Physics Exam 3 (HW 1)

Water settles to the bottom of a tank of gasoline. Which takes up more space: 1 kg of water or 1 kg of gasoline?

1 kg of gasoline because gasoline is less dense than water.

Water weighs about 10 N per liter on Earth. A 4.0 liter ball is pushed underwater on the space shuttle, where gravity was 1/10 that on Earth. What is the buoyant force on the ball?

4 N

Water weighs about 10 N per liter. A 4.0 liter ball is pushed underwater. What is the buoyant force on the ball?

40 N

The brake system in most cars makes use of a hydraulic system. This system consists of a fluid filled tube connected at each end to a piston. Assume that the piston attached to the brake pedal has a cross Section: al area of one half a square inch and the piston attached to the brake pad has a cross Section: area of two square inches. When you apply a force of 10 pounds to the piston attached to the brake pedal, the force at the brake pad will be,

40 pounds

You are in a hot air balloon and are hovering in the air so that the atmospheric pressure remains constant. At a temperature of 300 K your balloon holds 5x106 liter of air. The sun comes out and the temperature rises to 330 K. What is the new volume of the balloon? You may assume that no air can exit or enter the balloon.

5.5 × 106 liter

How does pushing on the plunger of a syringe cause medicine to flow into a patient through a hollow hypodermic needle?

Bernoulli's Equation: pressure on the fluid converts to kinetic energy (motion) of the fluid.

Waterproof watches have a maximum depth to which they can safely be taken while swimming. Why?

Deeper than that depth the water pressure is so great it might damage the watch.

Why will a rolled - up ball of aluminum foil float in water but solid aluminum will sink?

Due to the air inside, the average density of the ball is lower than water.

When someone pulls a fire alarm in a skyscraper, pumps increase the water pressure in the section of the building nearest that alarm box. How does this pressure change assist firefighters who must battle the blaze?

Greater water pressure means the water emerges from the hoses faster, so it can spray farther.

Some fish move extremely slowly, and it's hard to tell whether they are even alive. However, if a fish is floating at a middle height in your aquarium and not at the top or bottom of the water, you can be pretty certain that it's alive. Why?

If the fish is floating at a middle height, it must be be regulating its density to be equal to the density of the water, which a dead fish couldn't do

A helium-filled balloon floats in air. What will happen to an air-filled balloon in helium? Why?

It will sink because air is denser than helium.

You use your breath to inflate a large rubber tube and then ride down a snowy hill on it. After a few minutes in the snow the tube is underinflated. What happened to the air?

Its pressure decreased. Lower temp means lower pressure.

A popular drink is composed by pouring several types of beverage (one on top of the other) in order to get a layering effect. In what order should the individual beverages be poured into the glass so that your drink is stable?

Most dense first and then in order of decreasing density.

Gravity creates

Pressure gradients in fluids

If you seal a soft plastic bottle or juice container while hiking high in the mountains and then return to the valley, the container will be dented inward. What causes this compression?

The air inside the container is at a lower pressure than the air outside. The higher pressure causes the container to collapse until the air inside is at the same pressure as the air outside.

Many jars have dimples in their lids that pop up when you open the jar. What holds the dimple down while the jar is sealed, and why does it pop up when the jar is opened?

The air outside is at higher pressure than the air inside the jar, which holds the dimple down. When the pressure equalizes, the dimple pops back to its normal position.

You seal a rigid container that is half full of hot food and put it in the refrigerator. Why is the container's lid bowed inward when you look at it later?

The air pressure of the air outside the container pushes the lid in.

If you place a hot, wet cup upside down on a smooth counter for a few seconds, you may find it difficult to lift up again. What is holding that cup down on the counter?

The air pressure of the air outside the cup.

Ice tea is often dispensed from a large jug with a faucet near the bottom. Why does the speed of tea flowing out of the faucet decrease as the jug empties?

The gravitational potential energy due to the depth of the tea decreases as the height of the tea decreases. Decreased potential energy on one side of Bernoulli's Equation means decreased speed on the other.

Why must tall dams be so much thicker at their bases than at their tops?

