Why Orbits Happen - Middle School Earth and Space Science

1. What formula gives the centripetal force needed for circular motion at speed $v$ and radius $r$?

   $F_c=\frac{mv^2}{r}$. Force needed for circular motion depends on mass, speed squared, and radius.

2. Which orbit requires a greater speed: a smaller radius orbit or a larger radius orbit (same central mass)?

   A smaller radius orbit requires greater speed. Closer orbits need more speed to balance stronger gravity.

3. What provides the centripetal force for the Moon orbiting Earth in the standard model?

   Earth's gravitational force. Earth's gravity provides the inward pull for lunar orbit.

4. What is the name of the minimum speed needed to leave a planet's gravity without returning?

   Escape velocity. Speed where kinetic energy equals gravitational potential energy.

5. Which orbit shape results when an object has the correct speed and gravity but not a perfect circle?

   An ellipse. Non-circular orbits form when speed doesn't match circular requirement.

6. What happens to an orbit if the object suddenly has no forward (tangential) velocity?

   It falls straight toward the body due to gravity. Without sideways motion, only gravity acts, pulling it down.

7. What is the correct model statement linking orbit shape to constant "falling" around a body?

   An orbit is continuous free-fall around the central body. Objects fall toward but miss the body due to tangential velocity.

8. In a planetary orbit, which force provides the centripetal force?

   Gravitational force. Gravity acts as the center-seeking force in orbital motion.

9. Which orbit has a longer period: a smaller-radius orbit or a larger-radius orbit around the same planet?

   A larger-radius orbit has a longer period. Farther orbits move slower, so take longer to complete.

10. Which model describes an orbit as continuous free fall around a body?

    Newton's cannonball (free-fall) model. Objects fall toward Earth but miss due to Earth's curvature.

11. What happens to gravitational force if the distance halves from $r$ to $\frac{r}{2}$?

    It becomes $4$ times as large. Halving distance means $r^2$ becomes $\frac{r^2}{4}$, quadrupling force.

12. Which point in an elliptical orbit has greater speed: periapsis or apoapsis?

    Periapsis has the greater orbital speed. Closer distance means stronger gravity and faster motion.

13. Which law states that an object in motion stays in motion unless acted on by a net force?

    Newton's First Law of Motion. Describes inertia: objects maintain velocity without net force.

14. Identify the correct statement: In a stable orbit, gravity acts mainly as which kind of force?

    A centripetal (inward) force. Gravity provides the inward force needed for circular motion.

15. Which path best describes the motion without gravity: straight line or closed orbit?

    Straight line. Without gravity, inertia causes motion in a straight path.

16. Which diagram best models orbit: (A) straight-line motion, (B) inward fall, or (C) curved path from both?

    Option (C): a curved path from gravity plus forward motion. Orbits result from combining gravity's pull with forward motion.

17. If forward speed is too high for a given distance, what outcome does a model predict?

    The object moves to a higher orbit or escapes. Excess speed overcomes gravity's ability to curve the path enough.

18. What is inertia in the context of orbital motion?

    The tendency to keep moving in a straight line at constant speed. Newton's first law: objects maintain their motion unless acted upon by a force.

19. Which model best represents an orbit: a straight-line path, or a curved path from constant inward pull?

    A curved path caused by constant inward gravitational pull. Gravity continuously bends the straight-line inertial path.

20. Which option correctly explains why astronauts feel weightless while orbiting Earth?

    They are in continuous free-fall with their spacecraft. Both astronauts and spacecraft fall together around Earth.

21. What is inertia in the context of an orbiting moon or planet?

    Tendency to keep moving in a straight line at constant speed. Newton's first law: objects in motion stay in motion unless acted upon.

22. What does a "cannonball orbit" model show when launch speed is increased enough?

    The object keeps falling around Earth instead of hitting it. High speed makes gravity curve the path instead of pulling it down.

23. Which force provides the centripetal force that keeps planets orbiting the Sun?

    Gravity. The Sun's gravitational pull provides the required centripetal force.

24. What is the main force that keeps the Moon in orbit around Earth?

    Gravity between Earth and the Moon. This attractive force pulls the Moon toward Earth continuously.

25. What happens to an orbiting object's velocity direction due to gravity during each moment of motion?

    It is continuously deflected inward toward the central body. Gravity constantly bends the path inward, preventing straight-line motion.

