# T4 Add Spring Force using Hooke's Law
#### Find getSpringForce
Search \[TODO 1] in ClothSim.cpp, you are expected to find the following:
```cpp
// TODO: calculate spring force
glm::vec3 ClothSim::getSpringForce(int direction, int id) {
// the rest spring length
float lenRest = 0;
if (direction == DIRS.NORTH || direction == DIRS.SOUTH) lenRest = restLengthZ;
else if(direction == DIRS.WEST || direction == DIRS.EAST) lenRest = restLengthX;
else lenRest = restLengthXZ;
// the spring vector
glm::vec3 vSpring = mesh->vertices[id].pos - mesh->vertices[getId(direction, id)].pos;
// the spring length
float lenSpring = glm::length(vSpring); // distance
// the unit vector of spring direction
glm::vec3 vSpringUnit = vSpring /lenSpring;
// [TODO 1]: calculate spring force vector based on Hooke's law F = -k delta_x
// 1.1 calculate the length change delta_len
// replace 0 with your formula
float delta_len = 0;
// 1.2 Use Hooke's law F = -k * delta_len * vSpringUnit
// k : spring_factor
// replace glm::vec3(0.0) with your formula
glm::vec3 spring_force = glm::vec3(0.0);
return spring_force;
}
```
#### Calculate the spring force using Hooke's Law
Complete 1.1 and 1.2 using the Hooke's Law
#### Add spring force in \[TODO 2] vertex forces
variable name
spring force: spring
Add that to forces\[v] under \[TODO 2]
#### Build and Run
Press the Space Bar, you should be able to see the cloth simulation and finally it looks like the following
In another view
#### Add the wind
You can also add the wind force under \[TODO 2], press Space Bar and press v, you are going to see the effects of the wind force.
