Here's why:

* Mechanical Advantage is the ratio of output force to input force.

* Force is measured in Newtons (N).

* Since it's a ratio of two forces (both measured in Newtons), the units cancel out.

Example: If you apply a force of 10 Newtons and the machine amplifies it to 50 Newtons, the mechanical advantage is 50/10 = 5.

Let's look at the other units you mentioned:

* Joules (J): Units of energy or work.

* Watts (W): Units of power, which is the rate of energy transfer.

These units are not relevant to mechanical advantage.