Why is slip necessary for an induction motor to produce torque?

In an induction motor, slip is necessary because torque is generated by the interaction between the stator's rotating magnetic field and the current induced in the rotor conductors.12

For current to flow in the short-circuited rotor conductors, a voltage must be induced within them. According to the principles of electromagnetic induction, this voltage is produced only when there is relative motion between the rotating magnetic field and the rotor conductors.1

If the rotor were to spin at the exact same speed as the magnetic field (the synchronous speed), there would be no relative motion, no voltage would be induced, and consequently, no rotor current or torque would be produced. Therefore, the rotor must rotate at a speed slightly lower than the magnetic field—a difference known as slip—to ensure that the magnetic field constantly "cuts" across the rotor conductors, maintaining the induced current and the resulting torque required to drive the load.1