Why I changed my order to a 07 Camry

did someone say breakfast?... I'm always into breakfast... and lunch and then dinner... and midnight snack ice cream floats!.... :huh:

 

Sorry for the confusion Jim. When I referred to 2 Prii at 25 mph, I meant each was moving at that speed. Closing speed of 50 mph, each at the Kinetic Energy of 25 mph.

If 1 Prius hit a wall at 50 mph, its Kinetic Energy is twice that of the 2 Prii closing at a RELATIVE speed of 50 mph. EACH 25 mph car, if compared to its KE at 50 mph, has 1/4 the KE. So, 1/4 + 1/4 = 1/2, yet the closing speed is 50 mph.

If one were moving at 40 mph and the other at 10 mph (50 mph relative), the total KE would be 68% of that for one vehicle at 50 mph. If the speeds are 49 and 1 mph, the total Ke is 96% of that for one vehicle at 50 mph. KE is a function of velocity squared, and therefore non-linear.

 

Denny,

You still didn't quite answer the questions. Simply put. Explain the difference in engergy absorbed by a signle vehicle between the following two scenarios:

1) Two prii, each travelling at 25mph in a direct head on collision
2) One Prius, travelling at 25mph into a immovable barrier.

I have been arguing tha the two scenarios are identical in terms energy abosorb by one car. Others are trying to tell me that the driver of a prius in scenario 1) would be twice as bad off as the driver in scenario 2. I still believe that I am correct.

/Jim

 

Jim, I got your back! Let me try to explain. I think the major source of confusion is that people want the wall to absorb half of the energy. If the wall absorbs any of the energy in case 2, then the driver is indeed better off than in case 1.

If 1 Prius hits a fixed, rigid, non-energy-absorbing wall at 25 mph, it dissapates all of its energy. If 2 Prii hit each other head on, each going 25 mph, there's twice as much energy, but there's twice as many cars to absorb that energy. Since the cars are symmetric about the collision, each car absorbs half of the energy, so it's the same as if each had hit a fixed, rigid wall at 25 mph.

Here is a movie of the NHTSA crash test. You'll see that the wall they use doesn't absorb much energy from the collision.

 

What Plasm said!

 

Thank you Denny... I can sleep now

It is remarkable how many people that I have talked to who really do believe that a head-on collision with two cars each driving at 25mph.... is the same as a driver hitting an immovable object at 50mph.

/Jim

 

That makes no sense as you're only talking about kinetic energy relative to the ground. A satellite can orbit the earth at 13000 mph and collide with another satellite going 12996 mph. Does it create an enormous explosion? Of course not! They're only going 4 mph relative to each other. Their kinetic energy is high relative to the ground and low relative to each other.

 

Yeah, Potential knows Kinetic. Good explanation.

Although, the "of course not" caught me off guard when I figured they were going 25996 mph relative to each other. You didn't mention the directions they were going.

 

Yea but your satellite illustration is not what he has been talking about..... if one prius going 26mph collided with another going 25 "going the same direction" then you only have 1mph difference... but if they are going opposite directions and hit each other head on.. then you have a problem.

Your satellites were both going the same direction as I understood it... thats not a direct head on as discussed.

This reminds me of something I heard once..... "knoweledge ever increasing, yet still unable to come to the knowledge of the truth"

I give up!.. you guys can start your own church and believe what you want.

 

Okay, take off your seat belt...

If you run into a brick wall at 25mph, you are going to hit the stopped windshield at 25mph.

If you run into another prius at 25mph that is able to stop you because it too was going 25mph, you are going to hit the stopped windshield at 25mph.

If you run into a brick wall at 50mph, you'll hit the stopped windshield at 50mph. That would seem to be worse than hitting it at 25mph.

 

The same reason it's hard to stand still and hold a sack of bricks.

 

I know you gave up, but I apparently have a thing for dead horses.

How about the intermediary step:

I agree that if you're going 25mph and hit a brick wall that is moving at 25mph (in your direction), That's the same as hitting a stationary brick wall at 50mph.

You can't just replace a car with a brick wall without changing something else in the equation, hitting a brick wall is worse. It has much more mass. Mass matters. Just ask all the bugs you had collisions with in your car today.

 

No thank you... my personal mass is quite a bit lower than a Prius

/Jim

 

This post may be a bit longish. The crash of Prii into each other and into walls got a bit out of control. I hope this is my last on the subject.

I always emphasized that the Kinetic Energy (KE) of vehicles (or vehicle) involved determined the severity of a crash. Since most folks seemed to miss the point, I will attack crash severity from another angle.

Impulse-Momentum exchange.

Momentum is P = m*v, where m = mass and v = velocity.
Impulse is I = F*t, where F= force and t = time.
Therefore, I = P, or F*t = m*v.

During a crash the time taken to reach zero momentum is t2-t1, and the velocity at t2 is assumed to be zero. Then during the crash v2-v1 represents the end velocity minus the starting velocity. Rearranging to solve the The Impulse equation to yield force, results in:

F = m*(v2-v1)/(t2-t1) , a simplified differential eqn.

Let’s crash a 3200 lbf Prius into a solid barrier at 40 Ft/sec.

