# Real World Collision at speed

1. Aug 4, 2006

### Matteh

Hi, new user here, just had to ask this question though to settle a debate! :)

What I'm saying is that for a collision between two vehicles with one stationary and the other travelling at 10mph the damage will be less than if one vehicle was travelling at 50mph and the other at 60mph - same 10mph difference.

My main belief for this is that at 60mph, the engine is driving the car forwards with more force w.r.t speed as it has to overcome wind resistance. If the two vehicles were to collide, the engine would keep pushing the vehicle forwards, into the other creating a 'bigger' crash. if it were at 10mph, the engine would be doing less work compared to the speed and as such would slow down easier.

In this case, a hatchback (at 60mph) went into a cement mixer (at 50mph) and the damage looks more than the claimed 10mph difference

Any thoughts on this? my view obviously seems logical to me, but everyone else dissagrees!

its odd, because you have to take into account the fact that the car is propelled and the drag forces associated with the system

Last edited: Aug 4, 2006
2. Aug 4, 2006

### eep

I guess it depends on the collision - is it a head on collision?

3. Aug 4, 2006

### Matteh

oh sorry, the car (travelling at 60mph) went into the back of the cement mixer (travelling at 50mph)

4. Aug 4, 2006

### rcgldr

At lower speed and in a lower gear, the torque multiplier is much higher than in a higher gear (probably overdrive) cruising at 60mph. The momentum effect of the engine will also be multiplied by the gearing. So the effect of the engine's momentum may be higher at the lower speed than at the higher speed.

5. Aug 5, 2006

### Matteh

You could also say that the flywheel would have more momentum as it would be turning much faster, this would increase the engines torque

6. Aug 5, 2006

### Office_Shredder

Staff Emeritus
Cement mixer and hatchback vs..... two sedans? Because the cement mixer is obviously (in terms of conservation of momentum) going to force the hatchback to slow down more, meaning it's going to take more damage

Ignoring that, more energy is required to slow down the 60 mph car to, say, 55, than for a 10 mph car to slow down to say, 5 mph (because it's v2)

7. Aug 5, 2006

### Matteh

but what about if you think of it with the cement truck as the inertial frame of reference? the car is then only doing 10mph

8. Aug 6, 2006

### rcgldr

Assume that top gear is 3 times as tall as 1st gear (typical, 1st gear at around 3.00, top gear at 1.00) Then the flywheel is turning twice as fast at 60mph in top gear than at 10mph in 1st gear, but in 1st gear, the torque is multiplied by 3, so there's more momentum effect in 1st gear at 10mph.

True if the tires were doing the slowing, but in this case it's the big truck. The energy's frame of reference should be the relative to the point of application, and in both of these situations, the point of application is the big truck, so the energy relative to the truck is the same.

Say the car was going 10mph forwards on a flat bed truck moving at 50mph. The amount of work it takes to slow from 10mph to 0 on the truck is the same as if the car were slowing from 10mph to 0 on pavement, even though the car slows from 60mph to 50mph relative to the pavement when it's on the flat bed truck.

Similarly, even though the earth surface is moving at 900mph to the East at specific latitudes, the work done to slow from 10mph to 0mph is virtually the same if the car is headed east or west. Not to mention that the Earth is orbiting the Sun at around 67,000mph, and the Sun orbits the Milky Way at around 560,000mph.