Solved: Football players measure their acceleration by

Problem 1P When water freezes to make a snowflake crystal, the entropy of the water A. Decreases. B. Increases. C. Does not change.

Problem 2P When thermal energy is transferred from a hot object to a cold object, the overall entropy A. Decreases. B. Increases. C. Does not change.

Problem 3P Do convection cells represent a reversible process? A. Yes, because they are orderly. B. No, because they transfer thermal energy from hot to cold. C. It depends on the type of convection cell.

Problem 4P In an isolated system far from thermal equilibrium, as time passes, A. The total energy stays the same; the total entropy stays the same. B. The total energy decreases; the total entropy increases. C. The total energy stays the same; the total entropy increases. D. The total energy decreases; the total entropy stays the same.

Problem 5P Big Air A new generation of pogo sticks lets a rider bounce more than 2 meters off the ground by using elastic bands to store energy. When the pogo’s plunger hits the ground, the elastic bands stretch as the pogo and rider come to rest. At the low point of the bounce, the stretched bands start to contract, pushing out the plunger and launching the rider into the air. For a total mass of 80kg (rider plus pogo), a stretch of 0.40 m launches a rider 2.0 m above the starting point. If you were to jump to the ground from a height of 2 meters, you’d likely injure yourself. But a pogo rider can do this repeatedly, bounce after bounce. How does the pogo stick make this possible? A. The elastic bands absorb the energy of the bounce, keeping it from hurting the rider. B. The elastic bands warm up as the rider bounces, absorbing dangerous thermal energy. C. The elastic bands simply convert the rider’s kinetic energy to potential energy. D. The elastic bands let the rider come to rest over a longer time, meaning less force.

Problem 6P Big Air A new generation of pogo sticks lets a rider bounce more than 2 meters off the ground by using elastic bands to store energy. When the pogo’s plunger hits the ground, the elastic bands stretch as the pogo and rider come to rest. At the low point of the bounce, the stretched bands start to contract, pushing out the plunger and launching the rider into the air. For a total mass of 80kg (rider plus pogo), a stretch of 0.40 m launches a rider 2.0 m above the starting point. Assuming that the elastic bands stretch and store energy like a spring, how high would the 80 kg pogo and rider go for a stretch of 0.20 m? A. 2.0 m B. 1.5 m C. 1.0 m D. 0.50 m

Problem 7P Big Air A new generation of pogo sticks lets a rider bounce more than 2 meters off the ground by using elastic bands to store energy. When the pogo’s plunger hits the ground, the elastic bands stretch as the pogo and rider come to rest. At the low point of the bounce, the stretched bands start to contract, pushing out the plunger and launching the rider into the air. For a total mass of 80kg (rider plus pogo), a stretch of 0.40 m launches a rider 2.0 m above the starting point. Suppose a much smaller rider (total mass of rider plus pogo of 40 kg) mechanically stretched the elastic bands of the pogo by 0.40 m, then got on the pogo and released the bands. How high would this unwise rider go? A. 8.0 m B. 6.0 m C. 4.0 m D. 3.0 m

Problem 8P Big Air A new generation of pogo sticks lets a rider bounce more than 2 meters off the ground by using elastic bands to store energy. When the pogo’s plunger hits the ground, the elastic bands stretch as the pogo and rider come to rest. At the low point of the bounce, the stretched bands start to contract, pushing out the plunger and launching the rider into the air. For a total mass of 80kg (rider plus pogo), a stretch of 0.40 m launches a rider 2.0 m above the starting point. A pogo and rider of 80 kg total mass at the high point of a 2.0 m jump will drop 1.6 m before the pogo plunger touches the ground, slowing to a stop over an additional 0.40 m as the elastic bands stretch. What approximate average force does the pogo stick exert on the ground during the landing? A. 4000 N B. 3200 N C. 1600 N D. 800 N

Problem 9P Big Air A new generation of pogo sticks lets a rider bounce more than 2 meters off the ground by using elastic bands to store energy. When the pogo’s plunger hits the ground, the elastic bands stretch as the pogo and rider come to rest. At the low point of the bounce, the stretched bands start to contract, pushing out the plunger and launching the rider into the air. For a total mass of 80kg (rider plus pogo), a stretch of 0.40 m launches a rider 2.0 m above the starting point. Riders can use fewer elastic bands, reducing the effective spring constant of the pogo. The maximum stretch of the bands is still 0.40 m. Reducing the number of bands will A. Reduce the force on the rider and give a lower jump height. B. Not change the force on the rider but give a lower jump height. C. Reduce the force on the rider but give the same jump height. D. Make no difference to the force on the rider or the jump height.

Problem 10P Testing Tennis Balls Tennis balls are tested by being dropped from a height of 2.5 m onto a concrete floor. The 57 g ball hits the ground, compresses, then rebounds. A ball will be accepted for play if it rebounds to a height of about 1.4 m; it will be rejected if the bounce height is much more or much less than this. Consider the sequence of energy transformations in the bounce. When the dropped ball is motionless on the floor, compressed, and ready to rebound, most of the energy is in the form of A. Kinetic energy. B. Gravitational potential energy. C. Thermal energy. D. Elastic potential energy.

