Glossary
Apparent weight
The force an object *feels* or exerts on its support, which can differ from its true gravitational weight due to acceleration.
Example:
When an elevator suddenly accelerates downwards, you experience a decrease in your apparent weight, making you feel lighter.
Apparent weight
The magnitude of the normal force acting on an object, which is what an observer perceives or feels as their weight, and can differ from the actual gravitational force when accelerating.
Example:
When an elevator accelerates downwards, you feel lighter because your apparent weight decreases.
Center of mass
The unique point where the weighted relative position of the distributed mass sums to zero, effectively representing the average position of all the mass in a system.
Example:
For a perfectly uniform planet, its center of mass is located at its geometric center.
Constant gravitational force
A gravitational force that can be considered unchanging because the relative position of the interacting systems changes negligibly over the observed motion.
Example:
When analyzing the trajectory of a baseball thrown across a field, the Earth's constant gravitational force on it is typically assumed.
Directly proportional
A relationship where two quantities increase or decrease together at the same rate, meaning their ratio is constant.
Example:
In F=ma, the force applied to an object is directly proportional to the acceleration it experiences.
Earth's gravitational field strength
The approximate value of gravitational field strength near the Earth's surface, typically taken as 9.8 m/s² or 10 m/s² for calculations.
Example:
When calculating the force required to lift a 5 kg object, we use Earth's gravitational field strength of approximately 9.8 N/kg.
Equivalence Principle
The fundamental idea that the effects of gravity are indistinguishable from the effects of acceleration.
Example:
According to the Equivalence Principle, being in a rocket accelerating at 9.8 m/s² in deep space would feel identical to standing on Earth.
Equivalence principle
States that an observer in a non-inertial (accelerating) reference frame cannot distinguish between apparent weight caused by acceleration and the gravitational force on an object.
Example:
The feeling of being pressed into your seat during rapid acceleration in a car is indistinguishable from an increased gravitational pull, illustrating the equivalence principle.
Fields
A physical quantity that has a value for each point in space, used to describe how non-contact forces affect objects without direct physical contact.
Example:
A magnetic field surrounds a magnet, exerting force on other magnetic materials without touching them.
Gravitational constant (G)
A fundamental physical constant that quantifies the strength of the gravitational force between masses in Newton's Law of Universal Gravitation.
Example:
The value of the gravitational constant (G) is tiny, which is why you don't feel a strong gravitational pull from your textbook.
Gravitational constant (G)
The universal constant of proportionality in Newton's Law of Universal Gravitation, representing the fundamental strength of the gravitational interaction.
Example:
The tiny gravitational force between two students in a classroom can be calculated using the gravitational constant.
Gravitational field model
A conceptual model used to visualize how gravity affects objects in space, predicting their motion without direct contact.
Example:
Using the gravitational field model, physicists can predict the trajectory of a comet as it approaches the Sun.
Gravitational field strength (g)
The gravitational force per unit mass at a specific location, numerically equal to the acceleration due to gravity.
Example:
On the Moon, the gravitational field strength (g) is much lower than on Earth, which is why astronauts can jump so high.
Gravitational field strength (g)
The gravitational force per unit mass at a specific location, numerically equal to the acceleration an object would experience due to gravity at that point.
Example:
On the surface of Mars, the gravitational field strength is significantly less than on Earth, meaning objects weigh less there.
Gravitational force (F)
The attractive force between any two objects possessing mass, as described by Newton's Law of Universal Gravitation.
Example:
The gravitational force keeps you firmly planted on Earth, preventing you from floating off into the atmosphere.
Gravitational mass
A measure of an object's ability to exert and experience gravitational force, determining the strength of its gravitational interaction with other objects.
Example:
The Earth's immense gravitational mass is why it exerts such a strong pull on everything around it.
Gravitational mass
A measure of an object's response to gravitational forces, determining the strength of its gravitational interaction with other objects.
Example:
An object with larger gravitational mass will experience a stronger gravitational pull from a planet.
Inertial mass
A measure of an object's resistance to changes in its state of motion (acceleration), as defined by Newton's Second Law (F=ma).
Example:
A bowling ball has more inertial mass than a tennis ball, making it harder to get moving or stop.
Inertial mass
A measure of an object's resistance to changes in its state of motion (acceleration) when a force is applied, as described by Newton's second law.
Example:
A massive truck has a much greater inertial mass than a bicycle, making it harder to start or stop its motion.
Inversely proportional
A relationship where one quantity increases as the other decreases, often by a factor related to the inverse of the other quantity.
Example:
The brightness of a light source is inversely proportional to the square of the distance from it.
Near-Earth gravity (g ≈ 9.8 N/kg or m/s²)
The approximate constant value of gravitational field strength and acceleration due to gravity near the Earth's surface.
Example:
When calculating the force needed to lift a book, we typically use near-Earth gravity (g ≈ 9.8 N/kg or m/s²) for simplicity.
Newton's Law of Universal Gravitation
The force of attraction between any two objects with mass, directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers.
Example:
The Newton's Law of Universal Gravitation explains why the Moon orbits Earth and doesn't just float away into space.
Newton's Law of Universal Gravitation
This fundamental law states that the gravitational force between two objects is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers.
Example:
The force keeping a satellite in orbit around Earth is described by Newton's Law of Universal Gravitation.
Newton's shell theorem
A theorem stating that a uniform spherical shell of mass exerts no net gravitational force on an object inside it, and acts as if all its mass is concentrated at its center for objects outside.
Example:
If you were inside a hollow planet, you would float freely with no net gravitational pull from the planet's mass, as predicted by Newton's shell theorem.
Normal force
The force exerted by a surface that supports an object, acting perpendicular to the surface.
Example:
When you stand on a scale, the reading you see is the normal force exerted by the scale on your feet.
Weight
The gravitational force exerted on an object by a celestial body, calculated as the product of the object's mass and the local gravitational field strength.
Example:
Your weight would be significantly less on Mars than on Earth, even though your mass remains the same.
Weight
The gravitational force exerted by a large astronomical body on a smaller nearby object, calculated as the product of the object's mass and the local gravitational field strength.
Example:
A 70 kg astronaut has a different weight on the Moon compared to Earth, even though their mass remains the same.
Weightlessness
A condition where an object feels like it has no weight, typically occurring when gravity is the only force acting on it (free fall) or when there are no forces acting on it.
Example:
Astronauts in orbit experience weightlessness because they are continuously falling around the Earth.
Weightlessness
The condition where an object experiences no normal force, often occurring when gravity is the only force acting on it, such as during freefall.
Example:
Astronauts in orbit around Earth experience weightlessness because they are continuously falling around the planet.