Motion Caused by Gravity - Middle School Physical Science

1. What two factors determine the strength of the gravitational force between two objects?

   The objects' masses and the distance between them. These are the only two variables in Newton's law of universal gravitation.

2. Which option best explains why planets orbit the Sun instead of moving in a straight line?

   Gravity provides centripetal acceleration toward the Sun. This inward force curves their path into elliptical orbits.

3. What is the acceleration of an object in free fall near Earth (ignore air resistance)?

   Approximately $9.8\ \text{m/s}^2$ downward. This value is constant for all objects regardless of mass.

4. What is the definition of gravity in physical science?

   Gravity is the attractive force between masses. All objects with mass attract each other through this fundamental force.

5. What is the acceleration of an object in free fall near Earth (ignoring air resistance)?

   $g \approx 9.8\ \text{m/s}^2$ downward. All objects fall with same acceleration regardless of mass.

6. If distance doubles from $r$ to $2r$, what happens to gravitational force (use inverse square)?

   It becomes $\frac{1}{4}$ of the original force. Since force ∝ $\frac{1}{r^2}$, doubling $r$ gives $\frac{1}{(2r)^2} = \frac{1}{4r^2}$.

7. What is the cause of tides on Earth in terms of gravitational interactions?

   Uneven gravitational pull from the Moon (and the Sun). Water bulges toward and away from the Moon due to differential forces.

8. Which has stronger gravitational pull on you: Earth or the Moon?

   Earth. Earth's mass and proximity create much stronger gravitational field.

9. If distance triples from $r$ to $3r$, what fraction of the original gravitational force remains?

   It becomes $\frac{1}{9}$ of the original force. Since force ∝ $\frac{1}{r^2}$, tripling $r$ gives $\frac{1}{(3r)^2} = \frac{1}{9r^2}$.

10. Which word best describes an object's motion when gravity is the only significant force acting on it?

    Free fall. No other forces act, so only gravity affects motion.

11. What is the formula for weight near Earth's surface?

    $W = mg$. Weight is gravitational force on an object, where $g$ is Earth's surface gravity.

12. Identify the key difference between mass and weight.

    Mass is amount of matter; weight is gravitational force. Mass stays constant everywhere; weight varies with gravity.

13. Choose the correct comparison: On the Moon, an astronaut's mass is $\_\_$ and weight is $\_\_$.

    Mass is the same; weight is smaller. Mass is intrinsic; weight depends on local gravity (Moon's is ~1/6 Earth's).

14. Which location has the greatest gravitational force from Earth: $r$, $2r$, or $3r$ from Earth's center?

    At $r$. Gravitational force decreases with distance squared from center.

15. What is weight, in terms of gravity and mass?

    Weight is the gravitational force on a mass. Measured in newtons (N), not kilograms.

16. What happens to gravitational force when the distance between two objects increases?

    It decreases as distance increases. Follows the inverse square law: force ∝ $\frac{1}{r^2}$.

17. State the inverse-square relationship for gravity with distance $r$.

    Gravitational force is proportional to $\frac{1}{r^2}$. This means force decreases by factor of 4 when distance doubles.

18. Identify the main gravitational cause of ocean tides on Earth.

    The Moon's gravity (with a smaller effect from the Sun). Moon's gravity creates bulges as Earth rotates beneath them.

19. Which option best explains why the Moon stays in orbit around Earth?

    Earth's gravity continually pulls it inward as it moves forward. Without this force, the Moon would fly off in a straight line.

20. What causes a thrown ball to curve downward after it leaves the hand?

    Earth's gravitational force accelerates it downward. Gravity acts continuously on projectiles, creating parabolic paths.

21. Find the weight of a $2\ \text{kg}$ object on Earth using $g = 9.8\ \text{m/s}^2$.

    $19.6\ \text{N}$. Calculate using $W = mg = 2 \times 9.8 = 19.6$ N.

22. What is gravity?

    An attractive force between masses. Gravity pulls all objects with mass toward each other.

23. What happens to an object's weight if $g$ decreases while its mass stays constant?

    Weight decreases. Since $W = mg$, weight is proportional to gravitational acceleration.

24. What is the action–reaction pair when Earth pulls on an object with gravity?

    The object pulls on Earth with equal gravitational force. Newton's third law: forces always come in equal and opposite pairs.

25. Identify the correct statement about gravitational forces between two objects.

    They pull on each other with equal magnitude and opposite direction. Newton's third law applies to gravitational forces.

26. What happens to gravitational force when the mass of one or both objects increases?

    It increases as mass increases. Force is directly proportional to the product of masses.

27. Which falls faster in a vacuum: a $1\ \text{kg}$ rock or a $10\ \text{kg}$ rock?

    They fall at the same rate. In vacuum, all objects accelerate at $g$ regardless of mass.

28. Which quantity stays the same on the Moon: mass or weight?

    Mass stays the same. Mass is intrinsic property; weight depends on local gravity.

29. What happens to $F_g$ if the distance between two objects is cut in half?

    It becomes $4$ times the original. Halving distance means $(\frac{1}{2})^2$ in denominator, so force quadruples.

30. What is the relationship between mass and gravitational force when distance is constant?

    Force is directly proportional to mass. From $F_g = G\frac{m_1m_2}{r^2}$, force increases linearly with either mass.

31. If one object's mass doubles while distance stays the same, what happens to gravitational force?

    It doubles. Force is directly proportional to each mass.

32. What provides the centripetal force that keeps the Moon in orbit around Earth?

    Earth's gravitational force. Gravity provides the inward force needed for circular motion.

33. What is the direction of the gravitational force on an object near Earth?

    Toward Earth's center. Gravity always pulls objects toward the center of mass.

34. State the formula that relates weight $W$, mass $m$, and gravitational field strength $g$.

    $W = mg$. Where $g$ is gravitational field strength in m/s².

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