Problem 11P Testing Tennis Balls Tennis balls are tested by being dropped from a height of 2.5 m onto a concrete floor. The 57 g ball hits the ground, compresses, then rebounds. A ball will be accepted for play if it rebounds to a height of about 1.4 m; it will be rejected if the bounce height is much more or much less than this. If a ball is “soft,” it will spend more time in contact with the floor and won’t rebound as high as it is supposed to. The force on the floor of the “soft” ball is _______ the force on the floor of a “normal” ball. A. Greater than B. The same as C. Less than

Problem 12P Testing Tennis Balls Tennis balls are tested by being dropped from a height of 2.5 m onto a concrete floor. The 57 g ball hits the ground, compresses, then rebounds. A ball will be accepted for play if it rebounds to a height of about 1.4 m; it will be rejected if the bounce height is much more or much less than this. Suppose a ball is dropped from 2.5 m and rebounds to 1.4 m. a. How fast is the ball moving just before it hits the floor? b. What is the ball’s speed just after leaving the floor? c. What happens to the “lost” energy? d. If the time of the collision with the floor is 6.0 ms, what is the average force on the ball during the impact?

Problem 13P Squid Propulsion Squid usually move by using their fins, but they can utilize a form of “jet propulsion,” ejecting water at high speed to rocket them backward, as shown in Figure II.1 . A 4.0 kg squid can slowly draw in and then quickly eject 0.30 kg of water. The water is ejected in 0.10 s at a speed of 10 m/s. This gives the squid a quick burst of speed to evade predators or catch prey. What is the speed of the squid immediately after the water is ejected? A. 10 m/s B. 7.5 m/s C. 1.3 m/s D. 0.75 m/s

Problem 14P Squid Propulsion Squid usually move by using their fins, but they can utilize a form of “jet propulsion,” ejecting water at high speed to rocket them backward, as shown in Figure II.1 . A 4.0 kg squid can slowly draw in and then quickly eject 0.30 kg of water. The water is ejected in 0.10 s at a speed of 10 m/s. This gives the squid a quick burst of speed to evade predators or catch prey. What is the squid’s approximate acceleration in g ? A. 10 g B. 7.5 g C. 1.0 g D. 0.75 g

Problem 15P Squid Propulsion Squid usually move by using their fins, but they can utilize a form of “jet propulsion,” ejecting water at high speed to rocket them backward, as shown in Figure II.1 . A 4.0 kg squid can slowly draw in and then quickly eject 0.30 kg of water. The water is ejected in 0.10 s at a speed of 10 m/s. This gives the squid a quick burst of speed to evade predators or catch prey. What is the average force on the water during the jet? A. 100 N B. 30 N C. 10 N D. 3.0 N

Problem 16P Squid Propulsion Squid usually move by using their fins, but they can utilize a form of “jet propulsion,” ejecting water at high speed to rocket them backward, as shown in Figure II.1 . A 4.0 kg squid can slowly draw in and then quickly eject 0.30 kg of water. The water is ejected in 0.10 s at a speed of 10 m/s. This gives the squid a quick burst of speed to evade predators or catch prey. This form of locomotion is speedy, but is it efficient? The energy that the squid expends goes two places: the kinetic energy of the squid and the kinetic energy of the water. Think about how to define “what you get” and “what you had to pay”; then calculate an efficiency for this particular form of locomotion. (You can ignore biomechanical efficiency for this problem.)

Problem 17P What is the change in momentum of the ball during the collision? A. 1.4 kg ? m/s B. 1.8 kg ? m/s C. 3.2 kg ? m/s D. 5.1 kg ? m/s

Problem 18P What is the speed of the club head immediately after the collision? A. 29 m/s B. 25 m/s C. 19 m/s D. 11 m/s

Problem 19P Is this a perfectly elastic collision? A. Yes B. No C. There is insufficient information to make this determination.

Problem 20P If we define the kinetic energy of the club head before the collision as “what you paid” and the kinetic energy of the ball immediately after as “what you get,” what is the efficiency of this energy transfer? A. 0.54 B. 0.46 C. 0.37 D.0.27

Problem 21AIP Football players measure their acceleration by seeing how fast they can sprint 40 yards (37 m). A zippy player can, from a standing start, run 40 yards in 4.1 s, reaching a top speed of about 11 m/s. For an 80kg player, what is the average power output for this sprint? A. 300 W B. 600 W C. 900 W D. 1200 W

Problem 22AIP The unit of horsepower was defined by considering the power output of a typical horse. Working-horse guidelines in the 1900s called for them to pull with a force equal to 10% of their body weight at a speed of 3.0 mph. For a typical working horse of 1200 lb, what power does this represent in W and in hp?

Problem 23AIP A 100 kg football player is moving at 6.0 m/s to the east; a 130 kg player is moving at 5.0 m/s to the west. They meet, each jumping into the air and grabbing the other player. While they are still in the air, which way is the pair moving, and how fast?

Problem 24AIP A swift blow with the hand can break a pine board. As the hand hits the board, the kinetic energy of the hand is transformed into elastic potential energy of the bending board; if the board bends far enough, it breaks. Applying a force to the center of a particular pine board deflects the center of the board by a distance that increases in proportion to the force. Ultimately the board breaks at an applied force of 800 N and a deflection of 1.2 cm. a. To break the board with a blow from the hand, how fast must the hand be moving? Use 0.50 kg for the mass of the hand. b. If the hand is moving this fast and comes to rest in a distance of 1.2 cm, what is the average force on the hand?

Problem 25AIP A child’s sled has rails that slide with little friction across the snow. Logan has an old wooden sled with heavy iron rails that has a mass of 10 kg—quite a bit for a 30 kg child! Logan runs at 4.0 m/s and leaps onto the stationary sled and holds on tight as it slides forward. The impact time with the sled is 0.25 s. a. Immediately after Logan jumps on the sled, how fast is it moving? b. What was the force on the sled during the impact? c. How much energy was “lost” in the impact? Where did this energy go?