A Child Can Push Harder Than a Lawn Mower Blade

AI image generated by Google Gemini.

Did you know that a child can probably produce more torque from their arm than a lawn mower engine can produce at the blade? Surprisingly, the figure for the torque of a 5 hp engine used on a mower is 7 to 10 foot-pounds (ft-lbs) or 11.5 to 13.6 newton-meters (Nm). Applied torque is simply force multiplied by distance, so a pound applied over a distance of a foot is 1 foot-pound. In the metric system, the unit of torque is the newton-metre (Nm). A torque of 1 Nm is equivalent to one newton—the SI unit of force—applied over a distance of one metre. The speed of a lawn mower blade is more important than torque, and blades are driven directly by the crankshaft without reduction or overdrive gearing. Blades typically spin at the engine speed of around 3000 RPM, giving a blade tip speed of around 200 mph, or 90 metres per second.

As we saw in the article Interesting Facts - What is Torque?, power = torque x angular rotation speed, so while a child may be able to exert more torque than a mower with their arm,  they wouldn't be able to match its power.

Note: You may be wondering why "newton-metres" isn't capitalised above. It's not a typo. In the International System of Units (SI), or Système international d'unités, units named after a person are not capitalised. However ,the symbol is: N, in this case after the English scientist Isaac Newton. Units are normally lower case, as are prefixes. However, prefixes over one million are capitalised—so kilometres is km, but gigahertz is GHz.