Physics 1050 - How Things Work - Fall 2013 Midterm Examination 2 Solutions

Running on soft dry sand is exhausting, so you switch to running on hard wet sand. The hard wet sand removes less energy from you because it

(A) barely moves downward as you push downward on it, so you do almost zero work on it. [60.5% picked]

(B) stops the downward motion of your foot faster and thus absorbs more of your momentum. [6.8% picked]

(D) stops the downward motion of your foot faster and thus absorbs less of your momentum. [24.0% picked]

Answer: (A) barely moves downward as you push downward on it, so you do almost zero work on it. [60.5% picked]

You and seven of your teammates are rowing in a boat race and are heading northward toward the finish line. During each stroke, you immerse the blade (flat surface) of your oar (long stick used to propel the boat) in the water and make that blade move rapidly southward through the water. As the blade moves through the water, your boat accelerates northward because

(A) the water-level behind the boat rises and the boat accelerates down the inclined plane that you are producing. [3.0% picked]

(B) you exert a southward force on the blade of your oar. [25.5% picked]

(D) the water exerts a northward drag force on the southward-moving blade. [68.8% picked]

Answer: (D) the water exerts a northward drag force on the southward-moving blade. [68.8% picked]

(A) travel much farther after being hit by a golf club. [1.5% picked]

(B) curve toward the right after being hit by the club of a right-handed golfer. [0.4% picked]

(D) curve toward the left after being hit by the club of a right-handed golfer. [1.1% picked]

(A) water's larger force favors disorderly turbulent flow. [0.4% picked]

(B) water's larger pressure favors disorderly turbulent flow. [0.4% picked]

(D) honey's larger density favors orderly laminar flow. [2.3% picked]

Answer: (C) honey's much larger viscosity favors orderly laminar flow. [97.0% picked]

You are inflating a shiny plastic balloon with helium. The balloon starts as a thin, flat bag but it gradually gets thicker and rounder as it fills. At first, the thin plastic balloon doesn't float. But as you keep adding helium to the balloon and it gets thicker and rounder, it eventually begins floating because

(A) the upward buoyant force on a balloon full of helium is larger than the buoyant force on an identical balloon full of air. [12.5% picked]

(B) the helium-filled balloon's weight decreases as you put more lightweight helium particles inside it and eventually it becomes weightless. [15.6% picked]

(C) the average pressure of the helium-filled balloon increases as it fills and its pressure eventually becomes greater than that of the air it displaces. [17.5% picked]

(D) the weight of the helium-filled balloon increases as it fills, but the buoyant force it experiences increases much faster and eventually the buoyant force exceeds the balloon's weight. [54.0% picked]

Answer: (D) the weight of the helium-filled balloon increases as it fills, but the buoyant force it experiences increases much faster and eventually the buoyant force exceeds the balloon's weight. [54.0% picked]

Water hammer occurs when you suddenly stop the rapid flow of water in a pipe and results in the pipe-end experience a huge force in the direction of the water's original motion. It happens because flowing water has

(A) angular momentum. [1.9% picked]

(B) momentum. [59.3% picked]

(D) acceleration. [3.4% picked]

The sculpture in a museum lobby includes a water fountain. Water flowing upward through a pipe passes through a short vertical nozzle and then rises to a height of 10 feet in the open air. As water flows along a streamline in this system, its energy changes forms several times. As water passes through the nozzle its energy transforms from

(A) kinetic energy to pressure potential energy and as it then rises to its peak height its energy transforms from kinetic energy to gravitational potential energy. [0.8% picked]

(B) kinetic energy to pressure potential energy and as it then rises to its peak height its energy transforms from pressure potential energy to gravitational potential energy. [5.7% picked]

(C) pressure potential energy to gravitational energy and as it then rises to its peak height its energy transforms from gravitational energy to pressure potential energy. [9.1% picked]

(D) pressure potential energy to kinetic energy and as it then rises to its peak height its energy transforms from kinetic energy to gravitational potential energy. [84.4% picked]

