in formula F=DP/DT, d stands for what?

This is a standard equation which describes the Newton's second Law of Motion; it relates Force with momentum.

F = dp/dt means that the force applied on a body is directly proportional to the change in its momentum. Here momentum refers to as the amount of motion contained in a body. So, in other words, the equation represents the fact that force causes a change in the motion (or state) of a body. This is a fundamental definition of force as far as classical Newtonian mechanics is concerned.

The above relation can also be expressed as

F = dp/dt = d(mv)/dt

or

F = ma

force is also directly proportional to the acceleration (or retardation) produced in a body.

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