Lesson 3 of 5 · Speed, direction & orbital paths
Falling around Earth
8 minute lesson · 3 checks along the way
Your goal for this lessonPredict why objects starting at the same altitude can hit Earth, remain in orbit, or escape, depending on their initial motion.
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Forward motion and a continuing turn
Imagine an object released high above a spherical Earth with a sideways motion. Without a force, it would continue along a straight path at constant speed. Earth’s gravity continually changes that motion toward Earth’s center. Give the object suitable sideways speed and its path can curve around the planet while remaining above the surface: it is falling around Earth.
In a , speed can stay constant while direction keeps changing. That change of direction is acceleration. Gravity provides the inward force required for the curved motion; it is not canceled by a second outward force in this description. With no atmospheric drag or other disturbances, an orbiting spacecraft does not need an engine running continuously to preserve the circle.
Check 1 of 3
10 pointsUse the reading above. You can try again without losing points.
Work through a decision
Make a prediction before changing speed
The activity holds the starting altitude and sideways direction fixed while changing speed. Its Earth is spherical; the vehicle is a tiny test object; atmosphere, engine burns, and other celestial bodies are omitted. These choices isolate the effect of starting speed. Every preset retains gravity, including the one that escapes. The model is an investigation, not a flight plan for a real rocket.
Predict each outcome before selecting it. A low-speed path can intersect Earth. One particular sideways speed produces a circle at the starting distance. A greater speed below escape can produce a longer whose far end is higher. Sufficient speed gives an open escape path. Look at the whole trajectory: the same starting height can lead to very different futures.
Try a thought experiment
Change the sideways speed
Start a spacecraft one Earth radius above the surface, then turn its engine off. Keep the starting place and direction fixed. What changes when you give it a different sideways speed?
Use the slider or arrow keys, or try a comparison below. “Circular speed” means the speed for a circle at this same starting distance.
Falling in a circle
At this starting distance, circular speed keeps the spacecraft the same distance from Earth’s center. Gravity continually turns its velocity; it does not switch off.
The entire orbit fits in the frame. The solid path begins at the marked starting point and travels upward first.
What stays the same, and what does this model leave out?
The starting distance is always two Earth radii from the center. Earth and the trajectory share one fixed distance scale. The arrow shows the initial direction; its length is illustrative. There is no animation or timing scale.
This is an idealized Newtonian model: a spherical Earth, no atmosphere, no rotation effects, no other gravitating bodies, and no further thrust. Changing a small satellite’s mass would not change its path at the same starting position and velocity. Real mission planning must include the effects this model omits.
Explore the physics: NASA: How orbits work ↗ and NASA: Flight to orbit ↗.
Check 2 of 3
10 pointsUse the reading above. You can try again without losing points.
Altitude is only part of the story
To specify a trajectory, we need a position and a velocity: both speed and direction. A vehicle momentarily far above Earth might be rising, falling, circling, or leaving. Even a suitable speed for a circular orbit must point in the right direction, perpendicular to the radius from Earth’s center. Simply reaching a high point does not establish an orbit.
Escape means the object has enough orbital energy to avoid returning in this isolated two-body model, provided its path avoids the planet. It does not mean gravity has disappeared. As an outward-moving object climbs away, Earth’s gravity still slows it. In the real Solar System, the Sun, Moon, and other bodies also influence the path, so leaving Earth is not leaving all gravity behind.
From a simple orbit to a real mission
A real spacecraft encounters effects missing from our picture. Earth is not a perfectly uniform sphere, and even very thin upper air can gradually alter a low orbit. Thrusters can change speed and direction deliberately. Navigation teams combine observations with models to estimate where the spacecraft is going and decide whether a correction is needed.
A useful explanation should say both what the model predicts and why it might differ from a real flight. If an unpowered satellite’s orbit changes slowly, first investigate neglected forces rather than conclude that gravity has failed. Our simple model teaches the main connection between motion and attraction. The next lesson gives more precise names and patterns to those curved paths.
Check 3 of 3
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Put it together
Your lesson progress
Each check connects to a part of the story. Complete all 3 to earn your 25-point lesson bonus.
Go to the source
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