# Car doors and gravity

1. Jun 19, 2011

### Luke1121

I was asked a relativley simple question by someone the other day and I was a little shocked that I didn't know. The question was why is it harder to open a car door when it's at an angle parked half on the pavement and half on the road. I had a go at answering the person but I have no idea if I'm right. I thought the answer lay in vectors and I thought when the car is level the angle is tiny (in terms of drawing a right angle triangle and using trigonometry) so the vertical component will be small and the horizontal component will be large. When it's at an angle the vertical component becomes larger so it is more difficult to open. Is this even remotely correct? Please correct me if I'm wrong, thank you very much

2. Jun 19, 2011

### Filip Larsen

Car doors are designed to be easy to open when the car is level, that is, the hinges are positioned "vertically" so that it takes very little effort to swing the door open horizontally. When the car is parked at an angle, the door no longer swings purely horizontally, but also a bit either up or down. Assuming you have to open a door so that it swings upwards you therefore have to put more effort into it because you are in fact lifting the door a little bit upwards against gravity, and as you probably know, lifting stuff upwards takes far more effort than just sliding it along a level floor. Also, as far as I know, most car doors also have spring loaded "bumps" along the way that help the door stay in 2-3 different positions along the way, even if the car is tilted a bit, and these "bumps" also takes a bit more effort to push the door past.

You can equate it with the situation where you have to roll a wheel along the ground. If the ground is flat, it takes very little effort to keep the wheel rolling (it may take a tiny bit to start and stop the rolling, though, depending on how heavy the wheel is) and even if there is a small bump in the ground the wheel will just travel over it if it is rolling fast enough. If you now try to push the same wheel up a small incline you find that you have to continue to put an effort into keeping the wheel rolling or it will stop and roll back down, and even more so if you hit a bump on the way up.

3. Jun 19, 2011

### Luke1121

Thanks for that, it seems I overcomplicated things, i feel silly for not knowing something so simple