This circuit is really a bad design and much too simple to work properly as it
will turn on/off the MOSFET only gradually, that is a bad thing as that will cause current to flow through and a voltage to be present across the MOSFET. As P = V * I that means Power is dissipated in the MOSFET. Not a good thing. Even worse with an 2N7000 which cannot handle much power.
What is needed is an abrupt switch on/off of the MOSFET.
Also the 2N7000 by far isn't powerful enough to drive a motor. You should use a MOSFET which has a maximum current rating that is a lot higher than the 2N7000, it can only handle about 200 mA. Much less than what most motors require. Also see this question.
So probably you have already damaged that poor 2N7000. You really need a better circuit.
Here's an example using some CMOS inverters between the resistors and the gate of the MOSFET. As you also don't want the motor switching on/off constantly around the tripping point you should use schmitt-trigger inverters which will add hysteresis.
For the 2 NOT gates I suggest using the HEF40106 IC (or an equivalent one). Connect the inputs of the 4 unused NOT gates (the HEF40106 has 6 NOT gates) to ground.