What Do You Think? For You To Do GOALS

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What Do You Think?

Astronauts on many Shuttle flights study the effects of zero-gravity. Fish taken aboard the Shuttle react to “zero-gravity” by

swimming in circles.

• How would a fish's life be different without gravity?

• Does gravity hold a fish “down” on Earth?

Record your ideas about these questions in your

Active Physics log. Be prepared to discuss your responses with your small group and the class.

For You To Do

1. Place a projector 0.5 m from the

chalkboard. Insert a blank slide. Turn on the projector.

2. Use chalk to trace around the square of light on the board.

GOALS

In this activity you will:

• Explore the relationship between distance of a light source and intensity of light.

• Graph and analyze the relationship between distance of a light source and intensity of light.

• Describe the inverse square pattern.

• Graph and analyze gravity data.

• State Newton’s Law of Universal Gravitation.

• Express Newton’s Law of Universal Gravitation as a mathematical formula.

• Describe dowsing and state why the practice is not considered scientific.

Activity 2 Newton’s Law of Universal Gravitation

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3. Place the photocell in one corner of the light square.

Attach it to the galvanometer as directed by your teacher.

The photocell and galvanometer measure light intensity.

The more light that strikes the cell, the greater the current reading on the galvanometer.

a) Copy the table below in your log. Record the distance to the board, current in galvanometer, and length of a side of the square.

4. Move the projector to a position 1 m from the board. Adjust the projector so that the original square of light sits in one corner of the new square of light.

a) Enter the data into the table in your log.

5. Repeat Step 4 with the projector at distances of 1.5 m, 2 m, 2.5 m, and 3 m.

a) Enter the data into the table in your log.

6. Graph the current in the galvanometer versus distance.

Label this graph Graph 1.

a) Is Graph 1 a straight line?

b) What does a straight line on the graph tell you?

First square of light Second square of light

Galvanometer Photocell

Distance to Distance Current in Side of Area of board (m) squared galvanometers (A) square (cm) square (cm )²

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7. Light intensity decreases with distance as the light from the source spreads out over larger areas. The light is literally spread thin. The light intensity at any one spot increases as the area gets smaller and decreases as it gets larger. This observation is an example of a pattern called the inverse square relation. In an inverse square relation, if you double the distance the light becomes or as bright. If you

triple the distance, the light becomes or as bright.

If you increase the distance by 5 times, the light becomes , or as bright. If you increase the distance by 10 times, the light becomes , or times as bright.

a) How closely does your data reflect an inverse square relation?

8. Compute the distances from the center of the Earth (6400 km below sea level). Plot these distances vs.

acceleration in a graph. Draw the best possible curve through the points on the graph. Label this graph Graph 2.

a) Does the data form a pattern?

b) Is the pattern familiar to you? Give evidence for your conclusion.

1 25 1

5²

1 9 1 3² 1

4

1 10²

1 100

Acceleration Due to

the Earth’s Gravitational Field at Different Heights

Height above Acceleration due

Sea Level (km) to Gravity (m/s

²

)

0 9.81

3.1 9.76

11 9.74

160 9.30

400 (Shuttle orbit) 8.65

1600 6.24

8000 1.92

16,000 0.79

36,000 (geosynchronous orbit for 0.23 communications satellite)

385,000 (orbit of the Moon) 0.003

1 2²

Physics Words

acceleration: the change in velocity per unit time.

gravity: the force of attraction between two bodies due to their masses.

Inverse square relation:

the relationship of a force to the inverse square of the distance from the mass (for gravitational forces) or the charge (for electrostatic forces).

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FOR YOU TO READ An Important Pattern

You’ve seen one pattern in this activity.

But you’ve seen it in two different ways. In Steps 1 through 8 you found that light intensity becomes less as the light source is moved further away. In Step 7, you’ve seen that acceleration due to gravity becomes less as an object moves further from the surface of the Earth. Both are examples of the inverse square relation. Although light is not a force, the effect of distance on its behavior in this activity is like that of the effect of distance on the force of gravity.That is, the behavior of light in this activity is analogous to the behavior of gravity. In simple terms for gravity, the inverse square relation says that the force of gravity between two objects decreases by the square of the distance between them.

Mapping the Earth’s Gravitational Field

In Activity 1, you mapped the magnetic field around a bar magnet using a compass as a probe.You also read about electric fields. In this activity, you used data on acceleration due to gravity to map the Earth’s gravitational field.The probe is the acceleration of a falling mass.To see the pattern of Earth’s gravitational field, you needed data from satellites. The gravitational field changes very slowly near the surface of the Earth.The pattern is very difficult to see using surface data.

Newton’s Law of Universal Gravitation

describes the gravitational attraction of objects for one another. Isaac Newton first recognized that all objects with mass attract all other objects with mass.

