402 Unit 3: Motion and Forces

EXPLORE Forces in Liquid

How does water affect weight?

PROCEDURE

Tie a piece of string to the middle of the pencil. Tie 4 paper clips to each end of the pencil as shown.

Move the middle string along the pencil until the paper clips are balanced and the pencil hangs flat.

While keeping the pencil balanced, slowly lowerthe paper clips on one end of the pencil into thewater. Observe what happens.

WHAT DO YOU THINK?• How did the water affect the balance between

the two sets of paper clips?• Did the water exert a force on the paper clips? Explain.

3

2

1

MATERIALS• 3 pieces of

string• pencil• 8 paper clips• cup full of

water

VOCABULARY

buoyant force p. 402Bernoulli’s principle p. 404Pascal’s principle p. 406

BEFORE, you learned

• Pressure depends on force and area

• Pressure acts in all directions in fluids

• Density is mass divided by volume

NOW, you will learn

• How fluids apply forces to objects

• How the motion of a fluidaffects the pressure it exerts

• How forces are transmittedthrough fluids

KEY CONCEPT

Fluids can exert a force on objects.

Fluids can exert an upward force on objects. If you drop an ice cube in air, it falls to the floor. If you drop the icecube into water, it may sink a little at first, but the cube quickly risesupward until it floats. You know that gravity is pulling downward onthe ice, even when it is in the water. If the ice cube is not sinking, theremust be some force balancing gravity that is pushing upward on it.

The upward force on objects in a fluid is called or buoyancy. Buoyancy is why ice floats in water. Because of buoyantforce, objects seem lighter in water. For example, it is easier to lift aheavy rock in water than on land because the buoyant force pushesupward on the rock, reducing the net force you need to lift it.

buoyant force,

VOCABULARYCreate a four square diagram for buoyant force.

Chapter 12: Gravity, Friction, and Pressure 403

BuoyancyThe photograph on the right shows a balloon that has been pushedinto a beaker of water. Remember that in a fluid, pressure increaseswith depth. This means that there is greater pressure acting on thebottom of the balloon than on the top of it. The pressure differencebetween the top and bottom of the balloon produces a net forcethat is pushing the balloon upward.

When you push a balloon underwater, the water level risesbecause the water and the balloon cannot be in the same place at the same time. The volume of the water has not changed,but some of the water has been displaced, or moved, by the bal-loon. The volume of the displaced water is equal to the volume of the balloon. The buoyant force on the balloon is equal to theweight of the displaced water. A deflated balloon would displaceless water and would therefore have a smaller buoyant force on it.

check your reading Why does increasing the volume of an object increase the buoyant force on it when it is in a fluid?

Density and BuoyancyWhether or not an object floats in a fluid depends on the densities ofboth the object and the fluid. Density is a measure of the amount ofmatter packed into a unit volume. The density of an object is equal toits mass divided by its volume, and is commonly measured in gramsper cubic centimeter (g/cm3).

If an object is less dense than the fluid it is in, the fluid the objectdisplaces can weigh more than the object. A wooden ball that ispushed underwater, as in the beaker below and on the left, rises to thetop and floats. An object rising in a liquid has a buoyant force actingupon it that is greater than its own weight. If an object is floating in aliquid, the buoyant force is balancing the weight.

If the object is more dense thanthe fluid it is in, the object weighsmore than the fluid it displaces.A glass marble placed in the beaker on the far right sinks to the bottombecause glass is denser than water.The weight of the water the marbledisplaces is less than the weight ofthe marble. A sinking object has aweight that is greater than the buoyant force on it.

no netforce

buoyantforce

weight

net force

buoyantforce

weight

net force

reading tip

Remember that both air andwater are fluids, and waterhas a greater density thanair. Therefore, water has agreater buoyant force.

404 Unit 3: Motion and Forces

Bernoulli’s Principle

The motion of a fluid affects its pressure.The motion of a fluid affects the amount of pressure it exerts.A faster-moving fluid exerts less pressure as it flows over the surface of an object than a slower moving fluid. For example, wind blowingover a chimney top decreases the pressure at the top of the chimney.The faster air has less pressure than the slower-moving air in thefireplace. The increased pressure difference more effectively pulls the smoke from a fire out of the fireplace and up the chimney.

Bernoulli’s PrincipleBernoulli’s principle, named after Daniel Bernoulli (buhr-NOO-lee),a Swiss mathematician who lived in the 1700s, describes the effects offluid motion on pressure. In general, says that an increase in the speed of the motion of a fluid decreases thepressure within the fluid. The faster a fluid moves, the less pressure it exerts on surfaces or openings it flows over.

check your reading What is the relationship between the speed of a fluid and thepressure that the fluid exerts?

Bernoulli’s principle

How does the speed of air affect air pressure?PROCEDURE

Use the pen to mark off intervals of 1 cm along the length of one of the straws.

Put a drop of food coloring in the cup of water and stir it. Place the markedstraw into the cup and hold it upright so that the water level in the straw isat one of the marks. The straw should not touch the bottom of the cup.

Position the second straw as shown. Blow across the open end of themarked straw. Observe the level of the water in the marked straw as you blow.

Blow harder and then softer. Observe the water level as you change thespeed of the air.

WHAT DO YOU THINK?• What happened to the water in the straw as you blew?

• How did the speed of the air relate to the changes you observed?

