Gravitation

Zero (weightless)

Step 1: Weight is the force exerted by an object on a support (like a weighing scale). Step 2: When an object falls freely, both the object and any measuring device fall with the same acceleration g. Step 3: Since there's no relative motion between the object and measuring device, no force is exerted on the support. Step 4: This creates the condition of weightlessness - the apparent weight becomes zero. Step 5: Astronauts in orbit experience this same effect as they're in continuous free fall around Earth.

Weight of the fluid displaced

Step 1: Archimedes' principle states that when a body is immersed in a fluid, it experiences an upward force. Step 2: This upward force (buoyant force) equals the weight of the fluid that gets displaced by the object. Step 3: The displaced fluid volume equals the volume of the submerged part of the object. Step 4: Buoyant force = ρ_fluid × V_displaced × g, which equals the weight of displaced fluid. Step 5: This principle explains why ships float and helps in calculating the apparent loss of weight in fluids.

They reach the ground at the same time

Step 1: All objects near Earth's surface experience the same acceleration due to gravity (g = 9.8 ms⁻²). Step 2: From the equation s = ut + ½gt², time to fall depends only on height and acceleration, not mass. Step 3: Since both objects start from rest (u = 0) and same height, they take the same time. Step 4: Galileo demonstrated this principle by dropping objects from the Tower of Pisa. Step 5: Air resistance can affect this in real conditions, but in vacuum, all objects fall at the same rate regardless of mass.

Newton (N)

Step 1: Weight is the gravitational force acting on an object, calculated as W = mg. Step 2: Since weight is a force, it has the same SI unit as force, which is Newton (N). Step 3: Kilogram (kg) is the unit of mass, not weight. Step 4: ms⁻² is the unit of acceleration, and Joule (J) is the unit of energy. Step 5: Many people confuse mass and weight - mass is measured in kg, but weight (the gravitational force) is measured in Newtons.