material that forms coal was produced by the sun’s energy. Law of Conservation of Energy Coal is formed from plant matter that accumulates at the bottom of a water source, sediment piles on top, pressure and time. 13. How does the amount of work done on an automobile by its engine relate to the energy content of the gasoline? (8.6) The work done is less than the energy in the gasoline. The difference is in the form of heat energy. Work put into the system is transformed into thermal energy (heat) 14. In what two ways can a machine alter an input force? A machine can change the magnitude or direction of a force. A machine cannot alter the amount of energy in a system. 15. In what way is a machine subject to the law of energy conservation? Is it possible for a machine to multiply energy or work input? No, a machine cannot multiply energy or work input. Energy in a machine is not gained or lost, only transferred or transformed. 16. What does it mean to say that a machine has a certain mechanical advantage? This means that a machine has the ability to multiply a force by a certain amount. 17. In which type of lever is the output force smaller than the input? Type 3 lever, always Type 1 lever sometimes Type 2 lever, never THIS QUESTION WILL NOT BE ON YOUR TEST! 18. What is the efficiency of a machine that requires 100 J of input energy to do 35 J of work? 35% Efficiency = useful work output divided by total work input 19. Distinguish between the theoretical mechanical advantage (tma) and actual mechanical advantage (ama). How would these compare if a machine were 100% efficient? TMA-no friction AMA with friction The values would be the same, because the machine is 100% efficient. THIS QUESTION IS NOT ON YOUR TEST! 20. What is the efficiency of her body when a cyclist expends 1000 Watts of power to deliver mechanical energy to the bicycle at the rate of 100 Watts? 10% Efficiency = actual mechanical advantage divided by the theoretical mechanical advantage. Efficiency = work output divided by work input