Consider this following thought experiment: There are two tracks, one with a line of cameras and another on which a car with transparent sides moves (\*). Now, imagine attaching a pendulum to the roof of the car, as the car accelerates to gain speed, we see that the pendulum makes an angle with the vertical in a direction opposite to which the car is accelerating in from the photographs which the line of cameras will take.

Using the photographs, we can construct a position time curve and we would find that the acceleration of bob in the car has some sudden shifts. A person inside the car can understand this phenomenon as the effect of pseudo force(\*\*), that is the pendulum bob experiences a force opposite to the direction in which the car is accelerating. However, how does a person outside the car rationalize this phenomenon occurring?

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\*: We can think of it as ripping off the car door so that we can see the pendulum's motion inside the car at all times.

\*\*: If we attach a frame of reference to the car, then that frame is accelerating, and hence we have to consider the -Ma pseudo force on it. Where M is the mass of the pendulum and a is the acceleration of the car.