26. What happens to gravitational force if the distance between two bodies doubles from $r$ to $2r$?

    It becomes $\frac{1}{4}$ as large. Force follows inverse square law: $(2r)^2 = 4r^2$ in denominator.

27. What is the formula for centripetal acceleration of an object moving at speed $v$ in a circle of radius $r$?

    $a_c = \frac{v^2}{r}$. Shows how much acceleration is needed to maintain circular motion.

28. Identify the outcome: if a moon's speed is too high for its altitude, what happens over time?

    It moves to a higher orbit or can escape. Excess speed overcomes gravitational binding at that radius.

29. What is the escape velocity formula from distance $r$ from a mass $M$?

    $v_e = \sqrt{\frac{2GM}{r}}$. Factor of $\sqrt{2}$ larger than circular orbital speed at same $r$.

30. What happens to an orbiting object's path if gravity suddenly disappeared?

    It would move in a straight-line path tangent to the orbit. Without gravity, inertia carries objects straight forward.

31. Which model best represents why orbits persist: inward force, outward force, or inertia plus gravity?

    Inertia plus gravity (no outward force is required). Only inward gravity needed; "centrifugal force" is fictitious.

32. Which provides the centripetal force for the Moon orbiting Earth: gravity or Earth's rotation?

    Gravity. Earth's gravity, not its spin, provides the centripetal force.

33. What is the direction of the gravitational force on an orbiting moon relative to its path?

    Toward the center of the larger body. Gravity always pulls radially inward, perpendicular to tangential motion.

34. What is the main force that keeps a planet or moon moving in a curved orbital path?

    Gravity provides the inward (centripetal) force. Without gravity's pull, objects would fly off in straight lines.

35. What force provides the inward (centripetal) pull that keeps planets orbiting the Sun?

    Gravity between the Sun and the planet. This attractive force pulls the planet toward the Sun continuously.

36. What is inertia in the context of a planet or moon moving through space?

    Tendency to keep moving in a straight line at constant speed. Objects resist changes to their motion without external forces.

37. What happens to a planet's orbital speed as it moves farther from the Sun in an elliptical orbit?

    It decreases. Weaker gravity at greater distances results in slower orbital speed.

38. What provides the centripetal force for planets orbiting the Sun and moons orbiting planets?

    Gravitational force. Gravity acts as the centripetal force that curves the path into an orbit.

39. What two effects must balance to keep a planet or moon in a stable orbit?

    Forward inertia and inward gravitational pull. Inertia keeps it moving forward while gravity pulls it inward.

40. What does it mean in a model when an orbit is shown as constant "falling around" a planet?

    Gravity makes continual free-fall that curves the path. Object falls toward planet but forward motion curves the fall.

41. What happens in the no-gravity model if a moving object is released sideways near a planet?

    It travels in a straight line tangent to the orbit. Without gravity, objects continue in straight-line motion.

42. What is centripetal force in an orbital system?

    The inward net force that continually bends the path into a curve. For orbits, gravity provides this center-seeking force.

43. What speed formula for a circular orbit around mass $M$ follows from $G\frac{Mm}{r^2} = m\frac{v^2}{r}$?

    $v = \sqrt{\frac{GM}{r}}$. Solving the force balance for $v$ after canceling $m$ and one $r$.

44. What is the best word for the effect of gravity that continually bends an orbiting object's path?

    Centripetal force. This force constantly changes direction toward the center.

45. Which statement best describes why satellites do not need engines to keep orbiting?

    They keep moving by inertia; gravity continually bends their path. No fuel needed; natural forces maintain the motion.

46. Identify the correct statement: If distance becomes $3r$, gravity becomes what fraction of original?

    It becomes $\frac{1}{9}$ of the original force. Inverse square law: tripling distance reduces force ninefold.

47. What is the correct model statement linking orbit shape to forces on an orbiting body?

    Forward motion plus inward gravity produces a curved orbit. Combines Newton's first law (forward motion) with gravity's pull.

48. Identify the outcome: if a moon's speed is too low for its altitude, what happens over time?

    It spirals inward and may collide with the planet. Insufficient speed means gravity overcomes tangential motion.

49. What is the best model description of an orbit: falling straight down or continuously falling around?

    Continuously falling around the larger body. Objects in orbit fall toward the center but miss due to forward motion.

