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School Physics Notes: Turning Forces 5.2 Simple moment calculations

GCSE level physics exam revision notes on 5 Turning forces

5.2 Some simple examples of moment calculations

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[KEY POINTS and learning objectives for this page, after initial notes]

Index of physics notes: 5. Turning forces, calculating moments, problem solving


This page contains online questions only.

Jot down your answers and check them against the worked out answers at the end of the page.

5.2 Some simple moment calculations using the moment formula

These are not equilibrium situations, assuming you can turn the spanner!

Watch out for the units of the data and what units apply to the moment equation.

Q1

Calculate the moment if a force of 5.0 N is applied to a spanner 15 cm long.

Worked out ANSWERS

 

Q2 A force of 20 N is applied to a door causing a moment of 5 Nm.

Calculate the distance in cm from the hinge axle to the point on the door where the force was applied.

Worked out ANSWERS

 

Q3 What force must applied to a 30 cm long spanner to generate a moment of 6.0 Nm?

Worked out ANSWERS

 

Index of Forces 5. Turning forces, calculating moments, problem solving


Key points about turning forces - simple single moment calculations

Information sources for Doc Brown's key points: IGCSE-GCSE physics are based on textbooks & syllabus-specifications for students taking the UK AQA, Edexcel, OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA and CIE GCSE physics 9-1 level science examinations

For moment calculations

The rotational or turning force effect, the moment, has a magnitude easily calculated from the simple formula:

M = F x d, where M = the moment of a force (Nm), F = force applied (N)

and d (m) is the perpendicular distance from the pivot point to the line of action of the force.

Keywords, phrases and learning objectives for turning forces

Know the formula for calculating a moment and be able to do simple moment calculations.


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Index Forces 5. Turning forces, calculating moments, problem solving

Worked out ANSWERS to the moment calculations

Q1

Calculate the moment if a force of 5.0 N is applied to a spanner 15 cm long.

F = 5, d = 15/100 = 0.15 m

moment M = F x d = 5 x 0.15 = 0.75 Nm

 

Q2 A force of 20 N is applied to a door causing a moment of 5 Nm.

Calculate the distance in cm from the hinge axle to the point on the door where the force was applied.

moment M = F x d, so d = M/F = 5/20 = 0.25 m,

therefore d = 0.25 x 100 = 25 cm

 

Q3 What force must applied to a 30 cm long spanner to generate a moment of 6.0 Nm?

moment M = F x d, so F = M/d, d = 30 cm = 30/100 = 0.30 m

F = M/d = 6.0/0.30 = 20 N

Index Forces 5. Turning forces, calculating moments, problem solving

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