# Help w/ gravity question

1. Oct 15, 2004

### kemathen7

You'll have to excuse me - I'm nothing close to a physics expert.

I have a friend who was in a car accident. I was told they were going a couple of G's when they entered into the spin (the car flipped end over end). For argument's sake, lets say they were going 2 or 3 G's. Can someone explain to me what exactly this means and put it into idiot terms? And what could happen when going that fast? Two people were thrown from the car, and all the passenger's shoes stayed in the car.

Thanks!

2. Oct 15, 2004

### VantagePoint72

I'm not entirely sure what you mean by "going a couple of G's". "g's" (just an FYI, when refering to this, use the lower case 'g'. Uppercase refers to something else, a constant) aren't something you travel at, they're something you feel. Perhaps you just repeated what you were told incorrectly? At any rate, a 'g' is short for "gravitational force", which basically refers to the amount of force the people in the car were feeling due to their acceleration. Like when a car accelerates from a stop light, you feel pressed into your seat. The point is, when accelerating, you feel a force indistinguishable from gravity (this actually happens to be one of the core concepts in Einstein's general theory of relativity). Consider swinging a weight tied to string, like a yo-yo, around in circles. The weight seems to be pushed away from the centre of the circle. This is because it is accelerating (which refers to a change in velocity, which includes both speed and direction. If you're direction changes, you're accelerating), inward as it turns out, so, like when the car accelerates forward and you feel pushed backwards, since the weight is accelerating inward it feels an outward push. This is the same principle that works in those rides at amusement parks where the ride spins and the rider feels pressed backwards against the wall. The magnitude of this force is where the "g-force" comes in. A free-falling object will accelerate downward at, neglecting air resistance, approximately 9.81m/s^2 (the squared comes from the fact that just as speed is measured in metres per second, acceleration is measured in metres per second PER second, which is the same as saying metres per second squared). This acceleration is what we feel as the gravitational force. If your car is accelerating at 9.81m/s^2, the force pushing you into your seat would be equal to the force of gravity pushing you into the ground. Hence, the force experienced during acceleration is often put in terms of multiples of the force of gravity. So, if you're experiencing 1 g, the force you feel will be equal to the force of gravity. 2 g's is twice as strong as the force of gravity acting on you. To put all this into idiot terms, as requested, your friend's car underwent a lot of two kinds of acceleration: it slowed down a considerable amount in a short amount of time - acceleration can be negative - and it flipped end over end, thereby causing the change-in-direction acceleration I mentioned, like the yo-yo being swung in circles. Both of these translated into a force experienced by the passengers. If you say they experienced 2 or 3 g's, that means that this force was 2 or 3 times the force that you feel pushing you downward at this very moment. I hope that helps, I apologize if I didn't- I tend to be bad at explaining what I'm trying to say.

Last edited: Oct 15, 2004
3. Oct 16, 2004

### arildno

Another way of explaining this measuring in "g's" is simply:
Usually, the unit of acceleration is 1 m/s^2 (in the SI-system);
measuring acceleration in g's instead, is not particularly more different from going from cm as the length unit to "m" as our length unit.

4. Oct 16, 2004

### VantagePoint72

True, however the OP admitted to knowing very little about physics so I just wanted to give a little background on what acceleration, specifically centripetal acceleration, has to do with forces.

5. Oct 16, 2004

### arildno

There was no criticism towards your approach; in fact, I thought it very good.
However, I have occasionally met students which seem to think measuring accelerations in "g" is something mysterious and arcane. I tried to demystify this, by the analogy with cm's vs. m's