Gravity — AQA GCSE Physics
Test yourself on Gravity with AQA GCSE practice questions.
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Gravity explained
Weight is the force acting on an object because of gravity.
Read the full explanation
Near the Earth it exists because the Earth has a gravitational field around it, a region in which any mass experiences a force. The weight of an object always acts towards the centre of the Earth, which on a flat classroom diagram looks like straight down but on a diagram of a satellite in orbit does not. The field is strongest at the surface and gets weaker with distance, so a satellite high above the Earth weighs less than the same satellite on the ground, though the field never vanishes. The gravitational force on an orbiting object continually changes the direction of its motion and so its velocity.
The weight of an object depends on the gravitational field strength at the point where the object is.
Mass is the quantity of matter in an object and is the same everywhere; weight is a force and depends on where the object is. What changes is the gravitational field strength at that point. On the Earth's surface g is about 9.8 N/kg, on the Moon about 1.6 N/kg, so a 60 kg astronaut has a weight of about 588 N on Earth but only about 96 N on the Moon, with the same 60 kg of mass in both places. Field strength also falls as you move away from a planet, so the same object weighs slightly less at the top of a mountain or in orbit. Any value of g you need will be given in the question, so use the value supplied, not a remembered one.
The weight of an object can be calculated using the equation: weight = mass × gravitational field strength W = m g weight, W, in newtons, N mass, m, in kilograms, kg gravitational field strength, g, in newtons per kilogram, N/kg (In any calculation the value of the gravitational field strength (g) will be given.) The weight of an object may be considered to act at a single point referred to as the object’s ‘centre of mass’.
Weight is calculated from W = m g, with weight W in newtons, mass m in kilograms and gravitational field strength g in N/kg. Write the equation first, substitute, then evaluate: a 4.0 kg bag where g = 9.8 N/kg has a weight of 4.0 × 9.8 = 39.2 N. The value of g is always given, so use the one in the question. Mass supplied in grams must be converted to kilograms first, so 250 g becomes 0.250 kg. The weight of an object may be treated as acting at a single point called the centre of mass, the point from which the weight may be considered to act, or equivalently the point where the mass appears to be concentrated; on a diagram the weight arrow is drawn from there.
The weight of an object and the mass of an object are directly proportional.
Weight is directly proportional to mass, written W ∝ m, because W = m g and g is constant at a given place. Directly proportional has a precise meaning: double the mass and the weight doubles exactly, treble it and the weight trebles. On a graph of weight against mass, this shows as a straight line that passes through the origin, and the gradient of that line is the gravitational field strength. The relationship is not the same as weight being approximately equal to mass; they are different quantities in different units, and the constant linking them changes from place to place, so the same masses plotted on the Moon give a straight line through the origin with a much smaller gradient.
Weight is measured using a calibrated spring-balance (a newtonmeter).
Weight is a force, so it is measured with a calibrated spring-balance, also called a newtonmeter, which reads directly in newtons. Inside, a spring extends when the object is hung from it. Because the extension is directly proportional to the force applied, the scale is linear with evenly spaced divisions, which makes the instrument straightforward to calibrate. The spring must deform elastically so that it returns to its original length once the object is removed; if it is overloaded so far that it deforms inelastically it stays stretched and the readings are wrong afterwards; past the limit of proportionality the scale also stops being evenly spaced. To use it well, zero it first, hang it vertically, let the object come to rest and read the scale at eye level.
Your focus
- Define a gravitational field and state where an object's weight comes from.
- Draw the direction of the weight of an object shown in orbit around the Earth.
- Explain why the weight of a satellite decreases as it moves further from the Earth while its mass does not.
Show all 15 objectives
- Explain why an object's weight changes between the Earth and the Moon while its mass does not.
- Calculate the weight of a given mass on two different bodies using the field strengths supplied.
- State how gravitational field strength varies with distance from a planet.
- Calculate the weight of an object from its mass and a supplied value of gravitational field strength.
- Convert a mass given in grams into kilograms before substituting into W = m g.
- Define centre of mass and mark its position on a diagram of an object.
- Interpret the statement W ∝ m in words and describe the graph it predicts.
- Determine a gravitational field strength from the gradient of a weight against mass graph.
- Explain why a straight line not through the origin does not show direct proportion.
- Name the instrument used to measure weight and state the unit it reads.
