Students should be able to convert between newton-metres and joules.

The idea

This is the easiest conversion in the specification, because the numbers do not change: one newton-metre is one joule, so 35 N m of work is 35 J, and 6.0 kJ is 6000 N m. What the question really tests is whether you can see that force multiplied by distance produces the joule, so a value already given in newton-metres is an energy and needs no factor at all. The care is needed elsewhere. A moment is also measured in newton-metres but it is a turning effect, not an energy, so it is never rewritten in joules. And if a work done comes out in kilojoules or megajoules, convert it to joules by multiplying by a thousand or by a million before using the number in a later equation such as the kinetic energy one.

What earns the marks

• Using one newton-metre as equal to one joule, with no numerical change.
• Converting kilojoules or megajoules into joules before substituting.
• Leaving a moment in newton-metres rather than converting it to joules.
• The quantity quoted with the unit the question asks for.

Common mistakes and how to put them right

• Multiplying or dividing by a conversion factor between newton-metres and joules.

• Converting the moment of a force into joules because the unit looks the same.

• Leaving an energy in kilojoules when the next equation needs joules.

• Writing the unit as newtons per metre instead of newton-metres.

Exam tips

• If the unit is N m and the quantity is an energy, simply change the label to J.
• Check whether the newton-metre in front of you is a moment or a work done before doing anything with it.