50. What is the name for the point in an orbit closest to the body being orbited?

    Periapsis (perihelion for Sun, perigee for Earth). Prefix 'peri-' means near; suffix varies by central body.

51. What is inertia in the context of an orbiting planet or moon?

    The tendency to keep moving in a straight line at constant speed. Newton's first law: objects in motion stay in motion unless acted upon.

52. Which change increases gravitational pull more: doubling $m_1$ or doubling distance $r$?

    Doubling $m_1$ increases pull more (doubling $r$ reduces by $4$). Force is linear in mass but inverse square in distance.

53. What is the relationship between gravitational force and distance in the inverse-square model?

    Force decreases as distance increases, about $\frac{1}{r^2}$. Gravity weakens with square of distance in Newton's law.

54. Choose the correct effect: if orbital speed increases at the same radius, what happens to required $F_c$?

    It increases because $F_c\propto v^2$. Centripetal force is proportional to velocity squared.

55. What is the formula for gravitational force between two masses separated by distance $r$?

    $F_g = G\frac{m_1 m_2}{r^2}$. Newton's law of universal gravitation.

56. What is the velocity direction of a moon at any point in its orbit?

    Tangent to the orbital path at that point. Velocity is always perpendicular to the radius at any point.

57. Using $v = \sqrt{\frac{GM}{r}}$, what happens to required orbital speed when $r$ increases?

    Required orbital speed decreases. Speed is proportional to $\frac{1}{\sqrt{r}}$, so larger orbits are slower.

58. What happens to an orbit if an object's forward speed becomes too high at the same distance?

    It moves to a higher orbit or can escape. Excess speed overcomes gravity's ability to curve the path.

59. What does an inward-pointing arrow represent in a simple orbit force diagram?

    Gravity providing centripetal force. Shows the direction and source of the centripetal force.

60. What happens to the orbit size if an object's orbital speed decreases at the same distance?

    It drops to a smaller orbit and may spiral inward. Lower speed can't balance gravity, causing inward spiral.

61. Which option best describes why planets do not fall straight into the Sun?

    They have forward velocity that continually misses the Sun. Their sideways motion prevents direct fall into the Sun.

62. Which statement correctly compares orbital speed at periapsis and apoapsis?

    Speed is higher at periapsis than at apoapsis. Conservation of angular momentum requires faster speed when closer.

63. In a gravity-and-inertia model, what does the tangent arrow represent on an orbit diagram?

    The object's instantaneous velocity (inertia) direction. Shows the direction object would travel if gravity suddenly vanished.

64. What is the relationship between orbital speed and distance in a stable orbit around one body?

    Greater distance generally means lower orbital speed. Farther objects orbit slower due to weaker gravitational pull.

65. What happens if an object has too much tangential speed for its orbital distance?

    It moves to a higher orbit or can escape. Excess speed overcomes gravity's pull at that distance.

66. If forward speed is too low for a given distance, what outcome does a model predict?

    The object spirals inward or crashes. Insufficient speed can't maintain orbit against gravity's pull.

67. What is the name for the point in an orbit farthest from the body being orbited?

    Apoapsis (aphelion for Sun, apogee for Earth). Prefix 'apo-' means away; suffix varies by central body.

68. What two motions form the standard model that explains stable orbits in space?

    Forward motion (inertia) and inward pull (gravity). Balance between straight-line motion and gravitational pull creates curved paths.

69. What happens to gravitational force if the distance changes from $r$ to $2r$?

    It becomes $\frac{1}{4}$ as large. Inverse square law: doubling distance reduces force by $2^2 = 4$.

70. What model describes the path of many planets and moons as a stretched circle?

    An ellipse (elliptical orbit). Most real orbits aren't perfect circles but oval-shaped.

71. Which orbit has stronger gravity: one at distance $r$ or one at distance $2r$ from the same planet?

    At $r$; gravity there is $4$ times stronger than at $2r$. Gravity follows inverse square law: $F \propto \frac{1}{r^2}$.

72. What is the direction of the gravitational force on an orbiting moon relative to its path?

    Toward the center of the orbit (toward the planet). Gravity always pulls toward the massive body creating the orbit.

73. Identify the condition for a circular orbit by setting gravity equal to centripetal force.

    $G\frac{Mm}{r^2} = m\frac{v^2}{r}$. Gravitational force provides the centripetal force for orbit.