CHALLENGE What results would you expect if you blew over the top of a tube with a closed bottom instead of the straw? Explain.

4

3

2

1

Bernoulli’s PrincipleSKILL FOCUSObserving

MATERIALS• pen• ruler• two clear straws• clear plastic cup

filled with water• food coloring

TIME15 minutes

Chapter 12: Gravity, Friction, and Pressure 405

Applying Bernoulli’s PrincipleBernoulli’s principle has many applications. One important applicationis used in airplanes. Airplane wings can be shaped to take advantageof Bernoulli’s principle. Certain wing shapes cause the air flowingover the top of the wing to move faster than the air flowing under the wing. Such a design improves the lifting force on a flying airplane.

Many racecars, however, have a device on the rear of the car thathas the reverse effect. The device is designed like an upside-down airplane wing. This shape increases the pressure on the top of the car.The car is pressed downward on the road, which increases frictionbetween the tires and the road. With more friction, the car is less likelyto skid as it goes around curves at high speeds.

A prairie-dog colony also shows Bernoulli’s principle in action.The mounds that prairie dogs build over some entrances to theirburrows help to keep the burrows well-ventilated.

Air closer to the ground tends to move at slower speeds than airhigher up. The air over an entrance at ground level generally moves slower than the air over an entrance in a raised mound.

The increased speed of the air over a raised mound entrancedecreases the pressure over that opening.

The greater air pressure over a ground-level entrance produces anunbalanced force that pushes air through the tunnels and out thehigher mound entrance.

3

2

1

Bernoulli’s principle explains why having two entrances at different heights helps ventilate a prairie-dog burrow.

Bernoulli’s Principle in Nature

1Air moves moreslowly near the ground.

3

The pressure differencebetween the twoentrances moves airthrough the tunnel.

2

The air over the raised entrancemoves faster and has less pressure than the slower-movingair near the ground.

Forces can be transmitted through fluids.Imagine you have a bottle full of water. You place the bottle cap on it,but you do not tighten the cap. You give the bottle a hard squeeze andthe cap falls off. How was the force you put on the bottle transferredto the bottle cap?

Pascal’s PrincipleIn the 1600s Blaise Pascal (pa-SKAL), a French scientist for whom the unit of measure called the pascal was named, experimented withfluids in containers. One of his key discoveries is called Pascal’s principle. states that when an outside pressure isapplied at any point to a fluid in a container, that pressure is transmitted throughout the fluid with equal strength.

You can use Pascal’s principle to transmit a force through a fluid.Some car jacks lift cars using Pascal’s principle. These jacks containliquids that transmit and increase the force that you apply.

The part of the jack that moves down and pushes on the liquid iscalled a piston. As you push down on the piston, you increase thepressure on the liquid.

The increase in pressure is equal to your applied force divided bythe area of the downward-pushing piston. This increase in pressureis transmitted throughout the liquid.

2

1

Pascal’s principle

406 Unit 3: Motion and Forces

Pascal’s Principle

The pressure from the smaller piston is equal to the pressurepushing up the larger one. The large piston can exert moreforce because of its greater area.

The increase in pressureis transmitted throughoutthe liquid.

2

You apply a downwardforce, which increasespressure on the liquid.

1

The pressure increaseacts on a larger area toproduce a greater force,pushing the car up.

3

small area liquid

large area

Chapter 12: Gravity, Friction, and Pressure 407

The hydraulic arm on thegarbage truck lifts andempties trash cans.

The increased pressure pushes upward on another piston, whichraises the car. This piston has a large area compared with the firstpiston, so the upward force is greater than the downward force.A large enough area produces the force needed to lift a car.However, the larger piston does not move upward as far as thesmaller one moved downward.

check your reading Describe how pressure is transmitted through a fluid.

HydraulicsMachines that use liquids to transmit or increase a force are called hydraulic(hy-DRAW-lihk) machines. The advan-tage to using a liquid instead of a gas isthat when you squeeze a liquid, its volumedoes not change much. The molecules ina liquid are so close together that it ishard to push the molecules any closer.Gas molecules, however, have a lot ofspace between them. If you apply pressureto a gas, you decrease its volume.

Although hydraulic systems are used in large machines such asgarbage trucks, research is being done on using hydraulics on a muchsmaller scale. Researchers are developing a storage chip similar to acomputer chip that uses hydraulics rather than electronics. This chipuses pipes and pumps to move fluid into specific chambers on a rubber chip. Researchers hope that a hydraulic chip system will eventually allow scientists to use a single hand-held device to performchemical experiments with over a thousand different liquids.

3

KEY CONCEPTS1. Why is there an upward force

on objects in water?

2. How does changing the speedof a fluid affect its pressure?

3. If you push a cork into the neckof a bottle filled with air, whathappens to the pressure insidethe bottle?

CRITICAL THINKING4. Infer Ebony is a dark wood

that has a density of 1.2g/cm3. Water has a density of1.0 g/cm3. Will a block ofebony float in water? Explain.

5. Analyze When you use aspray bottle, you force air overa small tube inside the bottle.Explain why the liquid insidethe bottle comes out.

CHALLENGE6. Synthesize If you apply a

force of 20 N downward on acar jack piston with an area of2.5 cm2, what force will beapplied to the upward pistonif it has an area of 400 cm2?Hint: Remember that pressureequals force divided by area.