## Concept explanation **Gravity** is a force that always pulls one mass toward another mass. In this model, the Moon feels Earth's gravity at every position around Earth, so the pull always points inward toward Earth's center. Even when the Moon moves to a new location, the direction of the force changes so that it still aims at the center, and the arrow length shifts slightly to suggest that the pull depends on **distance**. ## What you see You are looking at Earth in the middle and the Moon around it. The teal arrow starts at the Moon and points toward Earth's center, showing the direction of the gravitational force. A faint guide line helps you see exactly where the center is, and the readout changes as the Moon's position changes. ## Try it yourself - **Drag the Moon** to the top, bottom, left, and right of Earth, and notice that the arrow always turns to point inward. - **Place the Moon closer to Earth** and watch how the arrow becomes a little longer. - **Move the Moon farther away** and see the arrow shorten slightly while still aiming at the center. - **Adjust the Moon distance slider** to reposition the Moon while keeping the same inward force idea. - **Adjust the arrow size slider** to make the force arrow easier to compare. - **Press `Reset Moon`** to return the Moon to its starting position and test the pattern again. ## Concept explanation **Gravity** pulls the object inward toward Earth, but the object's **sideways motion** keeps carrying it forward at the same time. The actual path comes from both effects together: if sideways speed is too small, gravity wins quickly and the object falls in; if the sideways speed is just right, the object keeps missing Earth and curves around it; if sideways speed is larger, gravity still bends the motion inward, but the path becomes a much wider arc. ## What you see Earth sits at the center, the Moon-like object starts off to one side, and two arrows show the competing influences on its motion. The teal arrow shows the object's forward, tangent direction, while the red arrow points toward Earth to show gravity. The blue curve is the predicted path for the current slider value, and the moving dot follows that path so you can see how the same inward pull leads to very different outcomes at different sideways speeds. ## Try it yourself - **Drag the sideways speed slider all the way down** and notice how the object quickly bends inward and hits Earth. - **Move the slider to the middle range** and watch the path wrap around Earth, showing an orbit-like curve. - **Push the slider higher** and compare how gravity still turns the path inward, but over a much broader arc. - **Sweep the slider slowly from low to high** to see the transition from falling, to curving around, to taking a wide path. ## Concept explanation An **orbit** happens when an object is pulled inward by **gravity** while also moving sideways fast enough that it keeps missing the surface below it. You can think of the Moon as constantly **falling** toward Earth, but because it has forward motion at the same time, its path keeps curving around Earth instead of crashing straight down. ## What you see You are looking at Earth in the center and the Moon moving around it along a curved path. The short trail shows the Moon’s recent motion, while the teal arrow always points inward to show gravity pulling toward Earth. The pale tangent arrow shows the Moon’s sideways motion, and together these make the path bend smoothly without the Moon getting closer to Earth’s surface. ## Try it yourself - **Press the play/pause button** and watch how the Moon keeps moving while the gravity arrow always points toward Earth. - **Pause the motion** at different points in the orbit and notice that gravity is still directed inward everywhere. - **Adjust the orbit speed slider** to see how faster sideways motion changes how quickly the Moon sweeps around Earth. - **Change the trail length slider** to make the recent curved path easier to compare with the Moon’s current direction. - **Press reset** and replay the motion while focusing on this idea: the Moon is always falling inward, but its sideways motion keeps it in orbit. ## Concept explanation Under **gravity**, an object near Earth is always pulled inward, but its starting **sideways speed** decides what happens next. If that speed is too small, the object curves inward and crashes. If the speed is in the right range, gravity keeps bending the path while the object keeps missing Earth, creating an **orbit**. If the speed is too large, the path stays open and the object escapes instead of circling back. That is why the Moon does not fall straight down: it is continually falling toward Earth while moving sideways fast enough to keep missing it. ## What you see Earth sits at the center, and the Moon-like object always starts from the same launch point on the left. The three speed choices let you compare outcomes from the same distance: the red path falls inward, the teal path wraps around into orbit, and the gold path swings away on an open curve. The faint circular ring marks the starting distance, so you can focus on how changing speed alone changes the motion. ## Try it yourself - **Click `Too slow`** and watch how the path bends sharply inward until it hits Earth. - **Click `Orbit speed`** and notice how the object keeps falling toward Earth without colliding, because its sideways motion keeps carrying it around. - **Click `Very fast`** and see how the curve opens up instead of closing into a loop. - **Toggle `Show guide paths`** to compare the three outcomes side by side before or after launching one. - **Press `Reset`** and replay the same case, paying attention to how the starting point stays fixed while only the speed changes. ## Concept explanation An orbit happens because **gravity** pulls the Moon inward while its sideways motion keeps it from falling straight into Earth. When the Moon starts at a smaller **distance** from Earth, gravity is stronger, so the orbit can be tighter and the Moon moves around more quickly. When the Moon starts farther away, gravity is weaker, so the path is wider and the trip around Earth takes longer. ## What you see You are looking at Earth in the center of the scene, a dashed circular orbital path, and a Moon-like object that can travel along that path after you start the animation. The teal arrow always points inward to show gravity pulling toward Earth, while the readout card summarizes how the chosen distance changes the orbit’s size, relative speed, and approximate period. ## Try it yourself - **Move the distance slider** to a small value and notice how the dashed orbit shrinks before you start. - **Press Start orbit** and watch the Moon-like object sweep around Earth faster in the close orbit. - **Reset the animation**, then **move the slider farther out** and compare the larger orbit to the smaller one. - **Press Start orbit again** and observe that the inward gravity arrow is still present, even though the motion is slower and the orbital path is wider. - **Compare the readout values** for `Radius`, `Relative speed`, and `Approx. period` to connect what you see with how orbital behavior changes with distance. ## Concept explanation An **orbit** happens when **gravity** keeps pulling an object inward while the object already has enough **forward motion** to keep missing the planet. Instead of falling straight down, it continuously falls around Earth. If the sideways speed is too small, gravity bends the path into a crash. If the sideways speed is too large, the object curves away and escapes. The Moon stays in orbit because its distance from Earth and its forward speed are balanced. ## What you see Earth is fixed at the center of the main view, and the smaller object starts at a chosen distance with a teal arrow showing its sideways launch speed. When you launch, the glowing path traces the object's motion so you can see whether the path tightens into a collision, settles into a repeating orbit, or opens outward into escape. The right-hand control panel lets you set the starting distance and sideways speed before each test. ## Try it yourself - **Set the distance slider** to move the object farther from or closer to Earth, then notice how the same speed behaves differently. - **Adjust the sideways speed slider** and compare what happens when the launch is too slow, near the balance point, or too fast. - **Click Launch** to test your settings and watch the path reveal `Crash`, `Stable orbit`, or `Escape`. - **Drag the object** to a new starting position around Earth, then **launch again** to see that the direction of motion stays sideways to the radius. - **Use Reset**, then **try to find a stable orbit** by tuning both controls until the path loops around Earth instead of hitting it or flying away.