An orbit is the circular or elliptical path an object follows around a more massive body because gravitational attraction supplies the centripetal force. On AP Physics 1, you analyze orbits by setting gravity equal to mv²/r and by tracking the system's energy and momentum.
An orbit is the path one object traces around another because of their mutual gravitational attraction. Think of it as falling that never lands. The orbiting object is constantly pulled toward the central body, but it's moving sideways fast enough that the ground keeps curving away beneath it. The result is a closed loop, usually a circle or an ellipse.
In AP Physics 1, the workhorse case is the circular orbit. Gravity is the only force acting, and it points toward the center, so it plays the role of the centripetal force. Setting Newton's law of gravitation equal to the centripetal requirement (GMm/r² = mv²/r) gives you orbital speed, v = √(GM/r). Notice the orbiting object's mass cancels. A pebble and a space station at the same radius move at the same speed. Because gravity is a conservative force, the work it does depends only on the initial and final positions, which is exactly why mechanical energy is conserved as a satellite moves around its orbit.
Orbits sit at the intersection of two big AP Physics 1 ideas. In Unit 3 (Topic 3.2, Fundamental Forces), gravity is your go-to example of a conservative force, and LO 3.2.A asks you to describe the work a force does on a system. An orbiting satellite is the cleanest test case. If it returns to the same point in its orbit, gravity has done zero net work, and in a circular orbit gravity does no work at all because the force is always perpendicular to the motion. In Unit 4 (Topic 4.1), LO 4.1.A asks you to describe the linear momentum of an object or system. A planet-and-moons system far from outside influences is effectively isolated, so its total momentum is conserved even as the individual bodies trade momentum through gravity. Orbits are also where the exam loves to combine concepts, asking you to reason about force, energy, and momentum in a single scenario.
Acceleration due to gravity (Unit 3)
The g you use near Earth's surface is just the gravitational field at one particular radius. An orbiting object still experiences gravitational acceleration, just a smaller value because it's farther from Earth's center. That acceleration is what bends its path into a circle.
Total Mechanical Energy (Unit 3)
Gravity is conservative, so a satellite's kinetic plus gravitational potential energy stays constant around the orbit. In an elliptical orbit, the object speeds up as it falls closer and slows down as it climbs away, trading KE for PE the whole way around.
Escape Velocity (Unit 3)
An orbit is what you get when an object doesn't have enough energy to leave. If total mechanical energy is negative, the object is bound and orbits. Give it speed √(2GM/r) or more and it escapes instead, never coming back.
Isolated System (Unit 4)
A planet and its moons form an isolated system if external forces are negligible, so total linear momentum is conserved. This is exactly the setup of the 2022 long FRQ, where two moons orbit a planet whose mass is comparable enough that you have to treat everything as one system.
Kepler's Laws of Planetary Motion (Unit 3)
Kepler's laws describe what orbits look like (ellipses, equal areas, the period-radius relationship). Newton's gravity explains why. The equal-area law is really just conservation at work, since gravity can't torque the orbiting object around the central body.
Orbit problems show up in both multiple choice and free response, usually as a derivation-plus-reasoning combo. The 2018 short answer FRQ gave you a spacecraft of mass m in a clockwise circular orbit of radius R around Earth and expected you to work from GMm/R² = mv²/R. The 2022 long FRQ put two moons of significant mass in orbit around a planet, pushing you to treat the whole thing as a system and reason about momentum and forces between all three bodies. Typical tasks include deriving orbital speed or period in terms of given variables, predicting what happens to speed or energy if the radius changes, explaining why gravity does no work in a circular orbit, and justifying whether momentum or mechanical energy is conserved for the system you've defined. The mass-cancellation result (orbital speed doesn't depend on the satellite's mass) is a favorite MCQ trap.
Orbital speed and escape velocity answer different questions. Orbital speed, v = √(GM/r), is the exact speed needed to keep circling at radius r, where gravity bends the path into a closed loop. Escape velocity, v = √(2GM/r), is the minimum speed to break free entirely and never return. They differ by a factor of √2, and the physics behind them differs too. Orbital speed comes from a force balance (gravity equals the centripetal requirement), while escape velocity comes from energy (kinetic energy just barely cancels the negative gravitational potential energy). An object in orbit is bound; an object at escape speed is not.
An orbit is the circular or elliptical path an object follows around a massive body, with gravity acting as the centripetal force.
For a circular orbit, set GMm/r² equal to mv²/r to get orbital speed v = √(GM/r), and notice that the orbiting object's mass cancels out.
Gravity is a conservative force, so a satellite's total mechanical energy is conserved, and in a circular orbit gravity does zero work because the force is always perpendicular to the velocity.
A planet and its orbiting moons can be treated as an isolated system, which means their total linear momentum is conserved even as gravity transfers momentum between them.
Orbital speed √(GM/r) and escape velocity √(2GM/r) are different quantities; an orbiting object is gravitationally bound, while an escaping object is not.
In an elliptical orbit, the object moves fastest at its closest approach and slowest at its farthest point, trading kinetic and potential energy back and forth.
An orbit is the path an object follows around a more massive body because gravitational attraction supplies the centripetal force. For the circular orbits AP Physics 1 focuses on, you find the speed by setting GMm/r² = mv²/r, which gives v = √(GM/r).
No, gravity is absolutely acting in orbit. It's the only force on the satellite and the entire reason the orbit exists. Astronauts feel weightless because they and their spacecraft are in continuous free fall together, not because gravity has switched off.
Orbital speed (√(GM/r)) keeps an object circling at radius r, while escape velocity (√(2GM/r)) lets it leave the gravitational pull entirely. Escape velocity is √2 times the circular orbital speed at the same radius, so an orbiting object is always moving too slowly to escape.
In a circular orbit, no. Gravity points toward the center while the velocity is tangent to the circle, so the force is always perpendicular to the displacement and does zero work. In an elliptical orbit gravity does do work along the way, but the net work over one full loop is zero because gravity is a conservative force.
Usually through derivations and system reasoning. The 2018 short answer FRQ asked about a spacecraft in a circular orbit of radius R around Earth, and the 2022 long FRQ involved two moons orbiting a planet, where you had to treat the bodies as a system and apply conservation of momentum. Expect to derive orbital speed, predict how changing the radius changes the motion, and justify which quantities are conserved.