Forces in Space

What is GRAVITY?

The force of attraction between any two objects.
Gravity is described by the Law of Universal Gravitation: every object in the universe attracts every other object.
Key dependencies:
- Mass of the objects: more mass -> stronger gravitational attraction.
- Distance between the two objects: greater distance -> weaker gravity; shorter distance -> stronger gravity.
Weight is gravity acting on a mass: weight changes with the strength of the gravitational field.

Statement: Every object attracts every other object in the universe.
Dependence on two factors: mass and distance between the objects.
Mathematical formulation (classic description):
F = G\frac{m1 m2}{r^2}
where $F$ is the force of attraction, $G$ is the gravitational constant, $m1$ and $m2$ are the masses, and $r$ is the distance between their centers.
Inverse-square nature: the force falls off with the square of the distance, $F \propto \frac{1}{r^2}$.
Implications: the force is larger for larger masses and shorter distances.

If the distance between two objects increases, gravity decreases.
If the distance between two objects decreases, gravity increases.
Practical takeaway: gravitational interaction weakens rapidly with distance due to the inverse-square law.

If the mass of an object increases, gravity increases.
If the mass of an object decreases, gravity decreases.
Note: the gravitational force depends on both masses; for a given companion mass, increasing the mass of one body strengthens the attraction.

Weight is the force exerted on a mass by gravity.
When you are on the Moon, your weight would be less than on Earth because the Moon’s gravitational field is weaker.
Relationship: $W = m g$, where $W$ is weight, $m$ is mass, and $g$ is the local acceleration due to gravity.
Important distinction: mass stays constant, weight can change with the strength of gravity.

Statement: An object at rest stays at rest and an object in motion stays in motion unless acted on by an outside force (such as friction).
Inertia is directly related to mass: More mass = more inertia; Less mass = less inertia.
Example: If you push a ball, it will continue rolling until friction or another external force stops it.
Relation to gravity: Newton concluded that gravity and inertia work together to produce planetary motions:
- Gravity provides the central attraction that pulls objects toward each other.
- Inertia resists changes to the motion of the objects, keeping them moving.
Specific takeaway: Gravity keeps planets in orbit around the Sun; inertia keeps them moving along their orbital paths.

Gravity is the central force that keeps planets in orbit around the Sun.
Inertia keeps planets moving in their orbits; the combination of gravitational attraction and forward inertia results in stable orbits.
Conceptual picture: an object in orbit is continually falling toward the attractor (e.g., Earth toward the Sun) but has tangential velocity that keeps it from colliding; gravity continually changes the direction of motion to produce an orbit.

1) You travel to the Moon. It is smaller than the Earth (has a smaller mass), therefore gravity decreases. What happens to your weight?

Answer: Decrease
Explanation focus: Weight depends on the local gravitational field; with a weaker gravity on the Moon, the force acting on your mass is smaller, so your weight is reduced accordingly.

2) You are standing up on a bus and the bus suddenly stops. Your body continues in motion forward and you fall. Why did you fall?

Answer: Gravity (with inertia in play)
Explanation: Inertia keeps your body moving forward when the bus stops; gravity pulls you downward, contributing to the fall. The combination of forward inertia and downward gravity leads to the fall.

3) A satellite continues in the motion of orbiting around the Earth. What keeps the satellite from going off into space?

Answers: Gravity keeps it bound; inertia keeps it moving
Explanation: Gravity provides the centripetal pull toward Earth, while the satellite’s forward inertia keeps it moving along its path. The balance between these effects sustains the orbit.

4) If the Earth suddenly moved farther away from the Sun, would the force of the Sun's gravity increase or decrease?

Answer: Decrease
Explanation: Gravitational force between two bodies decreases with increasing distance, per the inverse-square law: $F \propto \frac{1}{r^2}$.

5) Explain how gravity and inertia keep the Earth in orbit around the Sun.

Explanation: Gravity provides the attraction that pulls Earth toward the Sun; inertia (the Earth's forward motion) keeps Earth moving forward. Together, gravity continually redirects the motion, producing a stable orbit rather than a straight line into space.