import matplotlib.pyplot as plt
import numpy as np

fig=plt.figure()
ax1=fig.add_subplot(111)
ax1.fill(np.r_[0.2,0.4,0.4,0.2,0.2],np.r_[0,0,1,1,0],'grey')
ax1.text(0.41,0.66,r'$\leftarrow p_{low}A_{valve}$',size=20,ha='left',va='center')
ax1.text(0.41,0.33,r'$\leftarrow k_{valve}x$',size=20,ha='left',va='center')
ax1.text(0.19,0.66,r'$p_{high}A_{valve}\rightarrow$',size=20,ha='right',va='center')
ax1.text(0.19,0.33,r'$\frac{1}{2}C_D\rho (V-\dot x)^2 A_{valve}\rightarrow$',size=20,ha='right',va='center')
ax1.set_xlim(0.1-1,0.1+1)
ax1.axis('equal')
ax1.axis('off')
ax1.set_title('Pressure-dominant Free-body-diagram')
plt.show()