Hewitt/Lyons/Suchocki/Yeh Conceptual Integrated Science

Copyright © 2007 Pearson Education, Inc., publishing as Pearson Addison Wesley

Hewitt/Lyons/Suchocki/Yeh Conceptual Integrated

Science Chapter 5 GRAVITY


This lecture will help you understand:

• The Legend of the Falling Apple• The Fact of the Falling Moon• Newton’s Law of Universal Gravitation• Gravity and Distance: The Inverse-Square Law• The Universal Gravitational Constant, G• Weight and Weightlessness• Center of Gravity• Gravity Can Be a Centripetal Force• Projectile Motion• Projectile Altitude and Range• The Effect of Air Drag on Projectiles• Fast-Moving Projectiles—Satellites• Elliptical Orbits


The Legend of the Falling Apple

According to legend, while Isaac Newton was sitting under an apple tree pondering the nature of forces, an apple fell and possibly struck his head. He reasoned that the Moon is falling toward the Earth for the same reason the apple falls—both are pulled by Earth’s gravity.


The Fact of the Falling Moon

In ancient times, Aristotle and others believed that stars, planets, and the Moon moved in divine circles, free from forces of Earth.


The Fact of the Falling Moon

The Moon falls around the Earth in the sense that it falls beneath the straight line it would follow if no force acted on it.

The Moon maintains a tangential velocity, which ensures a nearly circular motion around and around the Earth rather than into it. This path is similar to the paths of planets around the Sun.


Newton’s Law of Universal Gravitation

Newton’s law of universal gravitation:Every body in the universe attracts every other body with a mutually attracting force. For two bodies, this force is directly proportional to the product of their masses and inversely proportional to the square of the distance separating them,

F = Gm1 m2

d 2



Newton discovered that gravity is universal.

Every mass pulls on every other mass.


Newton’s most celebrated synthesis is of

A. earthly and heavenly laws.B. weight on Earth and weightlessness in outer space. C. masses and distances.D. the paths of tossed rocks and the paths of satellites.

Newton’s Law of Universal GravitationCHECK YOUR NEIGHBOR


Newton’s most celebrated synthesis is of

A. earthly and heavenly laws.B. weight on Earth and weightlessness in outer space. C. masses and distances.D. the paths of tossed rocks and the paths of satellites.

Comment:This synthesis provided hope that other natural phenomena followed universal laws, and ushered in the Age of Enlightenment.

Newton’s Law of Universal GravitationCHECK YOUR ANSWER



The greater m1 and m2 the greater the

force of attraction between them.

The greater the distance of separation d, the weaker is the force of attraction—weaker as the inverse square of the distance between their centers.

F = Gm1 m2

d 2


Gravity and Distance: The Inverse-Square Law

Inverse-square law:relates the intensity of an effect to the inverse square of the distance from the cause

The greater the distance from Earth, the less the gravitational force on an object.

No matter how great the distance, gravity approaches, but never reaches, zero.

Intensity 1distance2


The force of gravity between two planets depends on their

A. masses and distance apart.B. planetary atmospheres. C. rotational motions.D. all of the above.

Gravity and Distance: The Inverse-Square LawCHECK YOUR NEIGHBOR


The force of gravity between two planets depends on their

A. masses and distance apart.B. planetary atmospheres. C. rotational motions.D. all of the above.

Explanation:The equation for gravitational force,

cites only masses and distances as variables. Rotation and atmospheres are irrelevant.

Gravity and Distance: The Inverse-Square LawCHECK YOUR ANSWER

F = Gm1 m2

d 2


If the masses of two planets are each somehow doubled, the force of gravity between them

A. doubles.B. quadruples. C. reduces by half.D. reduces by one quarter.



If the masses of two planets are each somehow doubled, the force of gravity between them


Explanation:Note that both masses double. Then double double = quadruple.



If the mass of one planet is somehow doubled, the force of gravity between it and a neighboring planet

A. doubles.B. quadruples C. reduces by half.D. reduces by one quarter.



If the mass of one planet is somehow doubled, the force of gravity between it and a neighboring planet


Explanation:Let the equation guide your thinking:

Note that if one mass doubles, the force between them doubles.


F = Gm1 m2

d 2


The Universal Gravitational Constant, G G is the proportionality constant in Newton’s law of

gravitation.G has the same magnitude as the gravitational force

between two 1-kg masses that are 1 meter apart:6.67 10–11 N.

So G = 6.67 10–11 N m2/kg2.