Experiments show that objects have mass and that the Earth attracts all objects. Newton reasoned that the Moon must have mass, and that the Earth must also attract the Moon. He calculated the acceleration of the Moon in its orbit and saw that the Earth’s gravity obeyed the inverse square relation. It is a tribute to Newton’s genius that he then guessed that not only the Earth but all bodies with mass attract each other.

Almost 100 years passed before Newton’s idea that all bodies with mass attract all other bodies with mass was supported by experiments.To do so, the very small

gravitational force that small bodies exert on one another had to be measured. Because this force is very small compared to the force of the massive Earth, the experiments were very difficult. But in 1798, Henry Cavendish, a British physicist, finally measured the gravitational force between two masses of a few kilograms each. He used the tiny twist of a quartz fiber caused by the force between two masses to detect and measure the force between them.

Newton’s Law of Universal Gravitation states:

All bodies with mass attract all other bodies with mass.

The force is proportional to the product of the two masses and gets stronger as either mass gets larger.

The force decreases as the square of the distances between the two bodies increases.

➔

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Physics and Dowsing: Comparing Forces

Dowsing is a way some people use to locate underground water. It is claimed to work on an apparent “attraction” between running water and a dowsing rod carried by a person. All dowsers claim to feel a force pulling the rod towards water, and many claim to feel unusual sensations when they cross running water. In the 19th century, many dowsers described the force on the rod as an electric force. No evidence supports this idea. In fact, there is no scientific theory to explain any attraction between running water and a dowsing rod.

Despite the skepticism of the scientific community about dowsing, it is widely used in the United States. Even a national scientific laboratory has used dowsers! But the United States Geological Survey has investigated dowsing and finds no experimental evidence for it. Statistics show that the success rate could be a result of random events. Even if experimental evidence supported the success of dowsing, there is no theory to predict its operation. In order to be accepted as scientific, a phenomenon must be reproducible in careful experiments. Its effects must be predictable by a theory. Also, the theory must give rise to other predictions that can be tested by experiments.

PHYSICS TALK

Newton’s Law of Universal Gravitation in Mathematical Form

Complex laws like Newton’s Law of Universal Gravitation may look easier in mathematical form. Let F

_G

be the force between the bodies, d be the distance between them, m

₁

and m

₂

the masses of the bodies and G be a universal constant equal to 6.67 10

^-11

N

^•

m

²

/kg

²

.

You can express Newton’s Law of Universal Gravitation as

F

_G

=

You can see that the equation says exactly the same thing as the words in a much smaller package.

G m

₁

m

₂

d

²

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Reflecting on the Activity and the Challenge

In this activity you determined experimentally how light intensity varies with distance. By plotting measured data, you found that gravity follows an identical pattern. You detect gravity by measuring the acceleration of objects falling at specific locations. Patterns help you understand the world around you. Light follows the inverse square relation and so does gravity. You can detect gravity with masses. You can detect magnetic fields with compasses. But you cannot detect the

“attraction” claimed by dowsers. There are no detectors for that!

You will be required in the Chapter Challenge to differentiate between the measured gravity and its inverse square nature and the dowser’s claim of measurement. This activity helped you to understand one difference between science and pseudoscience.

Physics To Go

1. How would the light intensity of a beam from a projector 1 m from a wall change if the projector was moved 50 cm closer to the wall?

2. The gravitational force between two asteroids is 500 N. What would the force be if the distance between them doubled?

3. A satellite sitting on the launch pad is one Earth radius away from the center of the Earth (6.4 10

⁶

m).

a) How would the gravitational force between them be changed after launch when the satellite was two Earth radii (1.28 10

⁷

m) from the center of the Earth?

b) What would the gravitational force be if it was 1.92 10

⁷

m from the center of the Earth?

c) What would the gravitational force be if it was 2.56 10

⁷

m from the center of the Earth?

4. Why does everyone trust in gravity?

5. Why doesn’t everyone trust in dowsing?

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6. a) Which is closer to the Moon, the middle of the Earth or the water on the side of Earth facing the Moon?

b) Use your answer to a) to propose an explanation for the uneven distribution of water on Earth’s surface, as shown in the diagram.

c) Suggest an explanation for high tides on the side of the Earth facing the Moon.

Stretching Exercises

1. To locate underground water, a dowser uses a Y-shaped stick or a coat hanger bent into a Y. The dowser holds the Y by its two equal legs with the palms up and elbows close to his or her sides. The long leg of the Y is held horizontal. The dowser walks back and forth across the area he or she is searching.

When he or she crosses water, the stick jerks convulsively and twists so hard that it may break off in the dowser’s hands. Dowsers claim to be unaware of putting any force on the stick. Most observers think that the motion of the stick is probably due to the unconscious action of the dowser.

According to records of those who believe in dowsing, approximately 1 in 10 people should have the ability to dowse. Do you have dowsing ability? Try this activity to find out. Can you prove that you’re a dowser to a classmate? What would constitute proof?