- Explain two properties of a spring that make it suitable for a newtonmeter.
- Describe how to take an accurate reading from a spring-balance.
Gravity exam tips
Marking Points
- one mark for weight being the force acting on an object due to gravity
- one mark for a gravitational field being the region around a mass where another mass experiences a force
- one mark for the weight acting towards the centre of the Earth
- one mark for the field strength, and so the weight, decreasing as distance from the Earth increases
- one mark for the mass of the object staying the same wherever it is
- one mark for the weight changing because the gravitational field strength changes
- one mark for a correct comparison using the values given, such as a smaller weight on the Moon where g is smaller
- one mark for stating that field strength falls as distance from the planet increases
- one mark for writing W = m g before substituting
- one mark for a correct substitution with the mass in kilograms and the value of g given in the question
- one mark for the answer in newtons
- one mark for defining centre of mass as the point from which the weight may be considered to act, or where the mass appears to be concentrated
- one mark for reading W ∝ m as 'weight is directly proportional to mass'
- one mark for recognising a graph of weight against mass as a straight line through the origin
- one mark for identifying the gradient of that line as the gravitational field strength
- one mark for using the proportionality to scale a value, such as doubling the mass to double the weight
- one mark for naming the instrument as a calibrated spring-balance or newtonmeter, reading in newtons
- one mark for the extension of the spring being directly proportional to the force, giving an evenly spaced scale
- one mark for the spring deforming elastically, so it returns to its original length when the force is removed
- one mark for a correct technique, such as zeroing the instrument and reading the scale at eye level with the object stationary
Examiner Tips
- 💡Use the field definition wording, 'the region where another mass experiences a force', which mirrors the accepted electric field answer.
- 💡On any orbit diagram, draw the force arrow pointing at the centre of the Earth.
- 💡Keep weight in newtons and mass in kilograms throughout, even in a written answer.
- 💡Underline the value of g given in the question before you start, because questions deliberately vary it.
- 💡State clearly which quantity stays the same and which changes; that contrast is what is being marked.
- 💡Check your units at the end: newtons for weight, kilograms for mass, N/kg for field strength.
- 💡Set out equation, substitution and answer on three lines; the first two carry marks even if the arithmetic slips.
- 💡Convert grams to kilograms as your first line of working, in view of the examiner.
- 💡For centre of mass, 'the point through which the line of action of the weight acts' is also accepted.
- 💡In a multiple choice question, the phrase to look for is 'directly proportional to', not 'approximately equal to'.
- 💡Check the line passes through the origin before claiming proportionality from a graph.
- 💡Use a large triangle when finding the gradient, so the reading error is a smaller fraction of the result.
- 💡If asked why a spring suits this job, give the elastic deformation and the linear scale as two separate points.
- 💡Say the instrument is 'calibrated' in newtons; that word is in the specification and signals you know it reads force.
- 💡Mention eye level and a stationary object when a question asks how to improve accuracy.
Common Mistakes
- defining weight as how heavy something is, and quoting it in kilograms
- drawing weight straight down the page on a diagram of an object in orbit
- claiming there is no gravity in orbit because astronauts appear weightless
- confusing gravitational field strength with the weight itself
- saying weight is caused by the atmosphere pressing down
- saying an astronaut's mass decreases on the Moon
- using a remembered value of g when the question supplies a different one
- quoting a weight in kilograms or a mass in newtons
- claiming weight is zero in orbit because there is no gravity
- treating gravitational field strength as a force rather than a force per kilogram
- leaving the mass in grams, so the weight comes out a thousand times too large
- using a remembered value of g instead of the one supplied
- quoting the weight in kilograms
- describing the centre of mass as the middle of the object, which fails for irregular shapes
- drawing the weight arrow from the base of the object rather than from its centre of mass
- reading directly proportional as approximately equal
- accepting a straight line that does not pass through the origin as showing proportionality
- assuming the constant of proportionality is the same on every planet
- treating weight and mass as the same quantity in different units
- reading a single weight value off the graph as the gradient without dividing by the corresponding mass
- naming a top-pan balance, which measures mass in grams rather than weight in newtons
- reading the scale while the object is still swinging
- failing to zero the newtonmeter before hanging the object on it
- recording the reading in kilograms
- overloading the spring past its limit of proportionality and continuing to use the readings