Abstract: A simplified 2D model for system of passenger and carseat was created in order to study the Hpoint of polyurethane foam carseat， in which the interaction between human body and seat was treated as spring model with nonlinear and viscoelastic properties. The Matlab integrodifferentialalgebraic solver ode15s was used to solve equilibrium position of the passenger and carseat model. The results show that the harder the material is， the closer the static equilibrium position is to the initial position of human body before the foam compressed. The larger the K3 spring stiffness is， the final equilibrium position of the body is more close to the initial position. The more forward K3 is moving， the body will be more slip into the seat. If μ1， the coefficient of friction between human body and cushion， is too small， body tends to slip out of seat.

Key words: Hpoint, model for system of passenger and carseat, polyurethane foam, nonlinear and viscoelastic properties, Lagrange equation