Answer: (D) pressure potential energy to kinetic energy and as it then rises to its peak height its energy transforms from kinetic energy to gravitational potential energy. [84.4% picked]

An airplane cruises forward at constant velocity through a region of calm, motionless air. After the plane has passed through that region of air, the air's total momentum is

(A) nonzero and directed at an angle between downward and forward. [10.6% picked]

(B) nonzero and directed exactly downward. [72.6% picked]

(D) nonzero and directed at an angle between downward and backward. [3.0% picked]

To avoid tipping over while turning right on a bicycle, why must you lean the bicycle toward your right?

(A) You accelerate rightward as you turn right and your inertia tends to make you tip over toward your left. By leaning toward your right, you cancel that tendency and avoid tipping over. [62.0% picked]

(B) Leaning the bicycle decreases the downward force that the ground exerts on its wheels and reduces your downward acceleration. [1.1% picked]

(C) Leaning the bicycle reduces the frictional force that the ground exerts on its wheels and reduces its tendency to tip over during the turn. [24.3% picked]

(D) Leaning the bicycle increases the upward force that the ground exerts on its wheels and reduces your downward acceleration. [11.8% picked]

Answer: (A) You accelerate rightward as you turn right and your inertia tends to make you tip over toward your left. By leaning toward your right, you cancel that tendency and avoid tipping over. [62.0% picked]

A tall flagpole is topped by a metal ball. When a large bird leaps off the ball, the flagpole starts bending back and forth rhythmically about its straight equilibrium shape. At what moment(s) during its rhythmic motion is the ball moving fastest?

(A) Just before the flagpole is bent furthest away from equilibrium. [2.7% picked]

(B) Just after the flagpole is bent furthest away from equilibrium. [5.3% picked]

(D) When the flagpole is bent furthest away from equilibrium. [3.8% picked]

A 10-inch diameter wooden ball is floating on a calm fresh-water lake at sea level. The top of the ball is exactly 1 inch above the level of the water. When you transfer that ball to a fresh-water lake high up in the mountains, the ball

(A) sinks to the bottom of the lake. [1.5% picked]

(B) floats deeper in the water, so that its top is less than 1 inch above the level of the water. [75.3% picked]

(D) floats just as it did before, with its top exactly 1 inch above the level of the water. [1.1% picked]

Answer: (B) floats deeper in the water, so that its top is less than 1 inch above the level of the water. [75.3% picked]

You are an astronaut spacewalking independently near your ship when you discover that your suit's propulsion system has failed. You and the ship are both coasting forward and are maintaining a constant separation of 100 feet (30 meters). There is danger in the distance, so you need to get to your ship. Fortunately, you have a hammer in your hand. To get to your ship, you throw the hammer

(A) in the direction of your velocity (toward the distant danger). [4.6% picked]

(B) in the direction opposite your velocity (away from the distant danger). [4.6% picked]

(D) away from the ship (in the direction that points from the ship to you). [89.4% picked]

Answer: (D) away from the ship (in the direction that points from the ship to you). [89.4% picked]

An enormous asteroid is coasting straight toward your space station. With impact exactly 100 hours away, you use a tiny missile to blow the asteroid into two equal pieces. The space station is saved! Neglecting gravity and the tiny momentum of the tiny missile, what happens to the asteroid's total momentum and center of mass after the missile explosion?

