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**Unformatted text preview: **Physics 101 Classical Physics Spring 2010 Exam 3 Instructions : The answer sheets must be handed in as soon as time is called. You have until 10:20am. Answer the following four multiple choice questions. Each question is worth 4 points. 1. The astronauts in the space shuttle (which orbits the Earth) float around inside it because: (a) There is no gravity in space (b) They are so far away that the Earth’s gravity is extremely weak and doesn’t affect them (c) They are in free-fall, all accelerating toward the Earth due to gravity (d) They are not really floating, it is just a camera trick (e) The air pressure inside the space shuttle keeps them buoyed upward Like the “Newton’s Cannon” example we saw in class, the astronauts and the space shuttle (and all the air inside) are all falling toward the Earth, accelerating downward at the same rate. So the answer is c . 2. An object is floating in the middle of a sealed tank of water. The tank is 2 m high and the object is 1 m beneath the surface, in contact only with the water. At a certain time, the tank is dropped out of an airplane, and falls at an acceleration of 9.8 m/s 2 (in other words, air resistance can be neglected for this problem). After the object is dropped out of the plane, what are the forces on it? (a) Only the force of gravity (b) The force of gravity, and a buoyant force (c) The force of gravity, a buoyant force, and a normal force (d) There are no forces on the object, because it is at rest (e) Only the buoyant force This looks like it is a fluid question, but in fact it is the same question as the first one. Since everything in the tank is accelerating downward at g , there can be no other force on the object but gravity. 3. An object floating in water has 25% of its volume below the surface. When placed in another fluid, the object has 50% of its volume below the surface. What is the density of the second fluid, relative to that of water? (a) ρ fluid = 1 2 ρ H 2 O (b) ρ fluid = 2 ρ H 2 O (c) ρ fluid = 4 ρ H 2 O (d) ρ fluid = 1 4 ρ H 2 O (e) ρ fluid = 8 ρ H 2 O The buoyant force on an object is equal to the weight of the fluid it displaces, or m fluid g = ρ fluid V fluid g . For the first case, in water, the volume of displaced fluid is 25% of the volume 1 of the object. In the second case, in the new fluid, it is 50% of the volume of the object. In both cases, however, the object is not moving and therefore the buoyant force is exactly balancing the weight of the object. So we have ρ fluid V fluid g = ρ water V water g or ρ fluid . 5 V object g = ρ water . 25 V object g. Cancelling things we have ρ fluid . 5 = ρ water . 25 or . 5 ρ fluid = 0 . 25 ρ water so ρ fluid = 0 . 5 ρ water and the answer is a ....

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