Women and overweight people often experience backpain. Let us explore the reason for this. Assume
the women has a weight of 60.0 daN before pregnancy. Suppose she gains 10.0 daN during pregnancy,
due to weight of the fetus, placenta, amniotic fluid, etc. To make the calculation easy, but still realistic,
we shall model the unpregnant women as a uniform cylinder of diameter 0.30 m. We can model the
added weight due to the fetus as a 10.0 daN sphere with a 25.0 cm diameter and centered about 5.00
cm outside the woman’s original front surface.
(a) By how much does her pregnancy change the horizontal location of the woman’s center of gravity?
(b) How does the change in part (a) affect the way the pregnant woman must stand and walk? i.e.,
what must she do to her posture to make up for her shifted center of gravity?
(c) Can you now explain why she might have backaches?
To answer these questions, we need to understand the concept of center of gravity and how it relates to a person's posture and backaches.
(a) To calculate the change in the horizontal location of the woman's center of gravity, we need to consider the weights and positions of the unpregnant woman and the fetus.
The unpregnant woman can be modeled as a uniform cylinder. We are given its diameter, which is 0.30 m. To calculate the location of its center of gravity, we can consider it as a thin rod with its center of gravity at its midpoint. Therefore, the center of gravity of the woman can be taken as 0.15 m from her original front surface.
The added weight due to the fetus can be modeled as a 10.0 daN sphere with a diameter of 25.0 cm. The sphere is centered 5.00 cm outside the woman's original front surface. To calculate the location of the center of gravity of the fetus, we can consider it as a point mass located 5.00 cm outside the front surface.
To find the overall center of gravity of the pregnant woman, we can calculate the weighted average of the woman and the fetus. The horizontal shift of the center of gravity can then be determined by taking the difference between the original location of the woman's center of gravity and the new location.
(b) The change in the horizontal location of the woman's center of gravity affects the way the pregnant woman must stand and walk. To make up for her shifted center of gravity, she needs to adjust her posture to maintain balance and prevent falls.
Typically, a pregnant woman will tilt her pelvis forward and lean back slightly to shift her center of gravity backward. This compensates for the forward shift caused by the growing fetus. By tilting the pelvis, she brings her center of gravity closer to her original position and maintains a stable posture.
(c) The reason why pregnant women, as well as overweight individuals, may experience backaches is due to the strain placed on their lower back muscles and spine. The forward shift in the center of gravity during pregnancy or excessive weight can cause an increased curvature of the lower back, known as lordosis. This excessive curvature puts extra pressure on the lower back muscles and spine, leading to discomfort and pain.
Additionally, the ligaments supporting the joints in the lower back may become lax during pregnancy, further contributing to back pain.
Therefore, the combination of the shifted center of gravity and the resulting changes in posture can lead to backaches in pregnant women and overweight individuals.