Specify that v2 = 0, and v1 = 40 ft/sec (27 mph), and t2 = 0.1 and t1 = 0 sec
Also let m = 3200 poundals = 3200 lbf-sec^2/ft. Time to reach 0 velocity is 0.1 seconds. Then:

F = 3200*(0-40)/(0.1-0) lbf = -128,000/0.1 lbf = -1.28 million lbf.

This is the force applied to the vehicle over the 0.1 sec sudden stop. An unrestrained object will continue forward consistent with the vehicle deceleration of 400 ft/sec^2.

Let’s try the same conditions, except that the velocity is set at 80 Ft/sec (54.5 mph). The above Impulse-Momentum equation, solved to return force, is the same, but v1 =80 ft/sec.

F = m*(v2-v1)/(t2-t1) = 3200*(-80)/(t2-0). = -256,000/ t2 lbf.

So what shall we use for t2? The velocity of the vehicle is twice that of the previous example. Will the engine compartment collapse at the same rate as it did for 40 ft/sec.? Not likely. Within 10%, it will collapse at twice the rate. So, for t2, I will use 0.05 sec, rather than 0.1 sec.

Then F = -256,000/0.05 lbf = -5,120,000 lbf = -5.12 million lbf. Which, of course, is 4 times the force of the collision at half the speed.

My reason for using KE in previous posts is the if the kinetic energy is increased by a certain percentage, all other conditions being unchanged, the severity of the crash will be increased by the same percentage.

It follows, that if two Prii at 40 ft/sec collide, the severity will be as if each had run into a solid wall, since the other vehicles Impulse will be identical, where the Impulse = m*v (momentum). Each front end collapses at the same rate, ergo same result as strking an immovable wall.

Summary; A.) Momentum is dissipated over time. The longer the time, the lower the applied force (think auto shock absorbers). B.) If momentum is doubled, shock absorption time is halved. Therefore 2*mv/(1/2t) = 4*F. Quadrupled force!

Thanks for enduring (for those who did).

 

*sigh* They were moving towards each other... at 4 mph. Who cares what their velocity is relative to the earth? The universe does not revolve around the earth you know. Clapping your hands in a plane going 500 mph does not produce a louder sound than clapping them on the ground because they both are moving towards each other at the same speed, regardless of the speed relative to the ground.

 

Sorry for the misunderstanding... when you made the statement in your post #128:

... I thought we were still talking about head on collisions... the whole arguement here was two prius's in a head on going 25mph. .would it be nastier to either one than merely hitting a wall at 25mph...

My mistake if I misunderstood you.

and your right about:

Things is space have nothing to do with whats going on the earth.... one inch or one miles in space is the same as on earth....and it your going 1000mph on earth.... that same speed in space doesn't change.

Sometimes people get confused with all they know.... just let the dust settle and use common sense sometimes?

The more you through complicated equations out etc, the more you won't see the obvious because sound reasoning does not follow equations unless your a genius or autistic.
Sometimes the equations are wrong...... or at least wrong for what your talking about.

Algebra only works if the problem was set up right to fit the situation.

I'm amazed this totally highjacked thread hasn't gone to breakfast yet.....

 

PB,

Sorry, but I scanned your post too quickly and missed your point. After re-reading, I must say you are correct. I made a huge error in not rigorously checking my premises (or frame of reference).

So the point you make can be stated: must enegy be able to change relative to an external reference frame; or is it something that only changes relative to impact points? Clearly, forces can only exist relative to impact points—not relative to an external reference frame, which is infinitely variable. So the correct definition of kinetic energy should be the change in momentum (Del m*v) relative to impact points or the points where the forces act! Which is what I did in a previous post in this thread. My conclusion was wrong in that when analyzing an "elastic" collision using impulse-momentum, the results seemed to support the KE approach.

Say a low performance plane flying at 80 kts is overtaken by a high performance plane moving at 500 kts indicated, and both are bucking a 50 kt headwind. When the collision occurs, the relative speed at time of impact would be 420 kts. Speed relative to the ground is inapplicable. Kinetic energy of the "system", as in combined center of mass of the 2 airplanes defines the total KE! After that velocities and mass of each plane is used to determine the resulting forces of the collision.

 

That sounds good to me.... the way I see it and and interpret what you just said is that the "difference" in speed between the tow objects colliding is all that matters as far as velocity.

So 3 scenarios:
1. car #1 going 25mph hits a car going the "same" direction at 24mph.... there is only a 1mph impact!... hardly no damage at all.

2. Car #1 going 25mph hits a wall..... 25mph worth of energy.X its mass is dissipated upon impact.... both the wall and the car absorbing the difference in speed between the two objects of 25mph worth of force.
OR.....
2a. Car #1 going 25mph hits car #2 stopped at a red light....25mph worth of energy.X its mass is dissipated upon impact.... both the wall and the car #2 absorb the difference in speed between the two objects of 25mph worth of force.

3. Car #1 going 25mph hits car #2 also going 25mph but in the "opposite" direction thus committing ahead-on collision. Now the "difference in speed" between the two objects is now 50mph because they are heading towards each other, so both are damaged even more than in #2 scenario.

Double the mass "2 cars" and double the velocity "both going at 25" does not cancel each of them out... it multiplies them together.