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Honor Code Signature: Physics 102 Exam 1 Sample
Answers
20 February 2002 Prof L. Weinstein

Please give a short explanation for all of your non-numerical answers (unless otherwise noted). Show your work for all numerical answers.
 

Useful numbers: Atmospheric pressure = $10^5$ Pa = 100,000 N/m$^2$

Absolute zero = 0 K = -273 degrees C = -459 degrees F
 
 

1. (No explanation needed.) In an electrically neutral atom, the number of protons in the nucleus is balanced by the number of (circle all that apply)
1. neutrons
2. quarks
3. orbiting electrons
4. none of these


C - number of electrons
 
 
 
 

2. (No explanation needed.) The nuclei of atoms that make up a newborn baby were made in
1. the mother's womb
2. the food the mother eats while pregnant
3. ancient stars
4. the Earth
5. other


C - Ancient stars
 
 
 
 

3. (No explanation needed.) Pumice is a rock that floats. Therefore its density is
1. less than the density of water
2. greater than the density of water
3. equal to the density of water
4. you can't tell


A - if it floats it must have less density
 
 
 
 

4. (No explanation needed.) A balloon is buoyed up with a force equal to the
1. weight of air it displaces
2. density of surrounding air
3. atmospheric pressure
4. weight of the balloon and its contents
5. all of these


A - all objects have a buoyant force equal to the weight of the displaced fluid
 
 
 
 

5. How many different elements are in a water molecule (H$_2$O)?


2: H and O
 
 
 
 
 

6. The weight of matter comes mostly from its
1. protons and neutrons
2. electrons
3. other


A - protons and neutrons.  They are each about 2000 times heavier than an electron.
 
 
 
 

7. Water pressure is greatest against the
1. top of a submerged object
2. sides of a submerged object
3. bottom of a submerged object
4. is the same against all surfaces
5. none of these


C - It's greatest against the bottom since pressure increases with depth
 
 
 
 
 

8. What is the buoyant force acting on a 10 ton ship floating in the ocean?
1. less than 10 tons
2. more than 10 tons
3. exactly 10 tons
4. it depends on the density of seawater
5. it depends on the shape of the ship


C - exactly 10 tons.  The vertical acceleration of the ship is zero, therefore the net vertical force on the ship is zero, therefore the buoyancy force is exactly equal to and opposite the force of gravity on the ship (its weight).
 
 

9. Two identical buckets are filled to the absolute top with water. One of the buckets has a piece of wood floating in the water. Which bucket weighs more?
1. The one with the wood
2. the one without the wood
3. they weigh the same
4. need more information


C - The floating wood displaces its weight of water so it does not change the weight of the bucket.
 
 
 
 

10. A large motorboat floats in a swimming pool. If the boat sinks, the water level in the pool will
1. rise
2. fall
3. remain unchanged
4. need more information


B - fall.  When it floats, it displaces its weight in water.  After it sinks, it only displaces its volume of water (which is clearly less - or it wouldn't have sunk).  Since less water is displaced, the water level falls.
 
 
 
 

11. Air pressure is $10^5$ Pa = 100,000 N/m$^2$. Your frontal surface area is about 0.5 m$^2$. When you are lying down, what is the total mass (in kg or tons) of all the air on top of you?


The weight of water is 10^5 Pa * 0.5 m^2 = 50,000 N.  This is the weight of an object of mass 5,000 kg = 5 tons!
 
 
 
 
 

12. If there was no atmosphere, would it still be possible to drink water through a straw?


No.  When you drink through a straw, you are not sucking the water up the straw, atmospheric pressure is pushing it up.  If there is no atmosphere, then there is no atmospheric pressure and no liquid will be sucked up the straw.
 
 
 
 
 
 
 

13. When a gas in a container is squeezed to half its volume (at constant temperature), its density
1. quarters
2. halves
3. stays the same
4. doubles
5. quadruples


D - PV = constant.  If you halve V, then P must double.
 
