# Discussion regarding the frames of reference

1. Jan 7, 2014

### george ozua

A person is inside a car. This person is not using the seat belt. The car crashes with a solid wall and decelerates quickly. The person, due to inertia, keeps moving until stopped by the wind shield.
I want to analyze the movement of this person after the crash, but I am troubled because I do not know what frame of reference I should take.
A) If we take the car as the frame of reference, I think the person describes a uniformly accelerated motion after the crash. The reasons to state this are the following- Before the crash, the wind shield and the person move at the same speed (therefore, there is no movement between them). After the crash, the wind shield stops completely ( because the car stops completely) and the person starts to move in relation to the car. The movement of the person is uniformly accelerated because the person (in relation to the car) parts from absolute repose to an x speed.
B) If we take the road or a fixed observer outside the car as the frame of reference, the person inside the car moves at a uniform speed (non accelerated motion) after the crash. The reasons to state this are the following- in relation to the road and before the crash, the person moves at the speed of the car (the person passes through every point of the road at the speed of the car). After the crash, the car stops completely, but the person keeps moving at its previous speed (due to inertia) in relation to the road. Therefore, there is not really any change in the person s speed and the movement cannot be considered as accelerated (at least until he hits the windshield).
Taking into account all the previous information, if I want to analyze the movement of the passenger, what frame or reference should I take (A or B)? (By analyzing the movement of the passenger I mean obtaining data such as the impact velocity of the passenger with the windshield and the usage of SUVAT ecuations). Independently from the car and the road, how does the person moves? Is he moving all the time at uniform speed until he hits the windshield? or, is he moving at uniform speed before the car crash and in an accelerated motion after the car crash? Can his movement be considerated as accelerated and non accelerated at the same time? Most importantly, how can I calculate his speed when he hits the windshield?
Thanks!!!

2. Jan 8, 2014

### Staff: Mentor

You can't take example #1. This would ignore the very real acceleration on the car and attribute a false acceleration to the driver. Go with #2.

The speed of the driver when they hit the windshield will be equal to the speed the car was traveling before the impact, assuming that nothing else slows the driver down and the car comes to a complete stop before the driver hits the windshield.

3. Jan 8, 2014

### Simon Bridge

Seconded - when you have the option, go with an inertial frame.