(A) The asteroid's total momentum is reduced by a factor of two and its center of mass remains at a constant distance from the space station. [26.2% picked]

(B) The asteroid's total momentum remains unchanged after the explosion and its center of mass passes through the space station exactly 100 hours later. [54.8% picked]

(C) The asteroid's total momentum is zero after the explosion and its center of mass remains at a constant distance from the space station. [10.6% picked]

(D) The asteroid's total momentum is zero after the explosion and its center of mass passes through the space station exactly 100 hours later. [8.4% picked]

Answer: (B) The asteroid's total momentum remains unchanged after the explosion and its center of mass passes through the space station exactly 100 hours later. [54.8% picked]

An toddler's sipping cup has a round base (like the bottom half of a ball) that is weighed so that the cup always returns to upright after being tipped. The cup is in a stable equilibrium when it is upright and tipping it causes its total

(A) kinetic energy to decrease. [0.4% picked]

(B) potential energy to decrease. [28.9% picked]

(D) kinetic energy to increase. [3.8% picked]

(A) increases near the inside of the curve and increases near the outside of the curve. [0.0% picked]

(B) increases near the inside of the curve and decreases near the outside of the curve. [4.9% picked]

(D) decreases near the inside of the curve and increases near the outside of the curve. [95.1% picked]

Answer: (D) decreases near the inside of the curve and increases near the outside of the curve. [95.1% picked]

When you drop coins on a hard level surface, they sometimes roll for amazingly long time. Why does a rolling coin tend to stay upright for such a long time?

(A) The rolling coin cannot exchange angular momentum with the ground. [1.9% picked]

(B) The rolling coin is supported by the buoyant force. [0.4% picked]

(C) The rolling coin naturally steers so as to place its contact point with the ground directly under its center of gravity. [96.2% picked]

(D) The rolling coin cannot exchange momentum with the ground. [1.5% picked]

Answer: (C) The rolling coin naturally steers so as to place its contact point with the ground directly under its center of gravity. [96.2% picked]

You are designing a chair for a museum. While your chair will be a work of art, it must also be practical and it mustn't tip over easily. To ensure that it stays upright, you design it so that its

(A) velocity always remains constant if you tip it. [0.0% picked]

(B) center of gravity always descends if you tip it. [5.7% picked]

(D) angular velocity always remains constant if you tip it. [0.4% picked]

Firefighters are battling a fire on the 10th floor of an apartment building. When they stand on the ground, the steady stream of water from the short nozzle of their short fire hose rises vertically upward but reaches its peak height at only the 8th floor. Fortunately, they have a well-equipped fire truck with a longer hose, a longer nozzle, a more powerful pump, and a tall ladder. Which of these four tools could allow the firefighters to send a steady stream of water onto the 10th-floor fire?

(A) The longer nozzle [4.9% picked]

(B) The more powerful pump [93.9% picked]

(D) The tall ladder [1.1% picked]

You are jumping up and down on a trampoline. As you drop from great height onto the trampoline's surface, when is the first moment that your speed stops increasing?

(A) When you first touch the trampoline's surface. [9.5% picked]

(B) When you reach equilibrium in the dented trampoline's surface. [82.1% picked]

(D) Just before you first touch the trampoline's surface. [1.1% picked]

Answer: (B) When you reach equilibrium in the dented trampoline's surface. [82.1% picked]

A helium-filled balloon weighs only about 1/7th as much as the air it displaces. If you replace the helium gas in the balloon with hydrogen gas (which is half as dense as helium gas), the upward buoyant force on the balloon would

(A) stay the same and the net force on the balloon would increase slightly in the upward direction. [26.6% picked]

(B) increase and the net force on the balloon would approximately double in the upward direction. [41.1% picked]

(D) increase and the net force on the balloon would increase slightly in the upward direction. [25.5% picked]

Answer: (A) stay the same and the net force on the balloon would increase slightly in the upward direction. [26.6% picked]

You are trying to increase the pressure in your village's water distribution system by modifying the local water tower. You should make that water tower

(A) narrower (decrease its diameter). [1.1% picked]

(B) shorter (decrease its altitude). [1.5% picked]

(D) wider (increase its diameter). [0.8% picked]

You are practicing tennis alone by hitting a tennis ball forward toward a cement wall. Each time the ball hits the wall, it bounces backward at high speed so that you can hit it again. During its bounce, the ball

(A) retains approximately all of its energy but transfers most of its momentum to the wall. [34.2% picked]