F = 6.67 10–11 N m2/kg2(m1 m2)/d2


The universal gravitational constant, G, which links force to mass and distance, is similar to the familiar constant

A. .B. g. C. acceleration due to gravity.D. speed of uniform motion.

The Universal Gravitational Constant, GCHECK YOUR NEIGHBOR


The universal gravitational constant, G, which links force to mass and distance, is similar to the familiar constant

A. .B. g. C. acceleration due to gravity.D. speed of uniform motion.

Explanation:Just as relates the circumference of a circle to its diameter, G relates force to mass and distance.

The Universal Gravitational Constant, GCHECK YOUR ANSWER


Weight and Weightlessness

Weightis the force exerted against a supporting floor or weighing scale.

Weightlessnessis a condition wherein a support force is lacking—free fall, for example.


Weight and Weightlessness

Example of weightlessness: Astronaut in orbit

An astronaut is weightless because he or she is not supported by anything. The body responds as if gravity forces were absent, and this gives the sensation of weightlessness.


When an elevator accelerates upward, your weight reading on a scale is

A. greater.B. less. C. zero.D. the normal weight.

Weight and WeightlessnessCHECK YOUR NEIGHBOR


When an elevator accelerates upward, your weight reading on a scale is


Explanation:The support force pressing on you is greater, so you weigh more.

Weight and WeightlessnessCHECK YOUR ANSWER


When an elevator accelerates downward, your weight reading is




When an elevator accelerates downward, your weight reading is


Explanation:The support force pressing on you is less, so you weigh less. Question: Would you weigh less in an elevator that moves downward at constant velocity?



When the elevator cable breaks, the elevator falls freely so your weight reading is

A. greater.B. less.C. zero.D. the normal weight.



When the elevator cable breaks, the elevator falls freely so your weight reading is

A. greater.B. less.C. zero.D. the normal weight.

Explanation:There is still a downward gravitational force acting on you, but gravity is not felt as weight because there is no support force, so your weight is zero.



If you weigh yourself in an elevator, you’ll weigh more when the elevator

A. moves upward.B. moves downward. C. accelerates upward.D. all of the above.



If you weigh yourself in an elevator, you’ll weigh more when the elevator

A. moves upward.B. moves downward. C. accelerates upward.D. all of the above.

Explanation:The support provided by the floor of an elevator is the same whether the elevator is at rest or moving at constant velocity. Only accelerated motion affects weight.



Center of Gravity

Center of gravity is the point located at an object’s average position of weight.


Center of GravityThe position of object’s center of gravity relative to a base of support determines the object’s stability.

Rule for stability:• If the CG of an object is above or within the area of support,

the object is stable.• If the CG of an object extends outside the area of support,

the object is unstable—and it will topple.

Example: The Leaning Tower of Pisa lies above its base so it doesn’t topple.


Comparing the CG of women versus men, most women’s CG is

A. slightly higher than in men.B. slightly lower than in men. C. equal to that of men.D. can’t be determined.

Center of GravityCHECK YOUR NEIGHBOR


Comparing the CG of women versus men, most women’s CG is

A. slightly higher than in men.B. slightly lower than in men. C. equal to that of men.D. can’t be determined.

Explanation:Most women, but not all, tend to be slightly larger in the pelvis and smaller in the shoulders.

Center of GravityCHECK YOUR ANSWER


Gravity Can Be a Centripetal Force

Centripetal Forceis any force that causes an object to follow a circular path.

Examples: The Sun pulls its planets in a nearly circular path.It is possible to whirl an empty tin can in a circular path at the end of a string.

To keep the can revolving over your head in a circular path you must keep pulling inward on the string.


Projectile Motion

Projectileis any object that moves through the air or through space under the influence of gravity.

Curved path of projectile (parabola) Example:

A stone thrown horizontally curves downward due to gravity.


Projectile Motion

A projectile curves as a result of two motions:• constant motion

horizontally• accelerated motion

vertically


The velocity of a typical projectile can be represented by horizontal and vertical components. Assuming negligible air resistance, the horizontal component along the path of the projectile

A. increases.B. decreases. C. remains the same.D. not enough information.

Projectile MotionCHECK YOUR NEIGHBOR


The velocity of a typical projectile can be represented by horizontal and vertical components. Assuming negligible air resistance, the horizontal component along the path of the projectile

A. increases.B. decreases. C. remains the same.D. not enough information.

Explanation:Because there is no force horizontally, no horizontal acceleration occurs.

Projectile MotionCHECK YOUR ANSWER


When no air resistance acts on a fast-moving baseball, its acceleration is

A. downward, g.B. due to a combination of constant horizontal motion and

accelerated downward motion. C. opposite to the force of gravity.D. centripetal.

Projectile MotionCHECK YOUR NEIGHBOR


When no air resistance acts on a fast-moving baseball, its acceleration is

A. downward, g.B. due to a combination of constant horizontal motion and

accelerated downward motion. C. opposite to the force of gravity.D. centripetal.