The greater the depth of the water the greater the pressure pushing on the wall of the dam

You can inflate a plastic bag by holding it up so that it catches the wind. Use Bernoulli's equation to explain this effect.

The kinetic energy of the moving air converts into greater air pressure inside the bag, which inflates the bag.

Oil and vinegar salad dressing settles with the oil floating on top of the vinegar. Explain this phenomenon in terms of density.

The oil is less dense than the vinegar.

Each time you breathe in, air accelerates toward your nose and lungs. How does the pressure in your lungs compare with that in the surrounding air as you breathe in?

The pressure in your lungs is less than that of the surrounding air.

Suppose you are ill and have to go to the hospital for shot. When the nurse points the fluid - filled syringe skyward to clear the air out of it, he presses the plunger down but no fluid comes out because the needle is plugged. When the plunger is pressed down and no fluid is moving,

The pressure of the fluid in the needle and much larger syringe are the same

Some clear toys contain two colored liquids. No matter how you tilt one of those toys, one liquid remains above the other. What keeps the upper liquid above the lower liquid?

The upper liquid is less dense than the lower liquid.

A log is much heavier than a stick, yet both of them float in water. Why doesn't the log's greater weight cause it to sink?

The upward buoyant force on the log is greater than its weight. Wood is less dense than water.

Many grocery stores display frozen foods in bins that are open at the top. Why doesn't the warm room air enter the bins and melt the food?

The warm air is blocked from the food by the cold air above the food. This is true because cold air is denser than warm air and so stays close to the food.

For an object that sinks in a fluid,

The weight of fluid displaced is less than its weight.

For an object that is neutrally buoyant in a fluid,

The weight of the fluid displaced equals the object's weight.

When a fish is floating motionless below the surface of a lake, what is the amount and direction of the force the water is exerting on it?

Upward, equal to the weight of the fish. The buoyant force from the water exactly balances the weight of the fish.

An automobile will float on water as long as it doesn't allow water to leak inside. In terms of density, why does admitting water cause the automobile to sink?

Water is much denser than air, so replacing the air inside the car with water increases the car's average density.

Cities have to use water towers to supply water to because

Water pressure at the bottom of the tower is much higher than at the top

Why are failures in dams more likely to occur closer to the bottom of the dam?

Water pressure is higher at the bottom.

For your high school science project you are able to get community support and terraform Mars, so that liquid water could exist on its surface. Suppose you have two cans of soda - one barely floats and one barely sinks in fresh water on Earth. Taking them to Mars, with the gravitational acceleration being 1/3 its value on earth,

We can't say anything until we know if the water on Mars is freshwater or not

Suppose you had a very long straw that could reach from a cup of water on the ground level to the top of a very tall building. When you suck on the straw in order to bring water up,

You could bring the water up to only a certain height in the straw because the atmosphere pushes down on the surface of the water by a given amount.

You are floating along in a hot air balloon. You look up and notice that the bottom of the balloon is open. Neglecting any diffusion of molecules, hot air remains inside the balloon despite this opening because

convection naturally would make hot air rise, so it has no opening through which to escape.

A block of hardwood is floating easily on the surface of a lake. As a storm approaches, the density and pressure of the air above the lake begin to decrease. As a result of this decrease in air pressure and density, the block of wood

moves downward slightly and floats lower in the water.

You are trying to deliver water to a sink in the tree house in your backyard. You run an old hose from the spigot behind your home, across your yard, and up the tree to a sink inside the tree house. You let water fill the hose all the way to the tree house sink and then leave the hose pressurized overnight, with no water flowing in it. Unfortunately, the hose cannot tolerate high pressure anymore and it springs a leak around midnight. By morning your whole backyard is a swamp. The most likely site for the leak is

on the ground—the lowest point on the hose.

When driving down a level smooth road, your car tires flatten at the bottom and they nicely support the weight of your car. However when you run over a small pointed rock the tire is punctured and fails. The rock is able to puncture the tire because

the small surface area of the rock times the pressure in the tire is not enough force to support the car. Thus, the tire fails.