G = 9.81

class Pendulum:

	def __init__(self, M, b):
		self.M = M
		self.b = b
		self.speed = 0.0
		self.position = 0.0

	def evaluate(self,_input, delta_t): # f(t)
		new_position = self.position + self.speed * delta_t
		self.speed = self.speed - G * delta_t * self.position \
				- (self.b / self.M)*delta_t*self.speed \
				+ delta_t/self.M * _input
		self.position = new_position