View Full Version : If I drop a penny...
03-31-02, 12:02 AM
I might just be ignorant, because I have taken basic physics, but, I can only come up with semi-solid answers for this question of mine:
Sit on the rode and drop a penny. It falls straight down.
Sit in a moving car and drop a penny. Relative to the car, the penny falls straight down (or so it seems), but relative to the Earth, the penny's moving quite quickly as it falls.
What's happening here?
Nothing strange is going on, the penny falls just as quick to earth in the car as it does when you're not moving (the most stunning, but yet trivially true statement I've heard in ths kind of problems is that a bullet, when fired from a gun, falls just as quick on the ground, as when you just drop it on the ground).
The velocity of the penny can be decomposed in two components (one towards the earth and one in the car's movement direction). When you just drop the penny on the ground, not in the moving car, the gravitational force works on it and accelerates it towards earth. There is no initial velocity in the car's movement direction (perpendicular to the earth's surface). When you sit in the car, you now give it an initial velocity in the car's movement direction, but the only force working on it is still gravity, that will only accelerate the penny towards the earth. Whether the penny has a velocity component in the car's movement direction is irrelevant.
Hope this clarifies it a bit,
03-31-02, 12:39 PM
but crisp, if the car accelerates, we get a different situation no?
03-31-02, 01:10 PM
There are certain other points, Dropping a penny in a car is a closed system.
The windshield (Windscreen) and the rest of the cab closes the system to the outside movement of airflow, so inside the car the space is tranquil and calm.
The lack of friction on the falling penny is what makes it fall straight down. If you were to try it in a convertable (Notibly a jeep you can drop the windshield down on) then that penny isn't going to fall straight down.
As for acceleration, once that coin is falling through space, it's no longer adjusting to the cars speed. So if you break suddenly or accelerate that coin is going to shift forwards or back. (or bouncing around the car if you happen to roll)
The penny in your car, if your car is moving, is already moving forward because the car is moving. Common trajectory/momentum. The air in your car is also moving with the car and penny. If you could see the penny drop inside your car from outside, like an X-ray picture, you would see it trace a curved line downward as it rode along in the car. If you drop it outside the car, it would still retain that common momentum, and still be pulled down by gravity, but the curve of falling would be a tad steeper due to air resistance from the air outside the car, which will most likely not by moving along with the car.
This picture is crap, but it's better than nothing. The second line is supposed to follow the dots representing the falling penny.
Hi c'est moi
"but crisp, if the car accelerates, we get a different situation no?
I'd have to think on that one, but my first intuitive guess would be that nothing changes: the penny gets a velocity component in the car's movement direction that is just as big as the speed of the car at the moment you drop it. However, I'd have to look into it to be sure though.
Basic physics of movement.
Any object moving straight in constant speed or stay still, if there is no force drive it.
Since your car moving, It's the initial speed for the object in horizontal axes. Without friction (to surounding air), the speed of coin in horizontal axes (or any direction of your car) will remain constant since you drop it.