Chapter 2 Review Answer Key

Transcription

1 Chapter Review Answer Key Select the correct term to complete the sentences. Section.. force. Newton s first law. inertia. net force. newton Section. 6. acceleration 7. Newton s second law Section. 8. free fall 9. acceleration due to gravity. velocity. weight. terminal speed Section.. slope Reviewing Concepts Section.. A force is a push or a pull or any action that has the ability to change motion. Three examples of forces are gravity, friction, and air resistance.. Examples will vary. In order for a soccer ball to move, you must apply a force to it. There can be no change without the presence of a force.. Newton s first law states that objects tend to keep doing what they are doing, or that objects resist changes in their motion. Inertia is an object s ability to resist changes in its state of motion and is an important part of Newton s first law.. Units for measuring mass include: kilograms (kg), grams (g), atomic mass unit (amu) Units for measuring force include: pounds (lb) and newtons (N). Note that pounds and newtons are measures of force or weight, not mass.. force; speed; kg; m/sec Section. 6. Its speed changes by centimeters per second each second. 7. m/sec sec; m/sec 8. An object can accelerate if its direction changes. Even though an object traveling in a circle may be going a constant speed, it is accelerating because its direction is changing. 9. Answers are: a. F = m a b. a = F m c. m = F a. Zero. The car is not accelerating; its speed is not changing.. Examples will vary. If you try to throw a golf ball and a bowling ball using the same force, the golf ball will have a much greater acceleration than the bowling ball since it has a much smaller mass.. A newton is the force required to accelerate a -kilogram mass at a rate of meter per second per second ( N = kg-m/sec ). Section m/sec each second.. decreases; 9.8 m/sec; increases; 9.8 m/sec. Velocity is speed with direction. 6. An object cannot have negative speed, but it can have negative velocity. The negative sign indicates direction. 7. Yes, an object can have a speed of zero while it has a nonzero acceleration. For example, when you toss a ball up and then catch it, its acceleration at every point is 9.8 m/sec, but at the very top of its path, it is at rest with a speed of zero for an instant. 8. The rock s mass is the sam, since mass is a measure of the amount of matter an object contains. The rock s weight is different since the acceleration due to gravity on the moon is different from Earth. 9. straight up 66

2 CHAPTER REVIEW ANSWER KEY. The force of gravity and the force of air resistance must be the same. Section.. Divide the vertical change rise by the horizontal change run.. speed. Sam is accelerating by speeding up. Because the slope of a curving line is not constant, it indicates that the speed is not constant; it is changing. Because the line curves up, it indicates that the speed is increasing.. By finding the area under the line.. acceleration 6. Speed (m/sec) Solving Problems 6 Section.. First, convert all the masses to kilograms: a.. kg b., g, grams per kg = kg c. kg d. g, grams per kg =. kg g <. kg < g < kg d, a, b, c. Answers are: a. N + N = N net force b. N - N = N net force Section.. acceleration = change in speed change in time acceleration = ( - m/sec) ( sec) = m/sec. Answers are: a. change in speed = m/sec - m/sec = m/sec b. acceleration = change in speed change in time acceleration = ( m/sec) ( sec) =. m/sec. acceleration = change in speed divided by change in time acceleration = ( - km/h) sec = -7. km/h/sec His acceleration is negative since he is slowing down. 6. Answers are: a. m/sec sec = m/sec b. m/sec sec = m/sec c. m/sec sec = 6 m/sec d. m/sec sec = m/sec 7. Answers are: a. force = mass acceleration =, kg m/sec =, N b. mass = force acceleration = N m/sec = kg c. acceleration = force mass = 98 N kg = 9.8 m/sec Section. 8. Answers are: a. Time (sec) Speed (m/sec) b. v avg = (v i + v f ) = ( m/sec) =.9 m/sec c. v avg = (v i + v f ) = ( + 9. m/sec) = 9.6 m/se d. distance = d = v avg t = (9.6 m/sec)( sec) = 78. meters 67

