Gravity and Weight Lecture Review

Definition and Capacity of Force: * A force has the capacity to change the motion of an object. * A force can also change the shape of an object.
Interaction Mechanics: * One object does not need to physically touch another object in order to exert a force on it (non-contact forces).
Representational Standards: * The size and direction of a force are represented by an arrow.

General Definition: * Gravity is a non-contact force that pulls objects toward one another. * It is specifically defined as the force by which a planet or other celestial body draws objects toward its geometric centre.
Universal Scope: * Every object in the universe pulls on every other object with a force of gravity.
Functional Example: Earth and Moon: * The gravitational attraction between Earth and the moon is what keeps the moon in its orbit. * Without this gravity, the moon would fly past the Earth instead of circling it. * Gravity exists on the moon, but it is significantly smaller than Earth's gravity.

Definition: * Mass is a measure of the amount of material or substance contained within an object.
Standard Unit: * The standard unit of mass is the kilogram ( $kg$ ).
Universal Constancy: * The mass of an object remains identical regardless of its location in the universe.
Relationship with Gravitational Strength: * The greater the mass of an object, the greater the force of gravity with which it can attract other objects.
Comparative Examples: * The Moon has less mass than the Earth. * Consequently, the Moon's gravitational field is less than that of Earth. * The Moon does not possess a pull force as strong as the Earth's.

Gravity as a Weak Force: * Gravity is inherently a weak force unless dealing with massive objects.
Large-Scale Mass: * In objects with large mass (e.g., stars, planets, moons), the gravitational force is large enough to be dominant.
Small-Scale Mass: * In objects with small mass, the gravitational force is too small to notice or measure compared to other forces.
The Perspective Challenge: * Humans feel the Earth pulling them down because of the Earth's immense mass. * However, humans cannot feel the Earth being pulled to them because the mass of a human is too small to exert a noticeable gravitational force on the planet.

Definition: * Weight is a force. * It is the measure of the force of gravity pulling an object down toward the centre of a large celestial object (such as a star, planet, or moon).
Mathematical Formula: * $Weight = mass \times gravity$
Weight on Earth: * On Earth, the force of gravity is approximately $10\,newtons$ for every $kilogram$ of mass. * Calculation Example: A person with a mass of $50\,kg$ has a weight of $500\,N$ on Earth (calculated as $50\,kg \times 10\,N/kg = 500\,N$ ).
Earth vs. Moon Comparison: * A person with a mass of $56\,kg$ on Earth will have the same mass of $56\,kg$ on the moon. * However, while their weight on Earth is approximately $560\,N$ , their weight on the moon is only around $90\,N$ .

Gravitational Acceleration on Earth: * The gravitational strength (acceleration of gravity) on Earth is specifically $9.8\,m/s^2$ .
Factors Leading to Change: * Distance: As you move away from the Earth, the strength of gravity decreases. * Mass of the Observed Object: Gravity is not the same for every object. Objects with a larger mass experience a greater gravitational pull than objects with less mass. * Example: A $3\,kg$ medicine ball experiences a stronger pull of gravity than a regular basketball.

Free-Fall Factors: * The way objects fall depends on the net-force ( $F_{net}$ ) acting upon them, which includes more than just the pull of gravity.
Air Resistance: * The atmosphere contains air that pushes against all falling objects. * Air resistance is a specific example of fluid friction. * Example: A skydiver falling through the air involves an upward force of air resistance (e.g., $400\,N$ upward) opposing the downward pull of gravity.

Inertial Feedback: * The air resistance of an object increases as the object's acceleration increases. * Real-world Analogy: When cycling fast, you feel the air pushing against you even without wind. When you slow down, the sensation of the air push decreases.
The State of Equilibrium: * If an object travels fast enough, the air resistance eventually becomes as great as the force of gravity. * When air resistance equals the force of gravity, the object stops accelerating.
Definitions: * Terminal Velocity: The constant maximum velocity reached by a falling object. * Terminal Speed: The maximum constant speed reached by a falling body when the force of gravity pulling it down is balanced by the air resistance pushing against it.

Scenario: Mr. A. Lien arrives from outer space and investigates weight variations across the solar system using the resource: https://www.exploratorium.edu/ronh/weight/index.html.
Data Table (Mass = $30\,kg$ for all entries): * Mercury: Gravity: $3.78$ , Weight: $113.4\,N$ * Venus: Mass $30\,kg$ * Earth: Mass $30\,kg$ * Moon (Non-planet): Mass $30\,kg$ * Mars: Mass $30\,kg$ * Jupiter: Mass $30\,kg$ * Saturn: Mass $30\,kg$ * Uranus: Mass $30\,kg$ * Neptune: Mass $30\,kg$ * Pluto: Mass $30\,kg$
Summary Analysis: * Relationship: Weight and gravity are directly related; if gravity changes, weight changes, while mass stays the same. * Maximal Weight: A person would weigh the most on the planet with the highest gravity (Jupiter), because gravity is dependent on the mass of the planet.

Exit Ticket Question 1: Why does gravity change on different planets? * Context for Response: Gravity depends on the mass of the planet; larger planets generally exert more gravitational pull than smaller ones.
Exit Ticket Question 2: Why does your weight change on different planets? * Context for Response: Weight is the product of mass and gravity ( $W = m \times g$ ). Since gravity varies between planetary bodies and mass is constant, the resulting weight force must change accordingly.