# Rear-end collision - thrown forward at all?

1. Dec 10, 2012

### autodidude

If you're in a vehicle and are hit from behind, then you would feel the seat push against you right? If you didn't have your seatbelt on, would you be thrown forward at all? If so, why? Wouldn't you be experiencing the same accleration as the vehicle you're in?

Also, would you feel the impact from the car behind at all (from waves that travel through the car or something like that)

2. Dec 10, 2012

### SteamKing

Staff Emeritus
If you feel the seat pushing against you, doesn't it stand to reason that you are feeling the impact of the car behind you?

3. Dec 11, 2012

### autodidude

Seat pushing against you would be your body at rest and the seat accelerating into your back...what I meant by the last bit is would you pushed forward at all by the the impact of the car....you know how if you have three coins, you hold the first one down, put the second one on one side of the first coin, then smash the third into the opposite side of the first one, the second goes flying? That's what I'm talking about, would your body be a bit like the second coin? (think this has to do with conservation of momentum?)

4. Dec 11, 2012

### haruspex

Of course you would be thrown forward with and by the car, but then (assuming the brakes are on) the car will quickly decelerate and you may become dependent on your seatbelt to avoid overtaking the dash.

5. Dec 11, 2012

### K^2

Yup, that does happen in a collision like this. It's a little different when masses are different, but it does work. A good demo of the effect is dropping a basketball with a tennis ball on top of it. The impact with ground makes the tennis ball shoot way out higher than the point you dropped both from. With a little bit of imagination, I hope you can see how this is similar to a rear end collision.

So yes. When rear ended, you will first hit the seat, and then be thrown forward. So your seat belt will help avoid additional injuries.

6. Dec 14, 2012

### autodidude

This seems like a super basic question but why would the car decelerate faster?

So would the 'thrown forward' part be due to the push on you from the seat and then your body continuing forward or form the conservation of momentum thing? Or both?

7. Dec 14, 2012

### K^2

These are just two alternative ways to look at the same situation. Rate of change of momentum is equal to net force. You can look at it as Newton's 3rd requiring your push against the seat to be balanced by seat pushing against you and accelerating you forward, or you can look at an overall momentum and note that it must be conserved. That looks after the 3rd law internally. You'll get same result either way. Difference is, to work out acceleration, you need the time-dependence of the force between you and the seat, and that's generally difficult to obtain. On the other hand, momentum approach just requires you to compare initial and final states, which is pretty straight forward.

8. Dec 15, 2012

### justsomeguy

It doesn't (unless you go flying out the window), but it does start to decelerate *first*. You don't start to decelerate until something resists your forward momentum, be that the dash or the seatbelt.

You can see camera footage of rear crash testing on youtube.

Here's one as a for example:

Last edited by a moderator: Sep 25, 2014
9. Dec 16, 2012

### autodidude

Ah ok...I haven't really learned about momentum 'properly' yet so I'll get onto that. Thanks

Is the cause of the deceleration of the vehicle the friction between the road and the tyres? And why would the car decelerate faster if you go flying out the window (assuming that's what you meant)?

10. Dec 16, 2012

### justsomeguy

The friction of the brakes slows the wheels, and their friction with the road slows the car.

It was a joke, but the serious answer is that if you go flying out the window, nothing is acting against your momentum to slow you down. The car decelerates, but you don't.