3 9. Answers are: a. v f = gt = (9.8 m/sec )( sec) = 9. m/sec b. v avg = (v i + v f ) = ( + 9. sec) =.7 m/sec c. d = v avg t = (.7 m/sec)( sec) =. m. Answers are: a. F g = mg F g = ( kg)(9.8 N/kg) = 9 N b. F g = ( kg)(.6 N/kg) = 8 N c. F g = ( kg)(9.8 N/kg) = N d. Aya s mass is constant at kg no matter where she is. e. F g = ( kg)(.6 N/kg) = 7 N Section.. Slowest: Fastest.. speed = rise run = (m - m) sec =. m/sec Since an object in free fall is accelerating due to gravity, the speed is changing at each point and slope is a measure of speed. A curved line indicates a changing slope Speed (m/sec) Speed (meters/second) 6 Speed (meters/second)

4 CHAPTER REVIEW ANSWER KEY Applying Your Knowledge Section.. Answers will vary. Aristotle believed that objects at rest would stay at rest, which proved to be correct. However, he also theorized that objects in motion only remain in motion if a force constantly acts on them, which was later proven to be wrong. Galileo said that an object in motion possessed an inertia that caused it to remain in motion unless a force acted on it. This law of inertia became the basis for Newton s first law. Newton s second law defines a force further with his formula relating force to mass and acceleration. Newton built on Galileo s theories to develop his three laws of motion, which form the basis for classical physics. Encourage your students to notice how each of these great scientists was able to make the discoveries that he did because he built upon the previous work of other scientists. All scientists learn from others work and build upon others knowledge. Even though theories may be proven incorrect, important knowledge can still be learned and developed into correct explanations.. Answers will vary. For example: Running hurdles on the track team. Newton s first law: In order to jump over a hurdle and change the direction of your motion, a force must be applied to your body. Newton s second law: When you are in the air jumping over a hurdle, the force of gravity is pushing your body back to the ground. Newton s third law: When your feet push off the ground to jump over a hurdle, the ground pushes back on your feet with an equal and opposite reaction. Section.. Answers will vary. Examples include: time (sec) for acceleration Car model Mass (kg) -6 mph (mph/sec) Jaguar XJR 8.. Maserati Quattroporte 9.. Infinity G 6..9 Volvo Ford Mustang Cobra R Ferrari Enzo Lexus SC Dodge Neon SRT Toyota Prius You might think that the more mass the car has, the slower the acceleration. This would be true if all the car engines had the same horsepower, but the engine powers vary widely across the examples.. Answers will vary. Most races results are a time to complete an entire course, so students may take a race distance and calculate the acceleration as constant over the entire distance of the race. A more accurate approach would be to have splits for the race which would show the initial acceleration up to maximum speed. For example: At the World Championships in Athens, Greece in 997, Maurice Green ran the first meters of the -meter dash in.7 seconds, going from -8.7 m/sec in the first meters for an acceleration of.9 m/sec. Quarter horses can reach a speed of m/sec in just - strides, which is an acceleration rate of approximately 6 m/sec. The cheetah can accelerate from - m/sec in seconds, at an acceleration of m/sec. Section.. When an object is at terminal speed, its velocity is constant. According to Newton s first law, an object in motion will continue to move at the same speed in the same direction unless a force is applied to it. The object continues to move at terminal speed until a force changes its motion, for example, hitting the ground to stop. According to Newton s second law, if there is zero acceleration, there is zero force on the object. The object at terminal speed has constant velocity, and therefore zero acceleration and zero net force on it. 6. Examples will vary. If there was no such thing as air resistance, snow and leaves would not fall gently to the ground, but would come careening out the sky at the acceleration due to gravity, and would continue accelerating until they hit the ground. Parachutes would not slow down skydivers. Birds would have a difficult time without being able to swoop in against air resistance to gracefully land. 69

5 Section. 7. As Joseph starts to ride his bike, he accelerates at a constant m/sec from rest to final speed of m/sec. a. v = at Time (sec) Speed (m/sec) Speed (meters) b. d = vt. Time (sec) Position (m)