Problem I $-$ uncertainty $-$ DC motor

The mechanical power is measured indirectly by directly measuring the shaft speed and resistive force when a prony brake is applied

to the motor shaft. A prony brake simply acts like an automotive brake except that on the "fixed" side of the brake, a moment arm of

length, $r,$ is attached to the brake and the end of the moment arm is applied to a load cell to measure the force, $F.$ This allows the

braking torque to be measured: $T_{\text{b}\text{r}\text{a}\text{k}\text{z}}=Fr.$

The electrical power is measured indirectly by directly measuring the voltage and current to the motor.

Calculate the mean motor efficiency and the measurement uncertainty $(95\%$ confidence level$).$

Equation:

$=\frac{P_m}{P_s}=\frac{Fr}{vi}$