OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Apply formulae relating force, mass and relevant physical constants,…
Full specification wording
Forces in action: apply formulae relating force, mass and relevant physical constants, including gravitational field strength, g, to explore how changes in these are inter-related (content found only in the physics separate science qualification)
This criterion belongs to the physics separate science qualification only, so treat it as outside the combined-science route.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 16 of 21
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This criterion belongs to the physics separate science qualification only, so treat it as outside the combined-science route. The core relationship is weight equals mass times gravitational field strength, W = m g, where W is in newtons, m in kilograms and g in N/kg (about 9.8 N/kg near the Earth's surface). Because the equation is a product, doubling the mass doubles the weight at fixed g, and doubling g doubles the weight at fixed mass. Rearranged, m = W/g and g = W/m, so a measured weight and mass let you find g. On the Moon g is smaller, so the same mass has a smaller weight while its mass is unchanged. A typical multiple-choice task gives two of the three quantities and asks for the third, or asks how weight changes when mass or g changes.
What earns the marks
- Applies W = m g to calculate weight from mass and gravitational field strength, keeping mass in kilograms and weight in newtons.
- Rearranges the relationship to find mass as m = W/g or field strength as g = W/m when the other two quantities are known.
- Explains that weight is directly proportional to mass at constant g, so doubling the mass doubles the weight.
- Describes how a smaller gravitational field strength, such as on the Moon, reduces the weight of a fixed mass without changing that mass.
Common mistakes and how to put them right
Uses grams for mass in W = m g.
Put it right: Convert to kilograms first, since the newton result depends on SI units.
Claims mass changes when an object is taken to a place with different g.
Put it right: Recognise that mass stays the same and only weight changes.
Adds mass and field strength instead of multiplying them.
Put it right: Apply the product W = m g to combine the quantities.
Exam tips
- Write down the values of m and g with their units before substituting, so a unit slip is easy to spot.
- When a question asks how weight changes, decide first whether mass or g is the one being altered.
- Check the answer's unit: a weight answer must be in newtons, not kilograms.
Try the lesson
The start of the Forces in action: apply formulae relating force, mass and relevant physical constants, including gravitational field strength, g, to explore how changes in these are inter-related (content found only in the physics separate science qualification) lesson, made from checked teaching notes for this specification point.
Have a go first
A bag of flour has a mass of 2 kg. Taking g = 10 N/kg, calculate the weight of the bag using W = m × g.
Listen
This criterion belongs to the physics separate science qualification only, so treat it as outside the combined-science route. The core relationship is weight equals mass times gravitational field strength, W = m g, where W is in newtons, m in kilograms and g in N/kg (about 9.8 N/kg near the Earth's surface). Because the equation is a product, doubling the mass doubles the weight at fixed g, and doubling g doubles the weight at fixed mass. Rearranged, m = W/g and g = W/m, so a measured weight and mass let you find g. On the Moon g is smaller, so the same mass has a smaller weight while its mass is unchanged. A typical multiple-choice task gives two of the three quantities and asks for the third, or asks how weight changes when mass or g changes. A strong answer earns marks for these points. Applies W = m g to calculate weight from mass and gravitational field strength, keeping mass in kilograms and weight in newtons. Rearranges the relationship to find mass as m = W/g or field strength as g = W/m when the other two quantities are known. Explains that weight is directly proportional to mass at constant g, so doubling the mass doubles the weight. Describes how a smaller gravitational field strength, such as on the Moon, reduces the weight of a fixed mass without changing that mass. Exam tip: Write down the values of m and g with their units before substituting, so a unit slip is easy to spot.
See it
Key words
Say what each word means before you turn the card.
Weight
Try answering aloud or in your head before turning the card.
Gravitational field strength
Try answering aloud or in your head before turning the card.
Mass
Try answering aloud or in your head before turning the card.
Rearranging formulae
Try answering aloud or in your head before turning the card.
See every definition
- Weight
- Equals mass times gravitational field strength, W = m g, measured in newtons, with mass in kilograms and g in N/kg.
- Gravitational field strength
- The g in W = m g, about 9.8 N/kg near the Earth's surface and smaller on the Moon.
- Mass
- The m in W = m g, kept in kilograms, and unchanged by location while weight changes with g.
- Rearranging formulae
- Gives m = W/g and g = W/m, so a measured weight and mass let you find g.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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