Calculate weight mass and gravitational field strength
Rearrange W = mg and convert mass units before substituting.
Exam scope and assessment notes
Core and Extended · Physics 0972, 2026–2028, section 1.3. Mass is measured in kg; weight is a gravitational force measured in N. Use W = mg or g = W/m with the field strength stated in the question. A balance can compare weights and hence masses at the same location. Written method exercises support supervised practical learning but do not demonstrate practical competence. Written practice is self-checked; cumulative assessment and human subject-teacher review remain pending. Cambridge states that IGCSE 0972 has the same subject content as IGCSE 0625, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Rearrange a product equation.
- Convert grams to kilograms.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which equation finds mass from W and g?
What you will learn
- Use W = mg, m = W/g and g = W/m.
- Choose the supplied value of g and retain physical units.
Use field strength with mass
Gravitational field strength is gravitational force per unit mass: g = W/m. Rearranging gives W = mg. Where g is 9.8 N/kg, each kilogram has gravitational weight 9.8 N. A 0.650 kg sample therefore has W = 0.650 × 9.8 = 6.37 N.
If the mass is given in grams, divide by one thousand before using g in N/kg. For 750 g at 9.8 N/kg, W = 0.750 × 9.8 = 7.35 N. Using 750 directly would overstate the weight by a factor of one thousand.
Rearrange for the requested quantity
To find mass from a measured stationary weight of 24 N at g = 10 N/kg, use m = W/g = 2.4 kg. To find field strength when a 0.30 kg object has weight 2.4 N, use g = 2.4/0.30 = 8.0 N/kg. State which quantity is being calculated before selecting an equation.
Use the value of g supplied in the problem. Near Earth's surface it is approximately 9.8 N/kg, but a question may explicitly use 10 N/kg or give another location's value. Substituting 9.8 regardless of the question changes its stated model. The units provide a useful check: kg × N/kg leaves N, and N divided by N/kg leaves kg.
Pause and explain
A 750 g object is at g = 9.8 N/kg. What is its weight?
Worked example
Find the weight of a 650 g sample where g = 9.8 N/kg.
Show the worked solution
- Convert 650 g to 0.650 kg.
- W = mg = 0.650 × 9.8.
- Weight = 6.37 N.
Answer 6.37 N.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Calculate the weight of 650 g at 9.8 N/kg.
Give me a hint
Convert the mass to kg before multiplying by g.
Compare my reasoning
- 650 g = 0.650 kg.
- 0.650 × 9.8 = 6.37.
- Use the weight unit N.
6.37 N.
Look for these in your work
- Convert mass correctly.
- Use a force unit.
A body has weight 24 N where g = 10 N/kg. Find mass, then the weight of a 750 g sample at 9.8 N/kg.
Give me a hint
Use division for the first quantity and multiplication for the second.
Compare my reasoning
- First mass = 24/10 = 2.4 kg.
- Second mass = 0.750 kg.
- Second weight = 0.750 × 9.8 = 7.35 N.
2.4 kg; 7.35 N.
Look for these in your work
- Choose the correct rearrangement.
- Use each problem's g value.
A 0.30 kg object has stationary weight 2.4 N. Infer g and predict the weight of 0.75 kg at the same location.
Give me a hint
Find force per kilogram, then scale it to the new mass.
Compare my reasoning
- g = 2.4/0.30 = 8.0 N/kg.
- Use the same field for the new object.
- W = 0.75 × 8.0 = 6.0 N.
8.0 N/kg; 6.0 N.
Look for these in your work
- Infer the local field.
- Keep the same-location assumption.
Common mistakes
- Convert mass to kg when g is in N/kg.
- Use the supplied field strength.
- Weight is not measured in N/kg; field strength is.
What can you explain now?
An object weighs 24 N where g = 10 N/kg. What is its mass?
Compare with the explanation
2.4 kg.
Divide gravitational force by force per kilogram.
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Make it stick
Write all three rearrangements and check their units before calculating a new example.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.