(B) retains approximately all of its momentum but transfers most of its energy to the wall. [6.5% picked]

(D) retains approximately all of its energy but transfers more forward momentum than it had to the wall. [51.0% picked]

Answer: (D) retains approximately all of its energy but transfers more forward momentum than it had to the wall. [51.0% picked]

You are standing in the middle of a subway car that is moving forward at constant velocity when another passenger accidently spills an enormous container of olive oil. Suddenly, the floor cannot exert any frictional forces on your feet. Because nothing else is touching you, you

(A) remain in the middle of the subway car. [57.4% picked]

(B) shift toward the front of the subway car (in the direction of its velocity). [4.2% picked]

(D) shift toward the back of the subway car (opposite its velocity). [37.6% picked]

You forgot to tip the bartender who brought a Shirley Temple drink to your luxury hotel suite. So you're leaning out over the balcony of your 20th floor suite and dropping solid gold marbles into the tip jar on the ground floor patio bar. You notice that it takes 4 seconds for each marble to fall into the jar, 20 floors below. When each marble has fallen for only 2 seconds, which floor of the hotel is it nearest? [Note: neglect any effects due to the air.]

(A) The marble is near the 10th floor. [1.1% picked]

(B) The marble is near the 8th floor. [7.6% picked]

(D) The marble is near the 12th floor. [4.9% picked]

Water is flowing gently out of the end of a garden hose. You block off most of the hose's opening with your thumb and now the water sprays out at high speed because you

(A) have slowed the water flow through hose and plumbing so that the water wastes less energy doing work against viscous forces. [30.8% picked]

(B) are keeping air from getting into the hose and reducing the water pressure. [18.3% picked]

(D) are compressing the water and increasing its density. [24.3% picked]

Answer: (A) have slowed the water flow through hose and plumbing so that the water wastes less energy doing work against viscous forces. [30.8% picked]

You kick a soccer ball so that its path bends (curves) toward your left as it travels away from you and approaches the goal. The ball is able to bend its path (curve) toward your left because

(A) it is deflecting the airflow around it toward your right. [67.3% picked]

(B) the pressure in front of it is higher than the pressure behind it. [2.7% picked]

(D) you exerted a leftward force on it with your foot. [0.4% picked]

Answer: (A) it is deflecting the airflow around it toward your right. [67.3% picked]

You are jogging northward along the ocean on a calm, windless day and are experiencing a pressure drag force that pushes you southward. The average velocity of the air a few feet behind you is

(A) directed southward. [6.5% picked]

(B) zero, but that air is swirling rapidly in all directions. [10.6% picked]

(D) zero and that air is calm and motionless. [1.9% picked]

You are cleaning your car with water from a hose and nozzle. At the center of the stream of water, right where it hits the side of your car, the water is almost coming to a complete stop. The pressure in the water at the center of the stream as it touches the car is

(A) lower than atmospheric pressure, but more than zero. [6.1% picked]

(B) exactly zero. [0.4% picked]

(D) higher than atmospheric pressure. [87.8% picked]

As air flows around a fast-moving ball, that air has difficulty maintaining laminar flow from the ball's sides to the ball's back. During the air's passage from sides to back, the pressure along each streamline

(A) increases and the airflow slows down. [52.9% picked]

(B) decreases and the airflow slows down. [6.8% picked]

(D) increases and the airflow speeds up. [6.1% picked]

You are riding a rollercoaster with a loop-the-loop. You have just rolled up the side of the loop and are, at this moment, exactly at the top of the loop. You and the car are upside down, yet you are pressed tightly into your seat. At this moment, you are accelerating

(A) downward at the acceleration due to gravity. [2.7% picked]

(B) downward at less than the acceleration due to gravity. [3.0% picked]

(D) downward at more than the acceleration due to gravity. [94.3% picked]

Answer: (D) downward at more than the acceleration due to gravity. [94.3% picked]