def act(environment)
    elapsed_time = @clock.elapsed_time
    environment.objects.each do |object|
      #Slow object.
      acceleration = @force * elapsed_time
      speed = object.vector.speed
      if speed > 0
        speed -= acceleration 
        speed = 0 if speed < 0
      elsif speed < 0
        speed += acceleration
        speed = 0 if speed > 0
      end
      object.vector.speed = speed
    end
  end
def act(environment)
    elapsed_time = @clock.elapsed_time
    environment.objects.each do |object|
      #Slow object.
      acceleration = @force * elapsed_time
      speed = object.vector.speed
      if speed > 0
        speed -= acceleration 
        speed = 0 if speed < 0
      elsif speed < 0
        speed += acceleration
        speed = 0 if speed > 0
      end
      object.vector.speed = speed
    end
  end
Reduce each object's speed at the given rate.