Physics 010 Assignments 2007-2008 This document contains the five homework assignments given in the Physics 010 course in 2007-2008.
Physics 010 Assignment #1 1. A car is traveling along a straight road. The top curve represents the car's position (x) as a function of time (t). Which of the other curves (a,b,c,d,e) best represents the car's velocity? Explain your answer. 2. Mr. Bean starts in the middle of the room and moves, at a constant rate, 6m to the right in 10 sec. He then stops and looks around for 10 sec, turns around, and goes 12m left in ten sec. Plot position (x) vs time (t), and velocity (v) vs time (t) 3. A marathon runner runs at a steady 15 km/hr. When the runner is 7.5 km from the finish, a bird starts flying from the runner to the finish at 30 km/hr. When the bird reaches the finish line, it turns around and flies back to the runner, and then turns around again, repeating the back-andforth trips until the runner reaches the finish line. How many km does the bird travel? 4. (a) Can an object be moving (ie. have a non-zero velocity) when its acceleration is zero? If so, give an example. (b) Can an object be accelerating when its velocity is zero? If so, give an example. 5. A bicyclist makes a trip that consists of three parts, each in the same direction along a straight road. In part (1), she rides for 22 minutes at an average speed of 7.2 m/s. In part (2), she rides for 36 minutes at an average speed of 5.1 m/s. In part (3), she rides for 8.0 minutes at an average speed of 13 m/s. (a) How far has the bicyclist traveled during the entire trip? (b) What is her average speed for the trip?
6. Very few athletes can jump more than 2 feet (0.6 m) straight up. Use d=(1/2)gt 2 (where g is the acceleration due to gravity = 9.8 m/s 2 ), and solve for the time one spends moving upwards in a 0.6m vertical jump. How long would the hang time be (the time one's feet are off the ground)? 7. Why do you lurch forward in a bus that suddenly slows? Why do you lurch backward when it picks up speed? Which of Newton's Laws applies here? 8. A coin is tossed straight up into the air. It reaches the top of its path and starts to fall back down. It s acceleration on the way down, neglecting air resistance, is: (a) greater than when it was at the top of its path. (b) the same as when it was at the top of its path. (c) less than when it was at the top of its path. Explain. 9. If a mass of 1 kg is accelerated 1m/s 2 by a force of 1 N, what would be the acceleration of a 2 kg mass acted on by a force of 2 N? 10. In the amusement park ride known as Magic Mountain Superman, powerful magnets accelerate a car and its riders from rest to 45 m/s in a time of 7.0 sec. The mass of the car and riders is 5.5 x 10 3 (5500) kg. Find the average net force exerted on the car and riders by the magnets. 11. A 200 kg horse is hooked up a 400 kg wagon, and you tell the horse to pull forward. The horse says: "Why bother? As hard as I pull, the wagon is just going to exert an equal but opposite force. The wagon won't move." Will the wagon move? How do you explain this to the horse?
Physics 010 Assignment #2 1. You have to lift a 20 kg mass a vertical height of 5 m. Since you can only exert a maximum force of 100 N, a ramp seems to be the answer to your problem. How long a ramp will you need? What will be the mechanical advantage of this ramp? 2. Explain how pulley systems work. Why do they make it "easier" to lift objects? What is the trade-off? 3. Show that no matter what the mechanical advantage of a pulley system is, the work done by moving a mass a distance h off the ground is the same as if no pulleys were used. What is this work? (hint: refer to parts of your answer to the previous question) 4. In raising a 5000 N piano with a pulley system, the workers note that, for every 2 m of rope pulled down, the piano rises 0.2 m. Ideally, how much force is required to lift the piano? 5. A 6 kg piece of metal displaces 1 liter of water when submerged. What is the density of the metal? 6. Equal volumes of lead and aluminum are submerged in water. Which object experiences the greatest buoyant force? Which object accelerates downwards fastest? Explain. 7. How much force is required to hold in place a 70 kg rock submerged in the ocean? The rock has a volume of 0.05 m3, and the density of salt water is 1025 kg/m3. 8. A ship sailing from the ocean into a freshwater harbor sinks slightly deeper into the water. Does the buoyant force on the ship change? If so, does it increase or decrease? 9. In the hydraulic pistons shown in the sketch, the small piston has a diameter of 2 cm. The larger piston has a diameter of 6 cm. How far will the large piston move up if the small piston moves down 1 cm? How much more force can the larger piston exert compared with the force applied to the smaller piston?