Projectile MotionCHECK YOUR ANSWER


Projectile Altitude and Range

For equal launching speeds, the same range is obtained from two different projection angles—a pair that add up to 90.

Example:The same range occurs for a 75 launch and a 15 launch of the same initial speed.


A ball tossed at an angle of 30 with the horizontal will go as far downrange as one tossed at the same speed at an angle of

A. 45.B. 60. C. 75.D. none of the above.

Projectile Altitude and RangeCHECK YOUR NEIGHBOR


A ball tossed at an angle of 30 with the horizontal will go as far downrange as one tossed at the same speed at an angle of

A. 45.B. 60. C. 75.D. none of the above.

Explanation:Same initial-speed projectiles have the same range when their launching angles add up to 90. Why this is true involves a bit of trigonometry—which in the interest of time, we’ll not pursue here.

Projectile Altitude and RangeCHECK YOUR ANSWER


The Effect of Air Drag on Projectiles

• With air resistance,both range and altitude are decreased.

• Without air resistance,the speed lost going up is the same as the speed gained while coming down.


Fast-Moving Projectiles—Satellites

Satelliteis any projectile moving fast enough to fall continually around the Earth.

To become an Earth satellite, the projectile’s horizontal velocity must be great enough for its trajectory to match Earth’s curvature.


Fast-Moving Projectiles—Satellites

The Earth’s curvature drops a vertical distance of 5 meters for each 8000 m tangent to the surface. So to orbit Earth, a projectile must travel 8000 m in the time it takes to fall 5 m.


When you toss a projectile sideways, it curves as it falls. It will be an Earth satellite if the curve it makes

A. matches the curved surface of Planet Earth.B. results in a straight line. C. spirals out indefinitely.D. none of the above.

Fast-Moving Projectiles—SatellitesCHECK YOUR NEIGHBOR


When you toss a projectile sideways, it curves as it falls. It will be an Earth satellite if the curve it makes

A. matches the curved surface of Planet Earth.B. results in a straight line. C. spirals out indefinitely.D. none of the above.

Explanation:For an 8-km tangent, Earth curves downward 5 m. So, a projectile traveling horizontally at 8 km/s will fall 5 m in that time and follow the curve of the Earth.

Fast-Moving Projectiles—SatellitesCHECK YOUR ANSWER


When a satellite travels at constant speed, the shape of its path is

A. a circle.B. an ellipse. C. an oval that is almost elliptical.D. a circle with a square corner, as seen throughout your book.

Fast-Moving Projectiles—SatellitesCHECK YOUR NEIGHBOR


When a satellite travels at constant speed, the shape of its path is

A. a circle.B. an ellipse. C. an oval that is almost elliptical.D. a circle with a square corner, as seen throughout your book.

Fast-Moving Projectiles—SatellitesCHECK YOUR ANSWER


Elliptical Orbits

EllipseThe oval path followed by a satellite. The closed path taken by a point that moves in such a way that the sum of its distances from two fixed points (called foci) is constant.


Elliptical OrbitsSpeed of a satellite in an elliptical orbit varies.

Near Earth it initially starts greater than 8 km/s and overshoots a circular orbit and travels away from Earth. Gravity slows it down until it no longer moves away from Earth. Then it falls toward Earth gaining the speed it lost in receding. It follows the same oval-shaped path in a repetitious cycle.


The speed of a satellite in an elliptical orbit

A. varies.B. remains constant. C. acts at right angles to its motion.D. all of the above.

Elliptical OrbitsCHECK YOUR NEIGHBOR


The speed of a satellite in an elliptical orbit

A. varies.B. remains constant. C. acts at right angles to its motion.D. all of the above.

Elliptical OrbitsCHECK YOUR ANSWER


Elliptical Orbits

Escape Speed The speed that a projectile, space probe, or similar object must reach in order to escape the gravitational influence of the Earth or of another celestial body to which it is attracted.


Elliptical Orbits

The escape speed of a body is the initial speed given by an initial thrust, after which there is no force to assist motion.

From Earth’s surface, escape speed is 11.2 km/s.


When a projectile achieves escape speed from Earth, it

A. forever leaves Earth’s gravitational field.B. outruns the influence of Earth’s gravity but is never beyond it. C. comes to an eventual stop, eventually returning to Earth at some

future time.D. all of the above.

Elliptical OrbitsCHECK YOUR NEIGHBOR


When a projectile achieves escape speed from Earth, it

A. forever leaves Earth’s gravitational field.B. outruns the influence of Earth’s gravity but is never beyond

it. C. comes to an eventual stop, eventually returning to Earth at some

future time.D. all of the above.

Elliptical OrbitsCHECK YOUR ANSWER

Hewitt/Lyons/Suchocki/Yeh Conceptual Integrated Science

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