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class Car(box):
    """Box that can be pushed and have its properties changed, and can update its position"""
    def __init__(self, mass=1, velocity=vector(), **args):
        super(Car, self).__init__(**args)
        self.mass = mass
        self.velocity = velocity
        self.momentum = self.mass * self.velocity
    def changemomentum(self, newmomentum):
        """Recompute velocity based on new momentum"""
        self.velocity = newmomentum / self.mass
        self.momentum = newmomentum
    def changemass(self, newmass):
        """Recompute velocity based on new mass using conservation of momentum"""
        self.mass = newmass
        self.velocity = self.momentum / self.mass
    def updateposition(self, deltat):
        """Update position based on velocity and a deltat"""
        self.pos = self.pos + (self.velocity * deltat)
    def applyforce(self, deltat, force=vector()):
        """Change momentum by applying a force over a period of time"""
        momentum = self.momentum + (deltat * force)
        self.updatemomentum(momentum)
        self.updateposition(deltat)