# Combined Speed

## Combined Speed

OK, this might be a bit different to the usual stuff that gets discussed on here but I was having a discussion with a work colleague -

If a vehicle travelling 100klm/h North has a collision with another vehicle travelling South at 100klm/h, is the speed at the point of impact 200klm/h or is it 100klm/h?

I thought it was 100klm/h as I think it would be the same as a having a head on with an immovable object but my colleague believes it to be 200klm/h due to the kinnetic energy.

He said mythbusters did a episode on it but I cannot locate the episode.

Any pointers appreciated,

truck

If a vehicle travelling 100klm/h North has a collision with another vehicle travelling South at 100klm/h, is the speed at the point of impact 200klm/h or is it 100klm/h?

I thought it was 100klm/h as I think it would be the same as a having a head on with an immovable object but my colleague believes it to be 200klm/h due to the kinnetic energy.

He said mythbusters did a episode on it but I cannot locate the episode.

Any pointers appreciated,

truck

**truck**- Posts : 520

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## Re: Combined Speed

200. It is the combined speed. Imagine if one was going in reverse at 99kph. The impact would only be 1kph.

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**Hammy**- Posts : 4446

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## Re: Combined Speed

Yep, I seem to remember from Year 11 physics that kinetic energy is determined by the formula E = 1/2 x M x V x V (ie one half mass times velocity squared). Bring two lots of that together at 100kmph and it's quite a mess, as almost none of the energy is dissipated by crumple zones.

**Ewok1958**- Posts : 3842

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## Re: Combined Speed

I'll chuck in something to think about that was put to me in my yr 11 physics class.

The speed is 200km/h but my teacher posed the question (because we were young hoons ) would you prefer to take on another car at 100km/h or a big old red gum at 200km/h?

While not disputing Ewok's suggestion above some energy is absorbed by another vehicle. I think a tree would make more of a mess. The end result is probably the same but the kinetic energy is different.

The speed is 200km/h but my teacher posed the question (because we were young hoons ) would you prefer to take on another car at 100km/h or a big old red gum at 200km/h?

While not disputing Ewok's suggestion above some energy is absorbed by another vehicle. I think a tree would make more of a mess. The end result is probably the same but the kinetic energy is different.

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**2wheelsagain**- Admin
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## Re: Combined Speed

The problem with the tree is you are exerting the same force on a much smaller surface. Thus it'll tear the car in two rather than crumple

**reddog**- Posts : 2508

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## Re: Combined Speed

Here's a head scratcher, while we're on the subject.

Two cars traveling along a straight road in the same direction.

The first car is 1km in front of the second & traveling at X km/h

The second car is traveling at 2X km/h. ie. Twice the speed of the first car.

So, when the second car has traveled 1km and is now where the first car was. The first car has traveled 500m furher along the road. The distance between the cars has halved.

The second car travels another 500m to be where the first car was. The first car has traveled 250m further along the road. The distance between them has halved.

The second car travels 250m to be where the first Car was. The first car has done another 125m . The distance between them has halved.

You get to a point where the second car travels 1cm and the first 5mm and on and on

Will they ever collide?

Two cars traveling along a straight road in the same direction.

The first car is 1km in front of the second & traveling at X km/h

The second car is traveling at 2X km/h. ie. Twice the speed of the first car.

So, when the second car has traveled 1km and is now where the first car was. The first car has traveled 500m furher along the road. The distance between the cars has halved.

The second car travels another 500m to be where the first car was. The first car has traveled 250m further along the road. The distance between them has halved.

The second car travels 250m to be where the first Car was. The first car has done another 125m . The distance between them has halved.

You get to a point where the second car travels 1cm and the first 5mm and on and on

Will they ever collide?

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**Bosco15**- Posts : 1357

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## Re: Combined Speed

reddog

Yes, but what if the tree was a immovable oversize object with no crumple zone matching the same surface area of said vehicle? And that is where I am unable to comprehend the combined speed formulae. I see it as impact speed of 100 rather than combined speed of the 2 vehicles at point of impact, though I feel science disagrees with my point of view.The problem with the tree is you are exerting the same force on a much smaller surface. Thus it'll tear the car in two rather than crumple

How young are you blokes that you can remember physics?

Yes?@Bosco15 wrote:Here's a head scratcher, while we're on the subject.

Two cars traveling along a straight road in the same direction.