 

14. Wind blowing faster over the top of a hill
1. increases atmospheric pressure there
2. decreases atmospheric pressure there
3. does not affect atmospheric pressure there
4. need more information


B - decreases pressure.  Velocity is bigger so the Bernoulli effect is bigger and the pressure drops.
 



15. Water flows through the wide and narrow pipe as shown. Some of the water squirts upward from tiny leaks at point A (in the wide section) and point B (in the narrow section). The water squirts out fastest (ie: with greatest pressure) at point
1. A
2. B
3. the same at each
4. need more information


A - water flows fastest at point B, thus the pressure is lowest there.  A higher pressure at A makes the water squirt out faster there.
 
 

16. Compared to a giant iceberg, a hot cup of coffee has
1. more internal energy and higher temperature
2. higher temperature but less internal energy
3. a greater specific heat and more internal energy
4. none of these


B - the hot coffee has a higher temperature than the iceberg.  Temperature is kinetic energy per molecule.  Even though each molecule of the iceberg has less kinetic energy, there are a LOT more molecules in the iceberg so the total internal energy of the iceberg is much greater.
 
 
 

17. When water at 4 degrees C is heated it expands. When water at 4 degrees C is cooled by 1 degree it:
1. expands
2. contracts
3. neither expands nor contracts


A - expands.  At 4 degrees C, water is at its densest (ie: it takes up the least volume).  If you cool it or warm it, it becomes less dense and hence it must expand.
 
 

18. The desert sand gets really hot during the day and really cold at night. This indicates that it has a
1. low specific heat
2. high specific heat
3. other (please specify)


A - The Sahara Desert gets about the same amount of heat energy as the Mediterranean Sea during the day and it loses about the same amount of heat energy at night.  The temperature of the desert changes much more than that of the Mediterranean because it has much less thermal inertia (specific heat).
 
 

19. Suppose that you are served hot coffee at a restaurant before you are ready to drink it. The cream is at room temperature. To make sure that the coffee is as hot as possible when you drink it, you should add the cream:
1. immediately
2. just before you drink the coffee
3. it doesn't matter
4. need more information


A - add it immediately.  By adding the cream immediately, you decrease the temperature of the coffee immediately.  The cooler coffee loses heat more slowly than the hot coffee would.  (ie: add cream, coffee cools from adding cream, coffee cools only a little bit more after that.  Otherwise: hot coffee cools a lot, add cream much later, coffee cools from adding cream.)
 
 

20. Your feet feel warmer on a rug than on a ceramic tile floor because the rug (circle all that apply)
1. is usually warmer than the tile
2. is a better insulator than the tile
3. for the same mass has more internal energy than the tile
4. none of these


B - rug is a better thermal insulator than tile.  The tile conducts much more heat away from your feet.
 
 
 

21. A pot of clean snow and a pot of dirty snow are placed on a hot stove. The snow to melt first will be the
1. clean snow
2. dirty snow
3. both the same
4. need more information


This is a complicated question.  If you are inside in your house, or outside during the day when the sun is shining, the dirty snow will melt first because it is absorbing heat from the pot (by conduction) and from the room or the sun (by radiation).  The clean snow is absorbing very little heat by radiation since it is such a good reflector.

If you are outside at night, the clean snow will melt first.  The dirty snow is absorbing energy by conduction from the pot, but it is radiating energy energy to the night sky.  The clean snow is also absorbing energy by conduction but it is not radiating as much energy to the night sky since it is a very bad emitter (bad emitter <--> bad absorber).

Therefore the answer is D.  Aren't you glad this question is on the practice test and not the real one?
 
 

22. If you double the pressure of an ideal gas while keeping the temperature constant, the average kinetic energy of the molecules
1. does not change
2. increases by less than twice
3. doubles
4. more than doubles
5. need more information


A - does not change.  Kinetic energy depends only on temperature.