Gravity and Newton Online Test
Here is the test for you with 10 questions and 4 variants of answers for each question, where only one is correct.
Before you take this test, review gravity the way Newton taught the world to see it: not as a special “Earth-only pull,” but as a universal attraction between masses. Apples fall, moons orbit, tides rise, and planets trace ellipses for reasons that can be described with the same idea. Gravity is familiar because you feel it every hour of your life, and strange because it reaches across empty space without any rope you can grab.
Gravity gives you weight, helps keep the atmosphere from drifting away quickly, holds the Moon in orbit, and shapes the architecture of the Solar System. Modern physics has deeper theories of gravity, but Newton’s law of universal gravitation remains the right starting point for school and early college science—accurate enough for orbits, free fall, and the difference between mass and weight. This page is your pre-quiz briefing.
Mass and Weight
Mass measures how much matter an object contains and how strongly it resists acceleration (inertia). The SI unit of mass is the kilogram. Mass does not change when you travel from Earth to the Moon or into orbit. Weight is the gravitational force acting on that mass. Near a planet’s surface, weight has magnitude mg, where g is the local gravitational field strength or free-fall acceleration. Weight is a force, so its SI unit is the newton (N), not the kilogram.
This distinction causes endless confusion because everyday language says “I weigh 70 kilograms.” In physics class, that number is mass; the weight would be about 70 × 9.8 ≈ 686 N on Earth. On the Moon, mass stays 70 kg while weight drops to roughly one-sixth. In orbit, astronauts still have mass, and gravity is still acting, but they feel weightless for a reason explained carefully below. Bathroom scales respond to force but often display a calibrated mass reading for convenience.
- Mass → kilograms; inertia; amount of matter; same on Earth or Moon.
- Weight → newtons; gravitational force; depends on location and local g.
- Scales measure force (normal force) but may show kilograms after calibration.
- In elevators that accelerate, scale readings change even if mass does not.
If a quiz choice says mass becomes zero in space, reject it. If it says weight is measured in kilograms as a force unit, reject it. Precision here is not pedantry; it is the difference between a correct model and a word salad.
Free Fall and g
Near Earth’s surface, if air resistance is ignored, all objects fall with the same acceleration g ≈ 9.8 m/s² downward. A heavy hammer and a light feather dropped together in a vacuum hit the ground together—a result confirmed in classrooms with evacuated tubes and dramatically on the Moon by astronaut David Scott with a hammer and a feather. Air resistance is what makes feathers drift slowly on Earth, not a failure of gravity’s law or a special “lightness force.”
The value of g varies slightly with altitude, latitude, and local geology, but 9.8 m/s² (or sometimes approximated as 10 m/s²) is the standard figure for problems. Moving upward against gravity requires work and increases gravitational potential energy; falling converts that potential energy into kinetic energy. On other worlds, g differs: Mars is about 0.38 times Earth’s surface g; the Moon about 0.16. Science fiction low-gravity scenes are different numbers, not magic.
Newton’s Law of Universal Gravitation
Newton proposed that every two masses attract each other with a force proportional to the product of their masses and inversely proportional to the square of the distance between their centers:
F = G m₁ m₂ / r²
Here G is the universal gravitational constant, a tiny number, which is why you do not notice the gravitational pull between two ordinary books on a desk. Earth is enormously massive, so its pull on you is obvious. Double one mass and the force doubles. Double both masses and the force quadruples. Double the separation distance and the force falls to one-fourth, because of the inverse-square law. That distance dependence is one of the most important patterns in physics and appears across gravity, light intensity, and other inverse-square phenomena.
- More mass → stronger gravity.
- Greater distance → much weaker gravity (inverse square).
- The force is mutual: Earth pulls you, and you pull Earth (equal magnitude, opposite direction; Earth’s huge mass barely accelerates).
- The law is universal: it applies to apples, moons, stars, and galaxies in the classical description.
- G is the same everywhere; local g = GM/R² depends on the planet.
For spherical planets, you can often treat the mass as if it were concentrated at the center when you are outside the planet. Surface gravity relates to mass and radius roughly as g = GM/R². If a planet had Earth’s mass but a larger radius, surface g would be smaller. If it were denser and more compact, surface g would be larger. That is why a more massive planet is not automatically higher g at the surface if it is also much larger in radius.
Orbits: Falling While Missing the Ground
Newton’s thought experiment of a cannon on a high mountain remains the best mental model for orbits. Fire a cannonball slowly and it falls to Earth nearby. Fire it faster and it travels farther before hitting. Fire it fast enough, and as it falls, Earth’s surface curves away at the same rate—the cannonball keeps falling around the world. That is an orbit: continuous free fall with enough sideways speed. No mystical antigravity is required; only gravity plus horizontal velocity.
The International Space Station is not “beyond gravity.” Gravity at its altitude is still a large fraction of surface gravity. Astronauts float because they, and the station, are accelerating together in free fall around Earth. There is no floor pushing up with a normal force the way the ground pushes on you as you stand. “Weightlessness” in orbit is free fall, not zero gravity. The same idea explains why a skydiver feels nearly weightless after jumping (until drag builds) even though gravity still accelerates them.