The first car is 1km in front of the second & traveling at X km/h

The second car is traveling at 2X km/h. ie. Twice the speed of the first car.

So, when the second car has traveled 1km and is now where the first car was. The first car has traveled 500m furher along the road. The distance between the cars has halved.

The second car travels another 500m to be where the first car was. The first car has traveled 250m further along the road. The distance between them has halved.

The second car travels 250m to be where the first Car was. The first car has done another 125m . The distance between them has halved.

You get to a point where the second car travels 1cm and the first 5mm and on and on

Will they ever collide?

**truck**- Posts : 520

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Age : 52

Location : Queensland Proud!!

## Re: Combined Speed

@Bosco15 wrote:Here's a head scratcher, while we're on the subject.

Two cars traveling along a straight road in the same direction.

The first car is 1km in front of the second & traveling at X km/h

The second car is traveling at 2X km/h. ie. Twice the speed of the first car.

So, when the second car has traveled 1km and is now where the first car was. The first car has traveled 500m furher along the road. The distance between the cars has halved.

The second car travels another 500m to be where the first car was. The first car has traveled 250m further along the road. The distance between them has halved.

The second car travels 250m to be where the first Car was. The first car has done another 125m . The distance between them has halved.

You get to a point where the second car travels 1cm and the first 5mm and on and on

Will they ever collide?

They will only collide if driver 2 is texting

Your theory is only relevant if its based on time V space and not just time.

I dont know about you but I have overtaken a few vehicles in my time

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**2wheelsagain**- Admin
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## Re: Combined Speed

The combined speed is 200km/h regardless of crumple zones etc.

The crumple zones only effect the amount of energy absorbed by you.

The more crumple, the more energy absorbed by the car the less energy absorbed by your body.

Therefore, two cars, two crumple zones would be better than one car and a tree.

The crumple zones only effect the amount of energy absorbed by you.

The more crumple, the more energy absorbed by the car the less energy absorbed by your body.

Therefore, two cars, two crumple zones would be better than one car and a tree.

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**Bosco15**- Posts : 1357

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## Re: Combined Speed

The tree is also physically attached to the ground, another car isn't@reddog wrote:The problem with the tree is you are exerting the same force on a much smaller surface. Thus it'll tear the car in two rather than crumple

**Chook**- Posts : 3308

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## Re: Combined Speed

And if the tree fell over, would the driver in the car hear it ? Ah haa.

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**Hammy**- Posts : 4446

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## Re: Combined Speed

Truck, I can tell you from experience, that one vehicle travelling at 45kp/h, mine, and another travelling at 60kp/h, pissed young P plater, sure as hell feels like more than a 45kp/h impact.

Both vehicles were the same size and make.

Both vehicles were the same size and make.

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**Kiwisteve**- Posts : 1419

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## Re: Combined Speed

@2wheelsagain wrote:@Bosco15 wrote:Here's a head scratcher, while we're on the subject.

Two cars traveling along a straight road in the same direction.

The first car is 1km in front of the second & traveling at X km/h

The second car is traveling at 2X km/h. ie. Twice the speed of the first car.

So, when the second car has traveled 1km and is now where the first car was. The first car has traveled 500m furher along the road. The distance between the cars has halved.

The second car travels another 500m to be where the first car was. The first car has traveled 250m further along the road. The distance between them has halved.

The second car travels 250m to be where the first Car was. The first car has done another 125m . The distance between them has halved.

You get to a point where the second car travels 1cm and the first 5mm and on and on

Will they ever collide?

They will only collide if driver 2 is texting

Your theory is only relevant if its based on time V space and not just time.

I dont know about you but I have overtaken a few vehicles in my time

'Wheels' is right you are looking at an ever decreasing time interval, say you get down to 1 billionth of a second, sure you can look at half a billionth, but one billionth of a second later it's Game Over.

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**Re-Cycled**- Posts : 2206

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## Re: Combined Speed

@truck wrote:reddogYes, but what if the tree was a immovable oversize object with no crumple zone matching the same surface area of said vehicle? And that is where I am unable to comprehend the combined speed formulae. I see it as impact speed of 100 rather than combined speed of the 2 vehicles at point of impact, though I feel science disagrees with my point of view.The problem with the tree is you are exerting the same force on a much smaller surface. Thus it'll tear the car in two rather than crumple

How young are you blokes that you can remember physics?Yes?

Two cars traveling along a straight road in the same direction.