Physics 010 Assignment #3 (KE = kinetic energy PE = potential energy) (For questions #1 and #2, you will need to do some research. A couple of paragraphs for each question is fine.) 1. How exactly does a solar cell convert sunlight to electricity? What are typical efficiencies for a solar cell (ie. what is the fraction of input solar energy that is converted to electrical energy and thus useable)? 2. How does a glowstick produce light? What is the energy source? Why does the brightness of a glowstick depend on its temperature? Does the lifetime of a glowstick depend on its temperature? 3. The second floor of a house is 6 m above ground level. How much work is required to lift a 300 kg refrigerator to the second-floor level? Does the PE of the refrigerator increase or decrease, and by how much? 4. What is the kinetic energy (KE) of a toy cart of mass m moving at a velocity v? Compare the KE of: a) a cart with double the mass (2m) traveling with the same velocity (v) and b) a cart with the same mass (m) but traveling at double the velocity (2v) of the original cart. 5. Give three examples of Potential Energy and explain where they are used. 6. At what point in its motion is the KE of a pendulum bob at a maximum? At what point is its PE at a maximum? When its KE is at half its maximum value, how much PE does it have? 7. In the absence of air resistance, a ball thrown vertically upward with a certain amount of KE will return to its original level with the same KE. When air resistance is a factor affecting the ball, will it return to its original level with same, less, or more KE? Does your answer contradict the law of energy conservation? 8. Heat was transferred from the hot water to the cooler glowstick by thermal conduction. How does thermal conduction work?
9. At a certain location, the solar power per unit area reaching the Earth's surface is 200 Watts per square meter (200 W/m 2 ), averaged over a 24-hour day. In other words, on average, each square meter of the Earth's surface in that location receives 200 Watts of solar power. If you live in a house whose average power requirement is 3000 W, and you can convert solar power to electric power with 10% efficiency (ie. only 10% of the incoming solar power can be converted to electric power), how large a collector area will you need to meet all your household energy requirements from solar energy? Will it fit in your yard? Remember that power = energy/time (joules/sec = watts) 10. A poker used in a fireplace is held at one end, while the other end is in the fire. Why are pokers made of iron rather than copper? Ignore the fact that iron may be cheaper and stronger.
Physics 010 Assignment #4 1. (a) Explain how charging by contact and charging by induction work, and the differences between these two methods. (b) How can you charge an object negatively with only the help of a positively charged object? 2. An electroscope (see below) is a simple device consisting of a metal ball that is attached by a conductor to two thin leaves of metal foil protected from air disturbances in a jar, as shown below. When the ball is touched by a charged body, the leaves that normally hang straight down spread apart. Why? 3. When one material is rubbed against another, electrons jump readily from one to the other, but protons do not. Why is this? (think in atomic terms) 4. You want to determine if an object is neutral, positively charged, or negatively charged. You have another object known to be positively charged, and another object known to be negatively charged to use for this testing. How do you determine what charge your object has? 5. In which of the circuits below does a current exist to light the bulb?
6. (a) As more and more bulbs are connected in series to a flashlight battery, what happens to the brightness of each bulb? (b) What happens to the brightness of each bulb when more and more bulbs are connected in parallel? 7. In the circuit shown below: (a) how do the brightnesses of the identical light bulbs compare? (b) Which bulb draws the most current? (c) What will happen if Bulb A is unscrewed? (d) What will happen if Bulb C is unscrewed? 8. (a) What is the effect on the current in a wire if both the voltage across it and its resistance are doubled? If the voltage is doubled and the resistance is halved? (b) Calculate the current that moves through your fingers (resistance 1000 Ω) when you touch them to the terminals of a 6-volt battery. 9. If you had to bars of iron-- one magnetized and one unmagnetized-- and no other materials at hand, how could you determine which bar was the magnet? 10. In what ways are electric charges and magnetic poles similar and different?
Physics 010 Assignment #5 1. This question refers to the "floating paperclip" demo you saw in class. You can pass some materials through the gap between the floating paperclip and the paperclip doesn't fall. But if other materials are passed through the gap, the paperclip falls down. Which materials cause the paperclip to fall down, and why? 2. If you place a chunk of iron near the north pole of a magnet, attraction will occur. Why will attraction also occur if you place the same iron near the south pole of the magnet? 3. Do mirrors really reverse left to right? Explain. Try writing something on a plastic transparency and holding it up to a mirror. Then try flipping the transparency from left to right and hold it up to the mirror again. 4. Trucks often have signs on their back ends that say: "If you can't see my mirrors, I can't see you." Explain the physics here. 5. Briefly describe 3 applications of fiber optics. 6. The drawings below show two arrows, A and B, that are located in front of a plane mirror. A person at point P is viewing the image of each arrow. Which images can be seen in their entirety? Determine your answers by drawing a ray from the head and foot of each arrow that reflects from the mirror according to the law of reflection and reaches point P. Only if both rays reach point P after reflection can the image of that arrow be seen in its entirety. 7. When your eye is submerged in water, is the bending of light rays from water to your eyes more, less, or the same as in air? 8. Explain how a pinhole camera works. Include a sketch with your answer.
9. The image produced by a converging lens is upside down. Our eyes have converging lenses. Does this mean the images we see are upside down on our retinas? Explain. 10. The diagram below shows three rays that can be used to find the image of an object in front of a converging lens. The image will be located on the other side of the lens, where the three rays cross. Using this ray-tracing technique, we can find the location of the image (how far it is from the lens), whether it is right side up or upside down, and whether it is magnified (smaller or larger than the object). Using these three rays, find the image for the object below. (a) Roughly how far from the lens is the image? (b) Is the image right side up or upside down? (c) Is the image larger, smaller, or the same size as the object?