I was reading lecture notes about Special Relativity whilst being driven over Shap. It turns out that we were in an inertial frame of reference. Newton's First Law states that unless a resultant force acts, you stay doing what you are doing and have no acceleration ie you stay still if you were still and you carry on in a straight line at constant speed if you were moving. In other words, sitting still and being driven in a straight line at constant speed should feel the same and the laws of Physics should be the same. Our reindeer mascot was unmoved. He sat there as if at home. But if the brakes had been applied suddenly, it would have ceased to be an inertial frame of reference and the car would have stopped beneath him. We know that his momentum would have kept him moving forwards but it might have looked like he was "thrown" forwards. Only an external observer would know the truth but we in the car would have had to have invented a fictitious force to explain how it could seem normal and still have the reindeer moving around.
It actually looks like everything has stopped in the top photograph but if you look out of the side window, the picture is warped and you can tell you are moving. This is how you can tell if your inertial frame of reference is actually moving or not. And the view across to the snow-covered Coniston Fells was wonderful.
