OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Forces in action: recall the acceleration in free fall
Free fall means motion under gravity alone, with air resistance negligible.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 15 of 21
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Free fall means motion under gravity alone, with air resistance negligible. Near the Earth's surface the gravitational force on a mass m is W = m g, so the acceleration is a = W/m = m g / m = g. The mass cancels, so a heavy ball and a light ball dropped together accelerate at the same rate when air resistance is negligible. OCR expects you to recall the value of this acceleration: about 9.8 m/s², commonly rounded to 10 m/s² in calculations. Learn it as a magnitude with direction downward, and note that g is also the gravitational field strength in N/kg; 9.8 N/kg and 9.8 m/s² are numerically the same. A typical question asks which value matches free-fall acceleration, so distinguish it from 9.8 N (a force) and from speeds such as 9.8 m/s.
What earns the marks
- Selects the value 9.8 m/s² (or the rounded 10 m/s²) as the acceleration of an object in free fall near the Earth's surface.
- Recognises that free-fall acceleration is the same for all masses when air resistance is negligible, because the mass cancels in a = W/m.
- States that the acceleration acts downward, toward the Earth, and is caused by the gravitational force on the object.
- Links the numerical value to gravitational field strength, noting that 9.8 N/kg and 9.8 m/s² are equivalent in magnitude.
Common mistakes and how to put them right
Confuses the units and gives 9.8 N as the acceleration.
Put it right: Use 9.8 m/s², since newtons measure force, not acceleration.
Assumes a heavier object falls with greater acceleration.
Put it right: Recognise that with negligible air resistance all masses accelerate at g.
Treats 9.8 m/s as a speed reached in free fall.
Put it right: Understand that 9.8 m/s² is the rate of change of velocity, not a velocity.
Exam tips
- Memorise 9.8 m/s² and its rounded form 10 m/s², and check which the question's data implies before answering.
- Read the unit in each option carefully, because distractors often swap m/s² for m/s or N.
- If a question mentions negligible air resistance, apply the constant value g rather than reasoning about mass.
Try the lesson
The start of the Forces in action: recall the acceleration in free fall lesson, made from checked teaching notes for this specification point.
Have a go first
Near the Earth's surface, an object is dropped and falls freely with air resistance negligible. What is the acceleration of the object?
Listen
Free fall means motion under gravity alone, with air resistance negligible. Near the Earth's surface the gravitational force on a mass m is W = m g, so the acceleration is a = W/m = m g / m = g. The mass cancels, so a heavy ball and a light ball dropped together accelerate at the same rate when air resistance is negligible. OCR expects you to recall the value of this acceleration: about 9.8 m/s², commonly rounded to 10 m/s² in calculations. Learn it as a magnitude with direction downward, and note that g is also the gravitational field strength in N/kg; 9.8 N/kg and 9.8 m/s² are numerically the same. A typical question asks which value matches free-fall acceleration, so distinguish it from 9.8 N (a force) and from speeds such as 9.8 m/s. A strong answer earns marks for these points. Selects the value 9.8 m/s² (or the rounded 10 m/s²) as the acceleration of an object in free fall near the Earth's surface. Recognises that free-fall acceleration is the same for all masses when air resistance is negligible, because the mass cancels in a = W/m. States that the acceleration acts downward, toward the Earth, and is caused by the gravitational force on the object. Links the numerical value to gravitational field strength, noting that 9.8 N/kg and 9.8 m/s² are equivalent in magnitude. Exam tip: Memorise 9.8 m/s² and its rounded form 10 m/s², and check which the question's data implies before answering.
Key words
Say what each word means before you turn the card.
Free fall
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Acceleration due to gravity
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Gravitational field strength
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Independence of acceleration from mass
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See every definition
- Free fall
- Motion under gravity alone, with air resistance negligible.
- Acceleration due to gravity
- About 9.8 m/s², commonly rounded to 10 m/s² in calculations, acting as a magnitude with direction downward.
- Gravitational field strength
- Also denoted by g and measured in N/kg; 9.8 N/kg and 9.8 m/s² are numerically the same.
- Independence of acceleration from mass
- The mass cancels in a = W/m, so a heavy ball and a light ball accelerate at the same rate when air resistance is negligible.
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