You can write a simple verlet integrator, which will allow you to do many more fun things easily (this is the home-made version of using a 2D Physics engine).

#include <iostream>
#include <cmath>
float const degrees = std::atan(1.0f)/45.0f;
struct Vector2D {
float x, y;
Vector2D(float x, float y) : x(x), y(y) {
}
};
std::ostream &operator<<(std::ostream &os, Vector2D v) {
return os << '(' << v.x << ',' << v.y << ')';
}
Vector2D operator+(Vector2D v, Vector2D w) {
return Vector2D(v.x+w.x, v.y+w.y);
}
Vector2D operator-(Vector2D v, Vector2D w) {
return Vector2D(v.x-w.x, v.y-w.y);
}
Vector2D operator*(Vector2D v, float s) {
return Vector2D(v.x*s, v.y*s);
}
float length(Vector2D v) {
return std::sqrt(v.x*v.x+v.y*v.y);
}
int main() {
float const dt = 0.01f; // (s) (100 Hz simulation)
float const dt_squared = dt*dt;
float length_of_string = 0.1f; // (m)
float gravity = 9.81f; // (m/s^2)
float friction = 1.5f; // (s^-1), I believe, but you better ask a Physicist if you care ;)
float friction_coef = std::exp(-friction*dt);
float alpha = 15.0f*degrees;
Vector2D position(length_of_string*std::sin(alpha), -length_of_string*std::cos(alpha));
Vector2D previous_position = position;
Vector2D acceleration(0.0f, -gravity);
// For 10 seconds
for (float t = 0; t < 10.0; t += dt) {
// Verlet integration
Vector2D next_position = position + (position - previous_position) * friction_coef + acceleration * dt_squared;
// Constrain the point to be at the desired length from the origin
next_position = next_position * (length_of_string / length(next_position));
// Update
previous_position = position;
position = next_position;
// Output
std::cout << t << ' ' << position << '\n';
}
}

**Edited by alvaro, 08 May 2012 - 07:42 PM.**