Wikipedia:Reference desk/Archives/Science/2020 February 19

= February 19 =

Neonatal Stroke
Hello. Is there anything contentious about the definition of neonatal stroke? Thanks in advance. --Mayfare (talk) 10:55, 19 February 2020 (UTC)
 * "Neonatal Stroke similar to a stroke which occurs in adults, is defined as a disturbance to the blood supply of the developing brain in the first 28 days of life." from the article Neonatal stroke is taken from this source and does not seem contentious in any way. It compares with perinatal stroke (stroke around the time of birth). DroneB (talk) 14:44, 19 February 2020 (UTC)
 * Do you know of some reason why the term has been italicized? Or perhaps that's why you're enquiring. Anyway, I changed it to sentence case and removed the italics, but left the bolding, to follow our WP:MOS. Matt Deres (talk) 20:45, 19 February 2020 (UTC)
 * I agree with the edit by Matt Deres in the Wikipedia article. DroneB (talk) 00:52, 20 February 2020 (UTC)

Two cars travelling in one direction
Let's say two cars are travelling on the same side of the road, one in front of the other travelling in the same direction. The one behind is travelling at 30 mph the one in front is slightly slower at 25mph.

When the faster car behind eventually hits the car travelling at 25mph, will the damage incurred to the cars be the same as if the cars were travelling at 5mph and 0mph respectively.

194.66.175.89 (talk) 11:42, 19 February 2020 (UTC)


 * Are we talking Newtonian mechanics, Einsteinian special relativity or traffic planning?
 * For Newton, yes, it's all linear and so the relative difference of 5 mph would be the same, whether that would be between 0-5 or 25-30. For traffic planning, an accident at 25 mph will distract most drivers and there's a risk of them then going into the stationary side of the road.
 * If they were travelling near to light speed, the Newtonian assumption of linearity doesn't work any more and it gets more complicated. But Newton is a good approximation for anything slower than spaceflight. Andy Dingley (talk) 11:50, 19 February 2020 (UTC)
 * What about inertia? I've always understood that the force required to move a stationary object is greater than the force to keep it moving. 41.165.67.114 (talk) 12:30, 20 February 2020 (UTC)
 * The net force required to keep an object moving is zero per Newton's first law. The net force required to start moving an object is infinitely low: per Newton's second law, an object's acceleration (not speed) is proportional to the force applied to it, so if you wait long enough applying a small acceleration, the object can accelerate to whatever speed you want.
 * Now, in a real-world case (such as a moving car), the net force is the sum of what you apply (e.g. via the engine) and a load of friction forces. Friction always tends to slow down the object acted upon and dissipate motion into heat (the reason for this is complex). To simplify, fluid-solid friction (such as air resistance around the car) increases with relative motion speed (so keeping a car going at a higher speed will require more force to counteract friction - that is why driving slowly gives better fuel economy); and solid-solid friction (such as tires on the road) is (mostly) independent on relative motion speed as long as it is above zero.
 * To complicate things further, solid-solid friction is higher when there is no motion than where there is some motion. That is the cause of the stick-slip phenomenon (but don't click that link, the article is rather awful - see youtube videos instead). It might actually be harder to start moving the car than to keep it moving, at low speeds, because of solid-solid friction of the car's components. Tigraan Click here to contact me 16:51, 20 February 2020 (UTC)


 * For Einstein as well. Newton works fine for these calculations.  Unless one is moving stupidly fast or is stupidly large in mass, there really is no reason to use relativity to do any calculations.  At the speeds and masses noted above, the difference between the special relativity calculations and the Newtonian mechanics calculations comes out in the 12th or so decimal place, much smaller than any device known to man could measure.  -- Jayron 32 16:13, 19 February 2020 (UTC)
 * Indeed, as a theoretical exercise in one-dimensional kinematics, it does not make a difference – but only if the halted car does not have its brakes on. In actual collisions, the faster moving cars may leave the track and hit a heavy stationary obstacle, causing far greater damage. --Lambiam 13:03, 19 February 2020 (UTC)
 * It sounds like the OP is describing the conclusion of the recent Daytona 500. ←Baseball Bugs What's up, Doc? carrots→ 17:01, 19 February 2020 (UTC)
 * Lambiam raises a good point and it's an excellent example of how real-world things are not always easy to (remember to) take into account when we model. A stopped car almost always has brakes applied and/or the transmission set to park and that needs to be taken into account. Matt Deres (talk) 21:03, 19 February 2020 (UTC)
 * At the risk of belaboring the obvious, it's not just brakes. It's the fact that there are objects around besides the two cars.  First of all being the roadway itself.  The faster the cars are travelling relative to the roadway, the more of a problem it is if they are diverted from their intended path.  If a car going 30 mph taps a car going 25 mph, that's not very fast, so you hope both drivers will be able to maintain control, but still, it's often going to be more dangerous than the one going 5 mph hitting a parked car.  For a link, I don't know, maybe spherical cow? --Trovatore (talk) 21:17, 19 February 2020 (UTC)
 * ...or how about Crash Injury Research from NHTSA? Loss of control at speed is a contributing factor to a lot of motor vehicle crash injuries.  After the initial collision - even if it is kinematically minor - the vehicles need to eventually slow to a stop.  The excess speed has well-documented consequences for the vehicles and the occupants.  The initial impact may cause that slowdown to be uncontrolled, which is essentially a euphemism for "bad."  Nimur (talk) 16:18, 20 February 2020 (UTC)