The physics is simple:
The 'force' (linear impulse so one really wants to measure force-time) the person feels from being struck with the bullet is a consequence of the linear momentum. The force felt will be larger or smaller depending on impact time. This is because the linear impulse (which is roughly force times time but better expressed as an integral) is constant and equal to the linear momentum. When tearing into flesh, the impact time is large when compared to hitting a steel plate. Whence, if you were made of steel you would feel a large initial force (which may cause you to lose your balance) but in flesh it is a smaller constant force (constant being a relative term here as we are talking microseconds v. milliseconds).
Hence two objects with the same linear momentum (mass * velocity) depending on how long they impact the target (how long they take to give the target all of their momentum) completely determines how much 'force' is felt.
Such things are readily available in any elementary (Calculus-based) texts on physics. Also see (engineering) texts on dynamics / classical mechanics for more concrete examples.
Edit: I forgot to mention the relationship that force is the time-derivative of (linear) momentum. Hence all the need to talk about multiplying by time.
The 'force' (linear impulse so one really wants to measure force-time) the person feels from being struck with the bullet is a consequence of the linear momentum. The force felt will be larger or smaller depending on impact time. This is because the linear impulse (which is roughly force times time but better expressed as an integral) is constant and equal to the linear momentum. When tearing into flesh, the impact time is large when compared to hitting a steel plate. Whence, if you were made of steel you would feel a large initial force (which may cause you to lose your balance) but in flesh it is a smaller constant force (constant being a relative term here as we are talking microseconds v. milliseconds).
Hence two objects with the same linear momentum (mass * velocity) depending on how long they impact the target (how long they take to give the target all of their momentum) completely determines how much 'force' is felt.
Such things are readily available in any elementary (Calculus-based) texts on physics. Also see (engineering) texts on dynamics / classical mechanics for more concrete examples.
Edit: I forgot to mention the relationship that force is the time-derivative of (linear) momentum. Hence all the need to talk about multiplying by time.
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