The Force of (making) Love 

Engaging in sexual intercourse is one of the two most important behaviors for humans and other animals to perform in order to maintain a population. While in the act of consensual, heterosexual, missionary sex there are several forces acting on both the male and the female. Forces on the female (bottom), include the force of gravity acting from the person on the bed and the normal force that the bed is exerting back on the female. Assuming that this act was completed on a bed with box springs, the spring force would be applied from the bed on to the female. This allows the woman to more easily thrust because the spring force is helping with that upward motion. The padding on the bed allows the female to not injure her tailbone when hitting the bed each time because padding increases the time over which the momentum changes, which will decrease the force felt by the female (sum of the forces= change in momentum/change in time). There is also an applied force from both the male and female in the thrust going towards each other. There is also friction between the condom (because safe sex is the best option) and (mainly) the female genitalia. This will slow down the motion and result in the male and female needing to apply more force when thrusting toward each other to reach the desired speed/intensity. If the pullout method is used, during the male orgasm the penis will ejaculate, I would argue that this is a perfectly inelastic collision in the male system. The conservation of momentum could be used to find the velocity of the semen and man initially or the velocity of the semen after the collision (ejaculation) or velocity of the man after his mass is decreased from the ejaculation. Assuming that before and after the ejaculation occurs the velocity of the male is going away from the female (pulling out) and the velocity of the semen is going toward the female, the final velocity of the semen must be negative because it is going in the opposite direction of the initial velocity and the final velocity of the male. (shown in equation below)