74. Which option correctly describes why orbiting objects do not "fall straight down" into the central body?

    They have enough tangential velocity to keep missing it. Sideways motion prevents direct collision despite constant falling.

75. What force provides the centripetal force that keeps planets orbiting the Sun?

    Gravitational force. The Sun's gravity pulls planets inward, providing the needed centripetal force.

76. What is the name of the inward force required for any object to move in a circle or orbit?

    Centripetal force. Any circular motion requires a center-directed force.

77. Which path best describes the motion with gravity only (no forward speed): fall inward or orbit?

    Fall inward toward the larger body. Without forward motion, gravity pulls objects directly inward.

78. What happens to a planet's orbital speed as it moves closer to the Sun in an elliptical orbit?

    It increases. Closer distance means stronger gravity and faster orbital speed.

79. What is Newton's first law idea that helps explain why an orbiting object keeps moving forward?

    Inertia: it continues in straight-line motion unless forced. Objects maintain velocity unless acted upon by external forces.

80. What happens if an object has too little tangential speed for its orbital distance?

    It spirals inward and may collide with the central body. Insufficient speed can't maintain orbit against gravitational pull.

81. Which statement is correct for a stable orbit: $F_g = F_c$ or $F_g > F_c$?

    $F_g = F_c$. Stable orbits require gravitational force to exactly equal centripetal force.

82. What provides the centripetal force for planets orbiting the Sun in the standard model?

    The Sun's gravitational force. Sun's gravity provides the inward pull for planetary orbits.

83. What happens to the orbit size if an object's orbital speed increases at the same distance?

    It moves to a larger orbit or can escape if fast enough. Higher speed needs larger orbit to balance gravity's pull.

84. Which orbit shape is most accurate for planets and moons in the Solar System?

    An ellipse (nearly circular for many orbits). Kepler showed all planetary orbits are elliptical shapes.

85. What happens to gravitational force if the distance changes from $r$ to $3r$?

    It becomes $\frac{1}{9}$ as large. Inverse square law: tripling distance reduces force by $3^2 = 9$.

86. What is the formula for gravitational force between masses $m_1$ and $m_2$ separated by $r$?

    $F_g = G\frac{m_1 m_2}{r^2}$. Newton's law shows force depends on masses and inverse square of distance.

87. What formula gives the gravitational force between two masses separated by distance $r$?

    $F_g=G\frac{m_1m_2}{r^2}$. Newton's law: force depends on masses and inverse square of distance.

88. Which model correctly explains why the Moon does not crash into Earth: no gravity or gravity plus sideways motion?

    Gravity plus sideways (forward) motion. Moon falls toward Earth but forward motion keeps it from hitting.

89. Identify the correct statement about orbital speed when distance from the central body increases.

    Orbital speed decreases as orbital distance increases. Weaker gravity at greater distances requires slower speeds.

90. What is the standard classroom model that shows orbit as "falling around" a planet?

    A projectile model: forward motion plus downward gravity. Objects continuously fall toward but miss the planet due to forward speed.

91. Which model describes an orbit as continuous free-fall toward a planet while moving forward?

    The projectile (Newton's cannon) free-fall model. Newton showed orbits are objects falling while moving forward fast.

92. What is the main force that keeps a planet or moon in orbit around another body?

    Gravity. This attractive force pulls objects toward each other, creating orbital motion.

93. In an elliptical orbit, where is the orbiting object moving fastest: near periapsis or apoapsis?

    Near periapsis. Kepler's law: objects speed up when closer to the central body.

94. What two motions must be balanced for a stable orbit: inward pull and forward motion?

    Gravity inward and tangential (sideways) velocity forward. Orbit occurs when gravity pulls inward while object moves sideways.

95. What happens if an orbiting object loses too much speed while gravity stays the same?

    It spirals inward and may collide with the central body. Insufficient speed can't balance gravity's inward pull.

96. What is the name of the inward-directed acceleration needed to keep an object moving in a circle?

    Centripetal acceleration. This acceleration constantly changes direction to maintain circular motion.

97. Identify what happens if an orbiting object's forward speed becomes too high for its distance.

    It moves outward to a higher orbit or escapes. Excess speed overcomes gravity's pull at that distance.

98. Which option is correct for a stable orbit model: gravity equals what type of force?

    Gravity equals the required centripetal force. Stable orbits occur when gravitational and centripetal forces balance.

