What is the size of the force needed to change the momentum of an object 2000 kg m/s over a time period of 0.5 seconds?

Impulse is equal to change in momentum of an object.  The only way to change an object's momentum is to apply a force for certain amount of time.

Impulse is calculated as follows:  Δp = F * t

Put your values in the right spots and solve for F. In this case since F is multiplied by Time, you will divide both sides by the time to find your answer.

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