tomer1510: why would I need a diode? the motor is not a power source therefor will power back the arduino wouldnt a diode be useless in this scenario?
A motor has a large induction because of it's coil winding. The motor when running develops a large magnetic field and if you suddenly switch off the motor the magnetic field collapses, causing a transient voltage spike of opposite polarity to be imposed onto the attached wiring. This voltage can be up to 4X the originally applied voltage and can cause damage to other components in the circuit. The reversed biased diode across the motor winding allows the spike to be dissipated through the diode and the winding resistance, thus 'suppressing' and dissipating the voltage.