99. What two factors must be balanced for a moon or planet to remain in orbit?

    Gravity inward and inertia (forward motion) together maintain orbit. Without both forces, objects would either fall inward or fly away.

100. Which path best models an object with forward speed but no gravity: orbit, straight line, or spiral inward?

     Straight line. Without gravity, objects continue in straight lines per Newton's first law.

101. What is the formula for centripetal acceleration for speed $v$ in a circle of radius $r$?

     $a_c = \frac{v^2}{r}$. Relates circular motion to radius and speed.

102. Choose the correct effect: if orbital radius doubles, what happens to gravitational force $F_g$?

     It becomes $\frac{1}{4}$ as large because $F_g\propto \frac{1}{r^2}$. Gravitational force follows inverse square law with distance.

103. Which free-body diagram best represents a moon in orbit (ignore other forces): one inward arrow or two opposite arrows?

     One inward arrow toward the planet (gravity only). Only gravity acts on orbiting bodies; no outward force exists.

104. What is the correct direction of the gravitational force on an orbiting moon?

     Toward the center of the planet (radially inward). Gravity always pulls toward the center of mass.

105. What happens to an orbit if an object's forward speed becomes too low at the same distance?

     It drops to a lower orbit or falls into the body. Insufficient speed can't maintain orbit against gravity's pull.

106. What happens to an orbit if gravity suddenly became zero while the object is moving?

     It moves in a straight line tangent to the orbit. Without gravity, inertia carries the object straight forward.

107. Identify what happens if an orbiting object suddenly loses most of its forward speed.

     It falls inward toward the larger body. Without sufficient tangential velocity, gravity dominates and pulls object inward.

108. Which direction does gravity act on an orbiting moon relative to the planet it orbits?

     Toward the planet's center. Gravity always pulls toward the center of mass of the attracting body.

109. In a force-arrow model, how should the gravity arrow change as the object moves farther away?

     It should shorten (weaken) with increasing distance. Gravity follows inverse square law, weakening with distance.

110. Identify what happens to orbital speed for a stable circular orbit when orbital radius increases.

     Orbital speed decreases as radius increases. From $v = \sqrt{\frac{GM}{r}}$, larger $r$ means smaller $v$.

111. Identify the best model statement: An orbit is best described as a continuous ______ around a body.

     free-fall. Orbiting objects are in constant free-fall but never hit the surface.

112. What is centripetal force in an orbiting system?

     The inward net force that keeps an object moving in a circle. Any force causing circular motion points toward the center.

113. What is the key idea of the "cannonball" model used to explain stable orbits?

     Forward motion plus gravity produces continuous fall. Objects fall toward Earth while moving forward, creating a curved path.

114. What two influences must balance to produce a stable orbit in the gravity-and-inertia model?

     Gravity inward and inertia (forward motion) outward. Gravity pulls inward while inertia maintains forward motion.

115. Identify the orbit change: if gravity suddenly disappeared, what path would the planet follow next?

     A straight-line path tangent to the orbit. Without gravity, inertia carries object straight forward.

116. What is the formula for gravitational force between two masses separated by distance $r$?

     $F_g = G\frac{m_1 m_2}{r^2}$. Newton's law shows force depends on masses and inverse square of distance.

117. What two factors must be balanced for a planet or moon to stay in a stable orbit?

     Gravity inward and forward (tangential) velocity. Gravity pulls inward while velocity carries the object forward.

118. What is the relationship between orbital speed $v$ and required centripetal acceleration $a_c$ at fixed $r$?

     $a_c$ increases with $v^2$. Doubling speed quadruples the required centripetal acceleration.

119. Which law states that force causes acceleration, often written as $F=ma$?

     Newton's Second Law of Motion. Relates force, mass, and acceleration mathematically.

120. What is the shape of most natural planetary and lunar orbits?

     Elliptical (an ellipse). Planets follow oval paths, not perfect circles.

121. What happens to an orbiting object's path if gravity suddenly disappears?

     It moves in a straight-line tangent path. Without centripetal force, objects continue in straight-line motion.

122. What is the relationship between gravitational force and mass in the gravity model?

     Greater masses produce a stronger gravitational force. More massive objects exert stronger gravitational pull.

123. Identify the correct condition for a circular orbit using forces: what must be true about $F_g$ and $F_c$?

     $F_g=F_c$. Stable orbit requires gravitational force to equal centripetal force.

