# Homework Help: Work, Energy, Fluids, Temperature, and Kinetic Theory

1. Jan 23, 2006

### Anna-Banana

Okay, I need help on a lot questions. I most of the math parts, but this stuff just completely confuses me. Any help will be greatly appreciated. Please help me not fail my class.

Work/Energy
Two identical arrows, one with twice the speed of the other, are fired into a bale of hay. Assuming the hay exerts a constant frictional force on the arrows, the faster arrow will penetratehow much farther than the slower arrow? Explain.

Why is it easier to climb a mountain via a zigzag trail rather than to climb straight up?

You can use a plley and ropes to decrease the force needed to raise a heavy load. But for every meter the load is raised, how much rope must be pulled up? Account for this using energy concepts.

Fluids
A small amount of water is boiled in a one-gallon gasoline can. The can is removed fromt he heat and the lid is put on. Shortly thereafter the can collapses. Explain.

Explain how a siphon can transwer liquid from one container to a lower one even though the liquid must flow uphill for part of its journey.

Will an ice cube float in a glass of alcohol? Why or why not?

Will an empty balloon have precisely the same apparent weight on a scale as one that is filled with air? Explain.

Does the buoyant force on a diving bell deep beneath the ocean have precisely the same value as when the bell is just beneath the surface? Explain.

Explain why helium weather balloons, which are used to measure atmospheric conditiosn at high altitude are normally released while filled only 10%-20% of their maximum volume.

Roofs of houses are sometimes "blown" off (or are they pusshed off?) during a tornado or hurricane. Explain, using Bernoulli's principle.

If you dangle two pieces of paper vertically, a few inches apart, and blow between them, how do you think the paper will move? Try it and see. Explain.

With a little effort, you can blow across a dime on a table and make it land in a cup without touching either cup or dime. Explain (and try it).

Temperature/Kinetic Theory
Which has more atoms: 1 kg of iron or 1 kg of aluminum?

A flat bimetallic strip consists of aluminum riveted to a strip of iron. When heated, which metal will be on the outside of the curve?

Explain why it is advisable to add water to an overheated automobile engine only slowly, and only with the engine running.

When a cole mercury-in-glass thermometer is first placed in a hot tub of water, the mercury initially descends a bit and then rises. Explain.

Escape velocity for the Earth refers to the minimum speed an object must have to leave the Earth and never return. The escape velocity for the moon is about one tenth what it is for the Earth since the Moon is smaller. Explain, then, why the Moon has practically no atmosphere.

Last edited: Jan 23, 2006
2. Jan 23, 2006

### Astronuc

Staff Emeritus
Please try to answer the questions yourself first. Then we can point you in the right direction.

In the first part with the arrows, what is the relationship between kinetic energy and velocity, and then think about friction (a force) applied over a distance, and conservation of energy.

3. Jan 23, 2006

### Robine

adding water to overheated engine

The concept is to avoid so sudden a change in the temperature of the engine that metal components fracture. Adding cold water in a rapid large volume can crack the metal as it cools to quickly. Adding a small amount of water more slowly will more gradually allow the hot parts to cool.
I doubt any automotive mechanic (someone with practical experience) would continue to let an already overheated engine continue to run....there is no coolant so the heat continues to build. However, if you (conceptually) add water fast enough to prevent further overheating yet slow enough to avoid damage in theory the engine circulation will disperse the water and spread it to additional interior volumes.

4. Jan 23, 2006

### Robine

will an ice cube float in alcohol

The principle here is which is denser. Think about a similar question: does wood float in water? Does oil float in water? In all these cases the less dense material will float on the more dense. Because the weight per unit volume is greater in one than the other, it pushes the less dense to the top of the liquid.

Pulley and ropes, and zizag trail principal: Work (W) is force times distance. W = F x D. So for a given amount of work you can increase the force (F) and decrease the distance (D) or increase the distance and decrease the force. Same concept as a lever. One feels easier than the other.

Boiled water in a can principal: What happens when air is heated? What happens when it cools?

Bouyant force on diving bell principal: The principal is that an object immersed in a liquid is buoyed up by a force equal to the weight of the displaced liquid. So if you assume the diving bell is absolutely rigid you get one answer; if you assume that the pressure at a great depth compresses it ever so slightly you can argue a different answer.

bimetallic strip principal: metals expand and contract at different rates, their coefficient of expansion. Some metals expand more than others.

weather baloons principal: How does the balloon gas behave when external pressure changes? How does atmospheric pressure change with increasing altitude. You can also look at the dieal gas law pV=nRT. If nRT stays constant, how would pV vary??

lots of different principals here which take time to master. Seek the underlying principal first, then try to apply it to your situation (question).
Good luck.

5. Jan 23, 2006

### Anna-Banana

Thanks for helping me out.

6. Jan 24, 2006

### Astronuc

Staff Emeritus
The two main factors here are differential thermal expansion and the lower strength of the hotter regions. As a metal cools it shrinks, and somewhere in a mass, some part may undergo an increase in tension. Strength (yield and ultimate tensile) decrease with temperature, and in some cases where differential thermal expansion/contraction is involved, the local stress field may exceed the local tensile limit.