The use of push up bra is to go against the force of gravity which is

g = 9.8 m/s2 = 32.2 ft/s2

the up force made by the bra is to have an opposite force to hold what is suppose to be Milk sacks .....

so the downward force by the Milk Sacks would be F=ma=mg , where m is the mass of the milk sacks ( generally size as AA,A,B,C,D,E.... , but the actual mass still requires proper measurement)

The push up bra will then have a upward force to hold it with the straps/hook to the back, this opposite force must be F much great than F=mg , if it is aprox or equal is just a normal bra.

.... hope you can understand TQ