- Orbit = free fall + sufficient tangential speed.
- Gravity provides the centripetal force for planetary and satellite orbits.
- Higher orbits involve different speeds and periods; orbital mechanics builds on Newton’s law.
- If orbital speed is too low, the path intersects the planet; too high with extra energy can become escape trajectories.
Gravity in the Solar System
The Sun’s gravity dominates the Solar System’s traffic pattern. Planets travel on elliptical orbits with the Sun at one focus (Kepler’s laws, which Newton’s gravity explains). More distant planets feel a weaker solar pull and take longer to complete an orbit. Moons orbit planets for the same reason: gravitational attraction supplies the curved path. Binary stars orbit their common center of mass; galaxies bind stars with gravity across vast distances (with modern cosmology adding dark matter to the full story).
Gravity also raises tides. The Moon’s pull (and the Sun’s to a lesser extent) stretches Earth’s oceans into tidal bulges. As Earth rotates, coastlines move through those bulges and experience high and low tides. Tidal forces appear whenever gravity is stronger on the near side of an object than on the far side—important for moons, rings, and extreme systems like black hole tidal disruption in advanced astrophysics.
Escape velocity is the speed needed to coast away from a body without further thrust, ignoring atmosphere. It depends on mass and radius. Earth’s escape velocity is about 11 km/s from the surface—a reminder that gravity binds us firmly even though we can leap a little with our legs. Rockets do not need to hit escape velocity in one instant if they thrust continuously, but the energy requirement remains large.
Gravity, Weight, and Everyday Experience
Your muscles and bones constantly deal with weight. Sports, architecture, and transportation engineering all assume a roughly constant g. Airplanes and elevators create sensations of heavier or lighter weight when they accelerate, because the normal force from the floor changes. In free fall (a jump from a diving board, a brief drop in a roller coaster), that normal force vanishes and you feel weightless for a moment even though gravity is still accelerating you downward. Feeling and force are related but not identical words.
Planets with different mass and radius would feel different underfoot. On Mars, surface gravity is about 0.38 g; walking would feel springier and falls would hurt less from the same height (though suits and rocks still matter). On a super-Earth with higher g, every step would be harder. Understanding g as GM/R² lets you predict those differences instead of memorizing isolated trivia.
From Newton Toward Einstein (Briefly)
Newton described gravity as a force at a distance. Einstein’s general relativity describes gravity as curvature of spacetime, and it is needed for the highest precision (GPS corrections, Mercury’s perihelion) and for extreme objects like black holes and gravitational waves. For this quiz and for almost all everyday and planetary problems at introductory level, Newton’s law is the correct tool. Knowing that science refined gravity later should increase respect for Newton’s achievement, not discard it. Successful theories are stepping stones with enormous reach; they are not lies simply because a deeper model exists for special cases.
Common Misconceptions
Gravity does not switch off in space. It does not only pull “down” as a mysterious absolute; “down” means toward the center of the local dominant mass. Heavier objects do not fall faster in vacuum. Astronauts are not floating because they are outside Earth’s pull. Gravity is not the same as magnetism: it acts on mass/energy, not only on magnetic materials, and it is always attractive in Newton’s theory. Also, the Moon’s gravity does not “turn off” on the far side of Earth; tides and orbital dynamics are continuous effects.
Another misconception: “gravity is stronger because Earth is magnetic.” Earth’s magnetism is real and useful, but weight is gravitational, not magnetic. Nonmagnetic objects still fall. Keep the interactions separate in your mind.
What the Quiz Will Test
Expect questions contrasting mass and weight, the value of g near Earth, how gravitational force depends on mass and distance, why astronauts feel weightless in orbit (continuous free fall), the newton as the unit of weight-as-force, how a larger radius weakens surface gravity for fixed mass, gravity as the reason planets orbit the Sun, equal free-fall acceleration regardless of mass in vacuum, gravitational potential energy’s dependence on mass, g, and height, and the universality of Newton’s law between any two masses. Watch units: kilograms versus newtons are a classic trap.
When a question mentions location change (Earth vs Moon vs orbit), ask what happens to mass and what happens to weight. When it mentions distance between masses, remember the inverse-square weakening. When it mentions orbits, picture the cannonball that keeps missing the ground. When it mentions free fall, remember that g is independent of mass in vacuum near Earth.
Key Ideas to Remember Before the Quiz
- Mass (kg) ≠ weight (N); weight ≈ mg near a surface.
- g ≈ 9.8 m/s² near Earth; free fall acceleration is the same for all masses in vacuum.
- F = Gm₁m₂/r²; stronger with mass, weaker with distance squared.
- Orbits are continuous free fall with sideways speed; not zero gravity.
- Newton’s gravity is universal between masses in classical physics.
Gravity is the quiet organizer of worlds. It is gentle between people and overwhelming between planets. Learn Newton’s rules, keep mass and weight straight, and you will be ready for the ten questions—and for seeing the sky as a dynamical system rather than a painted ceiling.
Sources: introductory gravitation units in physics curricula; NASA education materials on gravity and orbits; standard secondary and first-year college physics textbooks on Newton’s law of universal gravitation.