124. What is the best model-based reason astronauts feel weightless while orbiting Earth?

     They are in continuous free fall with their spacecraft. Both astronauts and spacecraft fall together at the same rate.

125. What is the formula for centripetal force on mass $m$ moving at speed $v$ in radius $r$?

     $F_c = m\frac{v^2}{r}$. Newton's second law $F = ma$ with centripetal acceleration.

126. Identify the correct relationship for a circular orbit: gravity supplies which type of force?

     Centripetal force. Gravity acts as the centripetal force in orbital motion.

127. What model correctly describes orbiting as continuous free-fall around a planet or star?

     An orbit is continuous free-fall with sideways motion. The object falls toward the center while moving sideways.

128. What happens in the no-forward-velocity model if an object is only pulled by gravity?

     It falls straight inward and collides with the planet. Without sideways motion, gravity pulls objects directly down.

129. Which path best models an object with gravity but zero sideways speed: stable orbit or fall inward?

     Fall inward (collision with the central body). Without sideways motion, gravity pulls objects directly downward.

130. What shape is the orbit of a planet around the Sun according to Kepler's First Law?

     An ellipse with the Sun at one focus. Planets follow elliptical paths, not perfect circles.

131. Identify the direction of acceleration for an object in uniform circular orbit.

     Inward, toward the center of the circle. Centripetal acceleration always points to the circle's center.

132. What is the shape of most planetary orbits in our solar system according to Kepler?

     An ellipse. Kepler's first law states planets orbit in elliptical paths.

133. What two factors must be balanced for a planet or moon to remain in a stable orbit?

     Gravity inward and inertia (forward motion) outward. Gravity pulls inward while inertia maintains forward motion.

134. Which option correctly compares gravity at distances $r$ and $2r$ from the same planet?

     At $2r$, gravity is $\frac{1}{4}$ as strong as at $r$. Inverse square law: doubling distance quarters the force.

135. Which variable in $F_g = G\frac{m_1 m_2}{r^2}$ most strongly changes force when distance changes?

     $r$ (force varies with $\frac{1}{r^2}$). Distance appears squared in denominator, making it the dominant factor.

136. What happens if an orbiting object gains too much speed while gravity stays the same?

     It moves to a higher orbit or may escape. Excess speed overcomes gravity's ability to curve the path.

137. What is the formula for centripetal acceleration for speed $v$ in a circular orbit of radius $r$?

     $a_c = \frac{v^2}{r}$. Acceleration toward center depends on speed squared over radius.

138. Which has the stronger gravitational pull: a planet on a moon or a moon on a planet?

     Neither; the forces are equal in magnitude and opposite. Newton's third law: forces are action-reaction pairs.

139. What shape describes most real planetary and lunar orbits according to Kepler's first law?

     An ellipse, with the central body at one focus. Most orbits are slightly oval, not perfect circles.

140. What happens to an orbit if gravity suddenly disappears but the object keeps its speed?

     It travels in a straight line tangent to the orbit. Without gravity's pull, inertia carries it straight forward.

141. What is the name of an orbit that is not a perfect circle but still bound to the central body?

     An elliptical orbit. Oval-shaped orbit where distance from center varies.

142. What is the name of the inward acceleration needed to keep an object moving in a circle?

     Centripetal acceleration. Acceleration directed toward the center enables circular motion.

143. Identify the correct statement about orbital speed in an elliptical orbit.

     It is faster when closer and slower when farther away. Conservation of angular momentum causes speed variations.

144. What force provides the centripetal (inward) acceleration for orbital motion?

     Gravity provides the centripetal force. This inward force continuously changes the object's direction.

145. What is the main force that keeps planets in orbit around the Sun?

     Gravity between the Sun and the planet. This attractive force pulls planets toward the Sun continuously.

146. Which orbit is faster: a smaller-radius orbit or a larger-radius orbit around the same planet?

     A smaller-radius orbit is faster. Closer orbits need higher speeds to balance stronger gravity.

147. What is the main force that keeps planets and moons in orbit around a larger body?

     Gravity. Pulls objects toward each other based on their masses.

148. Which option best explains why the Moon stays near Earth instead of flying away?

     Earth's gravity continually changes the Moon's direction of motion. Gravity acts as centripetal force, curving the Moon's path.

149. What is the name of the inward acceleration required to keep an object moving in a circle?

     Centripetal acceleration. The acceleration that constantly changes direction toward the center.

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