# Falling to the ground vs. a flat surface hitting you (same speed) **URL:** \ **Category:** Factual Questions\ **Created:** [December 7, 2012, 9:36am UTC](https://boards.straightdope.com/t/falling-to-the-ground-vs-a-flat-surface-hitting-you-same-speed/643285 "2012-12-07T09:36:31Z")\ **Posts on this page:** 10\ **Page:** 2
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [December 8, 2012, 3:05am UTC](https://boards.straightdope.com/t/falling-to-the-ground-vs-a-flat-surface-hitting-you-same-speed/643285/21 "2012-12-08T03:05:11Z")
As others have said, so long as the truck is much more massive than the man, the differences between the cases will be small. Yes, the body will probably be in different positions in the two collisions, but then, the body will also be in different positions for two different falls. ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [December 8, 2012, 3:59am UTC](https://boards.straightdope.com/t/falling-to-the-ground-vs-a-flat-surface-hitting-you-same-speed/643285/22 "2012-12-08T03:59:45Z")
> [@ZenBeam](#): > > Now you’re talking about differences that are really no bigger than the differences you might see between two different people hitting the ground, or the difference between two different people getting hit by a truck How did you come to that conclusion on the differences ? I said the man hitting the ground results in different impact characteristics than the man hitting the ground. In a way, I am saying a (2000 lb) 70’s car travelling at 80 mph and hitting a wall and a (2000 lb) 00’s car travelling at the same speed hitting a wall will have a **“huge difference”** in impact and damage characteristics. Me being an engineer - the difference is huge and I stand by it. > [@ZenBeam](#): > > I and others have already said there’d be small differences. That falls out of the math for [partially elastic collisions](http://en.wikipedia.org/wiki/Inelastic_collision#Formula). Are you sure on this one ? From your link - “Partially inelastic collisions … In this type of collision, the objects involved in the collisions **do not stick** …” (emphasis mine). In the case of man hitting ground, I believe it is more like man sticks to the ground. In the case of truck hitting the man - parts of the man will fly off at speeds higher than the speed of the truck. Here is a video of a train colliding with a semi - you can see parts of the semi flying off faster than the train at around 1:17 [train and semi](http://www.youtube.com/watch?v=TjeUrfjB8dE) ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [December 8, 2012, 4:27am UTC](https://boards.straightdope.com/t/falling-to-the-ground-vs-a-flat-surface-hitting-you-same-speed/643285/23 "2012-12-08T04:27:37Z")
> [@am77494](#): > > In a way, I am saying a (2000 lb) 70’s car travelling at 80 mph and hitting a wall and a (2000 lb) 00’s car travelling at the same speed hitting a wall will have a **“huge difference”** in impact and damage characteristics. Me being an engineer - the difference is huge and I stand by it. This is not the same as the OP at all. You’ve got two different cars hitting a wall, and any differences come from differences between the cars. In the two collisions in the OP, you assume you don’t have differences between the two people. > [@](#): > > Are you sure on this one ? From your link - “Partially inelastic collisions … In this type of collision, the objects involved in the collisions **do not stick** …” (emphasis mine). Whether they stick or not is captured in the coefficient of restitution, C[sub]R[/sub]. Set it to 0 if you want a completely inelastic collision. > [@](#): > > In the case of man hitting ground, I believe it is more like man sticks to the ground. In the case of truck hitting the man - parts of the man will fly off at speeds higher than the speed of the truck. Here is a video of a train colliding with a semi - you can see parts of the semi flying off faster than the train at around 1:17 [train and semi](http://www.youtube.com/watch?v=TjeUrfjB8dE) If you dropped a semi onto the ground at the same speed, you don’t think pieces will fly off? If parts of the man will fly off in the one case in the OP, they would fly off in the other case as well. ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [December 8, 2012, 4:35am UTC](https://boards.straightdope.com/t/falling-to-the-ground-vs-a-flat-surface-hitting-you-same-speed/643285/24 "2012-12-08T04:35:11Z")
> [@ZenBeam](#): > > This is not the same as the OP at all. You’ve got two different cars hitting a wall, and any differences come from differences between the cars. In the two collisions in the OP, you assume you don’t have differences between the two people. You may assume you don’t have differences between the two people - but you cannot assume that the ground and the truck bumper have the same impact characteristics. > [@ZenBeam](#): > > Whether they stick or not is captured in the coefficient of restitution, C[sub]R[/sub]. Set it to 0 if you want a completely inelastic collision. Ah - but me wanting it and what really happens is not the same thing. I would like to see why you believe that the coefficient of restitution is the same in both cases. > [@ZenBeam](#): > > If you dropped a semi onto the ground at the same speed, you don’t think pieces will fly off? If parts of the man will fly off in the one case in the OP, they would fly off in the other case as well. I don’t believe they will fly off in the case of man hitting the ground and believe they will fly off in the case of truck hitting the man. I could be wrong though and would like to see some evidence to the contrary. ---
