Critical (physical sciences)

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There is a meaning of critical that is commoner in the physical sciences. (For its use in Arts and Social Sciences subjects, as well as in assignment and exam questions, see crit-.)

The meaning in physical sciences is, in relation to quantities, 'the amount that alters the balance of things'. For example, the critical mass in nuclear physics is the amount of uranium or other radioactive materials above which a self-sustaining reaction of nuclear fission is possible. In engineering, a critical speed is one above which something's behaviour will change - an aeroplane will take off, perhaps; or a road vehicle become unstable.