Having recently bought my son his first car, we began discussing car accidents. Later, he posed the notion of a timely jump out of the vehicle just before impact to save himself. The conversation morphed to the age old hypothetical about jumping up in a falling elevator at the right time to avoid impact and how a passenger in a crashing car is in a similar predicament. I explained to him that jumping would be impossible because theres nothing to spring off of given you'd be pinned to the roof of the elevator because of the free fall as just one reason you'd be good and dead. I then said two colliding trains or anything else would also doom the passenger to the same fate.

He thought for a moment then brought up this interesting point. He stated to me that in the case of two trains getting ready to collide however, that one could theoretically jump at the right moment before impact, and literally land in another spot on the floor after the energy of the crash has dissipated.

He reasoned that the difference with the train and the elevator, was that the direction of the jump and the direction of collision (energy) were not aligned and therefore independent!

I thought about what he had reasoned and thought to myself....

Whether his theory stands up to reason, he'll be OK in life!

A father / son moment for all you dad's out there.

HFD!

 

I am no physics expert but isn't the whole problem with the jump in the elevator / train scenario that you are merely reducing your momentum slightly - and still going to impact something at a large rate of knots?

Eg elevator descending at 50mph, you too are travelling at 50mph. If you were to jump your upward motion might reduce your downward speed to 45mph - and you would still be a splatter stain on the floor, just a few fractions of a second later.

Same for the train scenario - train moving forward at 100mph, you jump, you might decrease your speed by 1mph but would still be moving forward at 99mph - toward the now stationary front of train. Splat.

Ask him this one - is it possible for a plane to take off if it were on a treadmill with engines at full thrust??

 

I know, I know but I was proud of him anyways...let a dad think his son is a little smart will ya! I was proud he got off his video game long enough to think theoretically for a few minutes!

 

My guess is what I learned long ago that you need four elements for lift off... thrust, lift, drag, and one other...I forget and too lazy to cheat.

On a treadmill, you'd have zero thrust, therefore leaving you stuck. The more you revved the engine, the faster the treadmill would spin....right?

 

No. The engines don't drive the wheels at all. The thrust would push the aircraft forward irrespective of a treadmill.

 

Ahhh, nice, yea you're right, wheels don't torque...

 

While the physics might be off your son is thinking analytically and that is something to be proud of. 😃

 

Indeed.

Now I'm asking, he's off back to his video games...What if the train passenger was on a say unicycle? Theoretically, could the tires spin under the passenger to dissipate the energy? That is to say...Let's imagine a man on a unicycle holding a rope being lowered into a colliding train, (wouldn't the spinning or stopping after impact) wheels offset the momentum of the moving train?

We got good stuff out here in Cali by the way!

Oh wait, yea, splat again against the front back wall because the train is zipping along while you're still. So you can be safe as long as you never settle on the floor and catch up to the speed of the train.

OK I got one for you....

Three guys check into a hotel.
The room is $25.
Each guy places a $10 bill on the counter.
The cashier, being a thief, takes the $5 in change, gives each guy back $1, and puts the other $2 in change into her pocket.

Each guy spent $9 ($10-$1) x 3 guys plus $2 the chick stole, ($9x3=$27+$2=$29)

Wheres the missing dollar?

 

Depends on the speed of the treadmill.
If the treadmill were to maintain even half the speed in an opposing direction to the speed normally attained by the force/energy of the thrust, the plane itself might move forward a small amount, but not gain enough speed for the foil shape of the wings to provide any lift.

 

Nope.

All the treadmill would do is cause the wheels to turn 'backwards' while stationary. The thrust of the engines would cause the aircraft to move forward- while the wheels turn even faster in the 'wrong' direction.

 

Here's the real brain teaser: Why are y'all wasting brain cells over all of this?

 

Don't believe this is accurate. Still say it depends on the speed of the treadmill in relation/opposition to the force of the thrust.

The thrust of the engines move the wheels the exact same direction the treadmill moves the wheels. The two together makes the wheels spin faster.
Neither makes the wheels spin backwards.

Now I am making 2 assumption others may not:
1) the treadmill is moving in a direction opposite that which the front of the plane is pointed, just like a human walks/runs on a treadmill moving the opposite direction we are facing. What direction is your treadmill running?
2) the treadmill is moving under power. If you are assuming a free-spinning treadmill, that certainly changes things.

Plane under thrust on wheels (in the absence of treadmill) moves forward.
Plane on wheels on moving treadmill (in the absence of thrust) moves backwards due to the mass of the plane.

The wheels may rotate somewhat in the latter example, and of course if the plane could gain enough speed to attain lift, this would reduce the mass (i.e. Downforce) which at some point could negate the effects of the treadmill, which brings us full circle to my main contention... The speed of the treadmill in correlation to the opposing energy created by engine thrust matters.

 

The plane would take off.

The wings generate lift based on the velocity of air relative to the wings of the plane (Bernoulli effect). The jets move the wings forward relative to surrounding air and irrespective of the wheels (assuming that the wheel brake is off). The treadmill just transmits rotational energy to the free spinning wheels, which should result in de minimis motion of the chassis relative to the jets.

It would be different if the plane were -- like a car -- powered forward by the wheels rather than by jets...

 

 

Correct. I must learn not to post after a second wee dram any more.

Irrespective of treadmill direction or speed, it does not impede the thrust of the engines pushing the aircraft forward, and it will gain sufficient velocity to achieve take off.

 

No, because a person jumping inside a moving vehicle is still moving at the same speed as the vehicle. If this was not true, a person jumping inside an aircraft while at cruising speed would quickly crash into the rear of the cabin at 600 mph. They wouldn't feel a thing as the sudden deceleration form 600-0 when they first jumped would have killed them.

This might be a blessing in disguise given the abominable quality of service in economy these days.