Gravity — AQA GCSE Combined Science

The weight of an object is calculated using W = m × g, where m is the mass in kilograms and g is the gravitational field strength in newtons per kilogram (N/kg). Because g varies from place to place, the same object can have different weights in different locations. For example, g is about 9.8 N/kg on Earth, about 1.6 N/kg on the Moon, and about 24.8 N/kg on Jupiter. A 10 kg mass therefore weighs about 98 N on Earth, about 16 N on the Moon, and about 248 N on Jupiter. The mass stays 10 kg everywhere, but the weight changes because g changes. This shows that weight depends on the gravitational field strength at the point where the object is, while mass does not depend on location.

Weight is the force acting on an object because of gravity, and it is measured in newtons (N). It is not the same as mass, which is the amount of matter in the object and is measured in kilograms (kg). The equation that links them is W = m × g, where W is weight in newtons, m is mass in kilograms and g is gravitational field strength in newtons per kilogram (N/kg). On Earth g is about 9.8 N/kg, often rounded to 10 N/kg in calculations. For example, a 5 kg bag has weight 5 × 9.8 = 49 N on Earth, but only about 8 N on the Moon where g is about 1.6 N/kg. The mass stays 5 kg everywhere; only the weight changes. This statement introduces the equation you must be able to use and recall.

This equation links three quantities: weight (W) in newtons, mass (m) in kilograms and gravitational field strength (g) in newtons per kilogram. Weight is the force of gravity on an object, so it depends on where the object is. Mass is the amount of matter and does not change with location. For example, a 60 kg student has weight 60 × 9.8 = 588 N on Earth, but on the Moon g is about 1.6 N/kg, so the same student weighs 60 × 1.6 = 96 N. The equation can be rearranged: m = W ÷ g and g = W ÷ m. When using it, keep units consistent and remember that g is a property of the place, not of the object.

This equation links the weight of an object to its mass and the gravitational field strength at its location. Weight is the force of gravity acting on a mass, measured in newtons (N). Mass is the amount of matter, measured in kilograms (kg), and does not change with location. Gravitational field strength, g, is the force per kilogram, measured in newtons per kilogram (N/kg). On Earth g is about 9.8 N/kg, often rounded to 10 N/kg for estimates. To use the equation, identify the mass in kg and the value of g for the planet or moon, then multiply: for example, a 2 kg bag has weight W = 2 kg × 9.8 N/kg = 19.6 N on Earth, but only about 3.2 N on the Moon where g is about 1.6 N/kg. Rearranging gives m = W ÷ g or g = W ÷ m, so the same equation lets you find mass from weight or compare field strengths.

Weight is the gravitational force acting on an object, measured in newtons (N). It is a vector quantity because it acts towards the centre of the planet or moon producing the field. The newton is the SI unit of force; one newton is the force needed to give a 1 kg mass an acceleration of 1 m/s². Weight is calculated using W = m g, where m is mass in kilograms and g is gravitational field strength in N/kg. For example, a 50 kg student on Earth has weight W = 50 kg × 9.8 N/kg = 490 N, but on the Moon the same student weighs about 80 N because g is smaller. Weight is measured with a newton meter or force sensor, while mass is measured with a balance. Because weight is a force, it must always be reported in newtons, never in kilograms.

Mass measures how much matter an object contains and is measured in kilograms (kg). In gravity calculations you must use mass in kg, so convert grams to kilograms by dividing by 1000: 2500 g = 2.5 kg. Mass is a scalar and stays the same everywhere in the universe, unlike weight, which is a force in newtons and changes with gravitational field strength. In the equation W = m × g, m is the mass in kg, g is in N/kg and W comes out in N. For example, a 3 kg bag has W = 3 × 9.8 = 29.4 N on Earth. Always label the quantity and unit clearly, and check that the mass is in kg before substituting into any formula.

Gravitational field strength, g, is the force in newtons acting on each kilogram of mass, so its unit is newtons per kilogram (N/kg or N kg⁻¹). On Earth g is about 9.8 N/kg, on the Moon about 1.6 N/kg, so the same 10 kg object weighs about 98 N on Earth but only 16 N on the Moon. In the equation W = m × g, g tells you the weight per unit mass. In any calculation the value of g will be given in the question, so read it carefully and use the stated value rather than assuming 9.8 N/kg. For example, with m = 5 kg and g = 1.6 N/kg, W = 5 × 1.6 = 8 N. Always write the unit N/kg with the value and check that the weight answer is in newtons.

Weight is the gravitational force on an object, and for many calculations you can treat this force as acting at one point: the centre of mass. This point is the balance point of the object. For a uniform ruler, it lies at the midpoint; for a symmetrical shape, it lies on every axis of symmetry. If you support an object at its centre of mass, it balances because the total clockwise moment equals the total anticlockwise moment. This idea lets you draw a single downward arrow from that point when solving moment or equilibrium problems, rather than drawing many small arrows. It also explains why a low centre of mass improves stability. The statement says ‘may be considered’, so this is a model that simplifies real objects, not a claim that gravity only pulls at one point.

Weight and mass are directly proportional, which means their ratio is constant for a given gravitational field. The equation is W = m × g, where W is weight in newtons (N), m is mass in kilograms (kg) and g is gravitational field strength in newtons per kilogram (N/kg). On Earth, g is about 9.8 N/kg, so a 2 kg mass has a weight of about 19.6 N. If mass doubles, weight doubles; if mass triples, weight triples. A graph of weight on the y-axis against mass on the x-axis is a straight line through the origin, and its gradient equals g. On the Moon, g is smaller, so the same mass has less weight, but the mass does not change. Direct proportionality is therefore a mathematical relationship, not a statement that weight and mass are the same quantity.

Weight is the gravitational force acting on an object, measured in newtons (N). A spring-balance, also called a newtonmeter, measures this force directly. Inside, a spring stretches when a force pulls it; the extension is proportional to the force, so a scale can be calibrated in newtons. To use one, hang the object from the hook or pull the hook with the object, wait for it to settle, and read the value at eye level. The reading is the weight, not the mass. Mass is measured with a balance in kilograms (kg) and does not change with location, but weight changes with gravitational field strength g, because W = m × g. For example, a 2 kg mass has a weight of about 20 N on Earth (g ≈ 10 N/kg), but only 3.2 N on the Moon (g ≈ 1.6 N/kg). A calibrated spring-balance therefore gives different readings on different planets, while a mass balance gives the same mass everywhere.