Question

Why is the actual mechanical advantage of a machine always less than its ideal mechanical advantage ?
Why can no machine be 100% efficient?
You test a machine and find that it exerts a force of 5 N for each 1 N of force you exert operating the machine. What is the actual mechanical advantage of the machine?

Answer 1

Definitions:
- Ideal mechanical advantage (IMA) is the mechanical advantage a machine would have if there were no losses (IMA = distance in / distance out).
- Actual mechanical advantage (AMA) is what you actually get: AMA = output force / input force.

Why AMA < IMA:
- Real machines have friction in bearings, sliding surfaces, belts, gears, etc. Friction converts some input work into heat, so less is available as output force.
- Parts flex, deform, or vibrate and dissipate energy as heat or sound.
- There can be slipping, backlash, and other mechanical losses.
- From energy balance: if there were no losses, Fout/Fin = IMA. Losses reduce the output force for a given input, so AMA < IMA. Thermodynamically, irreversible processes always produce entropy, so some input energy is irreversibly lost as heat — hence no machine can be 100% efficient.

Numeric answer:
If the machine exerts 5 N of output force for each 1 N of input force, AMA = Fout/Fin = 5/1 = 5.