substition effect - comes into play  

• could cause MPRL to shift more than for a single factor
  • wage decrease >> more labor demanded >> increases MPK >> firm buys more machinery >> increases MPL >> firms buys even more labor
• wage rate decrease >> more labor >> more output >> more units of good in the market >> price would decrease
  • wage hardly ever changes w/o affecting market price

input supply - no limit in competitive purchase market  

• firms can buy as much of each input as they want at market price
• firm will not affect market price of input
• input supply perfectly elastic >> price (wage/rental) stays constant