In AP Physics C: Mechanics, weight is the gravitational force an astronomical body (like Earth) exerts on a nearby object, calculated as W = mg, where m is the object's mass and g is the local gravitational field strength. Weight is a force measured in newtons, not a property of the object itself.
Weight is the gravitational force pulling an object toward an astronomical body. Near Earth's surface, you compute it as W = mg, where g ≈ 9.8 N/kg is the local gravitational field strength. The big idea hiding in that formula is that weight is not a fixed property of an object. Mass is. Weight depends on where you are. The same 70 kg person weighs about 686 N on Earth, much less on the Moon, and a different amount at high altitude, because g changes while m doesn't.
In Physics C, weight is really a special case of Newton's universal gravitation. The force GMm/r² between Earth and a small object near its surface is what we casually call weight, and g itself is just GM/r² evaluated at Earth's surface. That's why Topic 2.6 treats weight, the gravitational field, and universal gravitation as one connected story. On free-body diagrams, weight always points toward the center of the attracting body and always equals mg, no matter what else is happening to the object.
Weight lives in Topic 2.6 (Gravitational Force), but it's the most-used force in all of Unit 2. Nearly every dynamics problem starts with drawing mg on a free-body diagram, so getting weight right is the price of admission for Newton's second law problems. The concept also forces you to separate two ideas the exam loves to test together. The true weight mg never changes just because an object accelerates, but the scale reading (the normal force, or apparent weight) absolutely does. Elevators, inclines, and orbiting satellites all exploit that distinction. Weight also bridges into gravitation later in the course, where g stops being a constant 9.8 and becomes a field GM/r² that varies with distance.
Keep studying AP® Physics C: Mechanics Unit 2
Apparent Weight (Unit 2)
Apparent weight is what a scale reads, which is the normal force, not gravity. In an elevator accelerating upward at 3.0 m/s², the scale reads more than mg even though your true weight hasn't changed at all. True weight is about the gravitational field; apparent weight is about your acceleration.
Gravitational Field (Unit 2)
The g in W = mg is the gravitational field strength, and it's only 9.8 N/kg near Earth's surface. Far from Earth, g = GM/r², so weight shrinks with distance. This is how the surface formula W = mg connects to Newton's universal gravitation.
Weightlessness (Unit 2)
An astronaut in orbit still has weight, since Earth's gravity is what keeps the satellite in orbit at all. What they lack is apparent weight, because they're in free fall and nothing pushes back on them. "Weightless" really means "normal force equals zero."
Inertial Mass and the Equivalence Principle (Unit 2)
The mass in W = mg (gravitational mass) and the mass in F = ma (inertial mass) are experimentally the same. That equivalence is why every object falls with the same acceleration g regardless of how heavy it is.
Weight shows up constantly as the mg term in free-body diagrams and Newton's second law setups. On released FRQs, you decompose weight into components mg sin θ and mg cos θ on inclined planes (2017 FRQ Q2 and the rolling cylinder in 2017 FRQ Q3), and you balance weight against string tension in Atwood's machine problems (2017 FRQ Q1). Multiple-choice questions love the weight versus apparent weight trap. Classic stems include a passenger on a scale in an accelerating elevator (the scale reads the true weight only when acceleration is zero), the apparent weight perpendicular to a frictionless incline (mg cos θ, not mg), and a satellite in elliptical orbit, where the gap between apparent weight and gravitational force depends on the orbital acceleration. The move the exam rewards is simple. Always draw mg first, then solve for the normal force separately instead of assuming N = mg.
True weight is the gravitational force mg, and it depends only on your mass and the local field. Apparent weight is the normal force a surface (like a scale) pushes back with, and it depends on your acceleration. In an elevator accelerating upward, apparent weight exceeds mg; in free fall, apparent weight drops to zero while true weight stays exactly mg. If a problem mentions a scale reading, it's asking about apparent weight, which means you solve N from Newton's second law rather than writing N = mg.
Weight is the gravitational force on an object, W = mg, measured in newtons, and it points toward the center of the attracting body.
Mass is an intrinsic property of an object, but weight changes with location because the gravitational field strength g changes.
Near Earth's surface g is approximately 9.8 N/kg, but in general g = GM/r², so weight decreases as you move away from Earth.
A scale measures the normal force (apparent weight), not true weight, so the reading only equals mg when the object has zero vertical acceleration.
Orbiting astronauts are not actually weightless; gravity still acts on them, but they feel weightless because they are in free fall with zero normal force.
On inclines, break weight into components along the slope (mg sin θ) and perpendicular to it (mg cos θ) before applying Newton's second law.
Weight is the gravitational force an astronomical body exerts on a nearby object, calculated as W = mg, where g is the local gravitational field strength (about 9.8 N/kg at Earth's surface). It's a force in newtons, covered in Topic 2.6.
No. Mass (in kg) is an intrinsic property that never changes, while weight (in N) is the gravitational force mg, which depends on where you are. A 70 kg person has the same mass everywhere but weighs about 686 N on Earth and far less on the Moon.
Weight is the gravitational force mg, which doesn't care about your motion. Apparent weight is the normal force from a scale or surface, which changes when you accelerate. In an elevator accelerating upward at 3.0 m/s², your apparent weight rises to about 1.3 times your true weight, but your true weight never moves.
No. Earth's gravity still pulls on orbiting astronauts; that gravitational pull is exactly what keeps them in orbit. They experience weightlessness because they're in continuous free fall, so the normal force on them is zero, but their true weight mg is not.
Only when your vertical acceleration is zero, meaning the elevator (or whatever you're standing in) is at rest or moving at constant velocity. Any upward acceleration makes the scale read more than mg, and any downward acceleration makes it read less.
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