The first car is 1km in front of the second & traveling at X km/h

The second car is traveling at 2X km/h. ie. Twice the speed of the first car.

So, when the second car has traveled 1km and is now where the first car was. The first car has traveled 500m furher along the road. The distance between the cars has halved.

The second car travels another 500m to be where the first car was. The first car has traveled 250m further along the road. The distance between them has halved.

The second car travels 250m to be where the first Car was. The first car has done another 125m . The distance between them has halved.

You get to a point where the second car travels 1cm and the first 5mm and on and on

Will they ever collide?

I wouldn;t be worried about the speed formulae. If the tree is immovable you are totally fucked!

Have a look at people that try driving past them toll road bollards and thats at slow speed. I've seen a 30-40cm redgum split cars in half before, they seldom snap compared to jarrah which are the two main eucalypts down in the south west. Then you head further down south and see the size of some of the karri trees. You see a lot of crosses on the road so I presume they dont move far when 2 tonne of car careers into them at 100kph

**reddog**- Posts : 2508

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## Re: Combined Speed

This is from the show Mythbusters:

Two cars crashing into each other at 50 mph will result in the same damage (for each car) as a single car hitting a wall at 50 mph. confirmed

In their small scale tests, the Mythbusters compressed clay at 1x and 2x speeds. Their results showed that two objects hitting each other at 1x speed will cause 1x damage. In their full scale tests, the Mythbusters crashed two cars into a wall at 50 and 100 mph as references. They then had two cars going at 50 mph collide into each other. After surveying the results, it was clear that the two cars suffered damage identical to the car that crashed into the wall at 50 mph. The Mythbusters explained that was possible through Newton’s third law of motion. Although the total force was doubled by having two cars, that force also had to be divided between both cars during the crash.

Two cars crashing into each other at 50 mph will result in the same damage (for each car) as a single car hitting a wall at 50 mph. confirmed

In their small scale tests, the Mythbusters compressed clay at 1x and 2x speeds. Their results showed that two objects hitting each other at 1x speed will cause 1x damage. In their full scale tests, the Mythbusters crashed two cars into a wall at 50 and 100 mph as references. They then had two cars going at 50 mph collide into each other. After surveying the results, it was clear that the two cars suffered damage identical to the car that crashed into the wall at 50 mph. The Mythbusters explained that was possible through Newton’s third law of motion. Although the total force was doubled by having two cars, that force also had to be divided between both cars during the crash.

**Cal**- Posts : 353

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## Re: Combined Speed

I am feeling your pain Kiwisteve.@Kiwisteve wrote:Truck, I can tell you from experience, that one vehicle travelling at 45kp/h, mine, and another travelling at 60kp/h, pissed young P plater, sure as hell feels like more than a 45kp/h impact.

Both vehicles were the same size and make.

I understand what you are saying Bosco15 but I was more questioning the claimed combined speed at point of impact@reddog wrote:@truck wrote:Have a look at people that try driving past them toll road bollards and thats at slow speed. I've seen a 30-40cm redgum split cars in half before, they seldom snap compared to jarrah which are the two main eucalypts down in the south west. Then you head further down south and see the size of some of the karri trees. You see a lot of crosses on the road so I presume they dont move far when 2 tonne of car careers into them at 100kph

Thanks Cal, I may be wrong but that last sentence is along the lines of what I was thinking@Cal wrote:This is from the show Mythbusters:

Two cars crashing into each other at 50 mph will result in the same damage (for each car) as a single car hitting a wall at 50 mph. confirmed

In their small scale tests, the Mythbusters compressed clay at 1x and 2x speeds. Their results showed that two objects hitting each other at 1x speed will cause 1x damage. In their full scale tests, the Mythbusters crashed two cars into a wall at 50 and 100 mph as references. They then had two cars going at 50 mph collide into each other. After surveying the results, it was clear that the two cars suffered damage identical to the car that crashed into the wall at 50 mph. The Mythbusters explained that was possible through Newton’s third law of motion. Although the total force was doubled by having two cars, that force also had to be divided between both cars during the crash.

**truck**- Posts : 520

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## Re: Combined Speed

Sorry to chime in so late in the conversation, but have you considered that if you were travelling faster then there would be no impact.... on account of the fact that you would be 0.5 km up the road.

Sorry... this conversation was getting too serious!

Sorry... this conversation was getting too serious!

**spanna**- Posts : 151

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