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Phys 3550 How Things Work Fall 2007 1st Midterm (A) – Tuesday October 23rd 2007 Concepts and Definitions 8. A car is traveling east. What would happen if 1. What is Newton’s 3rd Law of Motion? - For 1. there is a (net) force to the east? every action there is an equal an opposite 2. there is a (net) force to the west? reaction 3. there is a (net) force to the north? 2. What is the Principle of Conservation of In each case, does the change in kinetic Energy? - Energy cannot be created or energy of the car increase, decrease or stay destroyed the same? 1. Since the force is in the same direction 3. What is the term given to the energy as the motion the car speeds up. KE associated with vertical position? - increases gravitational potential energy 2. Since the force is in the opposite direction to the motion the car slows 4. When is it possible to say that the velocity down. KE decreases of a car is 30 mph? - Never. 30 mph is the 3. This the force is perpendicular to the speed. To get velocity you would also need motion. There is no change in speed, but direction the car will steer to the north (change of direction. The KE is unchanged. 5. Can an object have a constant speed and also have an acceleration? Under what 9. If you were told that a car has traveled 16 conditions might this occur? - Yes, when miles in ¾ hour, which quantity would you changing direction be able to deduce? 1. Its average acceleration 6. Imagine that you have stone tied to a 2. Its average speed string, and you are swinging the stone 3. Its average velocity around your head in a horizontal circle. 4. Its displacement (change in position) 1. Is the stone accelerating? Why? - Yes, because the velocity (direction) is This is really a definition question. You are constantly changing told a speed (a scalar) and time (a scalar). 2. Is there a force on the stone. If so You have no directional information at all. where does the force come from? - From that you can only calculate another From Newton's 2nd Law, there must be a scalar, and so the only option is 2) average force F = ma speed. The other three are vector 3. What would be the path of the stone if quantities, and need direction as well. you let go of the string? - Sideways, tangential to the circle 10. List all the circumstances when an object might experience an acceleration. 7. According to Newton’s Laws, what happens 1. speeding up to a moving object if all forces acting on it 2. slowing down are removed? - Continues to move in a 3. changing direction straight line at constant speed Phys 3550 How Things Work Fall 2007 1st Midterm (A) – Tuesday October 23rd 2007 11. Fill in the blanks also smaller. 1. In stopping a car kinetic energy is converted to heat by the brakes. Numerical Pr oblems 2. When water falls over a waterfall GPE energy is converted to kinetic energy 14. If a 2000 kg car has a KE of 25 kJ what is its and then to heat. speed? Can you say what its velocity is? 3. In a coal or oil fired power station 1. KE = ½ mv² Chemical energy is ultimately 2. 25,000 = ½ 2000 v² converted to electrical energy. 3. v² = 2 * 25,000 / 2000 = 25 4. When you lift a heavy suitcase 4. v = 5 m/s chemical energy is converted to GPE. 5. Cannot tell velocity without direction. 5. If you drive a car around a bend at constant speed, Kinetic energy is not 15. A man of mass 60 kg runs up a hill. If the converted to anything, there is no work top of the hill is 15 m above the ground, done. how much work does he do? What is the power which he needs if it takes him 40 12. If a car goes around a corner at constant seconds to run up the hill? speed, does either the momentum or the 1. W = GPE gain = mgh kinetic energy change? 2. W = 60 * 9.8 * 15 = 8820 J 1. Neither change 3. P = W/t = 220.5 W 2. The momentum is a constant but the kinetic energy changes 16. A 4 kg weight hangs from a string. You pull 3. The kinetic energy is a constant but the on the string so that the mass moves momentum changes upwards with a constant deceleration of 6 4. Both change m/s². What is the tension in the string? 1. Net force = m a = 4 * 6 = 24 N KE depends only on mass and speed. It does 2. Since the weight is moving up and not change. On the other hand momentum decelerating, the force must be depends on velocity and speed, and since downwards direction is change then so is momentum 3. W = mg = 4 * 9.8 = 39.2 N 4. Tnet = F – W 13. The most serious accidents for car 5. F = 39.2 – 24 = 15.2 N passengers who are not wearing a seat belt occurs when the passenger strikes the 17. A car has a mass of 1500 kg. The engine front windshield. Why does this occur, and generates a force of 600 N. Calculate what physical principle explains why an 1. The acceleration of the car. - a = F/m = air bag helps avoid injuries in these 600 N / 1500 kg = 0.4 m/s² collisions? 2. The change in velocity in a time of 6 s. - Δv = a t = 0.4 m/s² * 6 s = 2.4 m/s The air bag slows you down gently, taking a 3. If it starts this time interval moving at substantial time to do so. That means to for 6 m/s, what is its final speed? - vf = vi + a given change in velocity the acceleration Δv = 6 m/s + 2.4 m/s = 8.4 m/s is smaller, and so the forces involved are 4. Sketch the speed as a function of time. Phys 3550 How Things Work Fall 2007 1st Midterm (A) – Tuesday October 23rd 2007 5. How far does it move in this 6 s interval? 1. The sketch is a rectangle and a triangle 2. area of rectangle = 6 * 6 = 36 m 3. area of triangle = ½ * 6 * 2.4 = 7.2 m 4. total distance = 36 + 7.2 = 43.2 m 18. A 60,000 kg rocket is propelled forward with an acceleration of 4 m/s². 1. What is the force on the rocket? - F = ma = 60,000 * 4 = 240,000 N 2. if the rocket moves a distance of 60 km, how much work is done? - W = Fd = 240,000 N * 60,000 m = 1.44 x 1010 J 3. if the rocket starts from rest, what is the final speed of the rocket? 1. KE = ½ mv² 2. 1.44 x 1010 = ½ 60,000 v² 3. v² = 2 * 1.44 x 1010 / 60,000 = 480,000 4. v = 693 m/s

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the force, kinetic energy, section 2, Class Schedule, How Things Work, energy change, Tuesday October, Fall 2007, final speed, Need to Know

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posted: | 10/24/2010 |

language: | English |

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