**Author:** ![obfusciatrist](https://avatars.discourse-cdn.com/v4/letter/o/4af34b/32.png) [@obfusciatrist](https://boards.straightdope.com/u/obfusciatrist)\ **Post date:** [December 8, 2012, 6:27am UTC](https://boards.straightdope.com/t/falling-to-the-ground-vs-a-flat-surface-hitting-you-same-speed/643285/25 "2012-12-08T06:27:52Z")
> [@am77494](#): > > but you cannot assume that the ground and the truck bumper have the same impact characteristics. Except the OP assumes exactly that: > [@](#): > > assume the front of the truck that hits the person has a flat, square face whose hardness and other properties are, for all practical purposes, identical to those of the ground that the first person fell to. ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [December 8, 2012, 10:25am UTC](https://boards.straightdope.com/t/falling-to-the-ground-vs-a-flat-surface-hitting-you-same-speed/643285/26 "2012-12-08T10:25:50Z")
> [@am77494](#): > > I believe you are thinking the man as a rigid body when it is not. Different parts of the man will end up with different velocities. The moment of impact is very different and the stresses at the moment of impact are different too. No they are not. The moment of impact is fundamentally the same. In the mythbusters experiment looking into the difference between falling on pavement and falling on water, a g-meter rated for 500g maxed out in a fall from 23 meters with a flat landing on pavement. That’s a fall with a top speed less than a third of the OPs 140 mph. The two scenarios then differ in a component of the total force of, at most, .2 % of the total having a different direction, at the moment of impact. Once any part of the body is travelling at 140mph and the one G of force is the only player, the direction will matter and make for different aftermaths. But that’s not how the OPs question is being interpreted by most participants in this thread. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [December 8, 2012, 2:40pm UTC](https://boards.straightdope.com/t/falling-to-the-ground-vs-a-flat-surface-hitting-you-same-speed/643285/27 "2012-12-08T14:40:18Z")
> [@am77494](#): > > You may assume you don’t have differences between the two people - but you cannot assume that the ground and the truck bumper have the same impact characteristics. Yes, you can. From the OP: > [@](#): > > assume the front of the truck that hits the person has a flat, square face whose hardness and other properties are, for all practical purposes, identical to those of the ground that the first person fell to. > [@am77494](#): > > Ah - but me wanting it and what really happens is not the same thing. I would like to see why you believe that the coefficient of restitution is the same in both cases. From the OP: > [@](#): > > assume the front of the truck that hits the person has a flat, square face whose hardness and other properties are, for all practical purposes, identical to those of the ground that the first person fell to. > [@am77494](#): > > I don’t believe they will fly off in the case of man hitting the ground and believe they will fly off in the case of truck hitting the man. I could be wrong though and would like to see some evidence to the contrary. Yeah, that same quote from the OP again… And that part of the OP was already pointed out to you by **naita** back in post 12. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [December 8, 2012, 3:46pm UTC](https://boards.straightdope.com/t/falling-to-the-ground-vs-a-flat-surface-hitting-you-same-speed/643285/28 "2012-12-08T15:46:56Z")
Let’s make this as simple as possible. We are talking about a collision between a human body and a hard, flat surface with mass much greater than the human body, at a relative speed of 140 MPH. To me, that looks like a complete description of the collision, and it applies equally to both of the scenarios in the OP. What’s the difference? ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [December 8, 2012, 11:30pm UTC](https://boards.straightdope.com/t/falling-to-the-ground-vs-a-flat-surface-hitting-you-same-speed/643285/29 "2012-12-08T23:30:05Z")
> [@Chronos](#): > > Let’s make this as simple as possible. We are talking about a collision between a human body and a hard, flat surface with mass much greater than the human body, at a relative speed of 140 MPH. To me, that looks like a complete description of the collision, and it applies equally to both of the scenarios in the OP. What’s the difference? The one G of gravitational force which has different directions in the two scenarios, but is insignificant compared to the main forces during impact. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [December 10, 2012, 12:57pm UTC](https://boards.straightdope.com/t/falling-to-the-ground-vs-a-flat-surface-hitting-you-same-speed/643285/30 "2012-12-10T12:57:08Z")
> [@newme](#): > > So for a 200 lb man hit by a 10,000 lb truck, the final velocity will be reduced by ~2%. FWIW a tractor-trailer rig in the US typically weighs about 80,000 pounds (although you might have trouble getting it up to 140 MPH). [Previous page](https://boards.straightdope.com/t/falling-to-the-ground-vs-a-flat-surface-hitting-you-same-speed/643285.md?page=1)