A little physics to think about...

1. ## A little physics to think about...

Today I was thinking about kinetic energy (Ek), for no reason in particular.

I did a little bit of math to determine that the kinetic energy of the shell leaving the barrel of a 105mm artillery piece was approximately 10,000kg-m/s (somewhere around 15kg projectile with a muzzle velocity of approximately 670m/s), which is a fair bit of kinetic energy. Since, in the old days, the energy contained in the projectile was all that was used to knock buildings down and destroy people utterly (and fairly successfully), I assume that 10,000kg-m/s is a fairly destructive amount of Ek.

Then I started thinking about our bikes.

Let's say that the average sportbike, with a rider, weighs about 600 pounds. That's probably a little on the light side, but let's run with it. That's about 272 kilograms.

Now, how many of us can say we've never broken 100MPH (160km/h) on our bikes? Not many, I'm guessing.

100MPH is about 44.7 meters per second.

At 100MPH, the average sportbike therefore has in excess of 12,000kg-m/s of kinetic energy.

Ride safe.

-Matt

2.

3. Haha...ur avatar mentions geek in it...man...

you'd better hope Scheme doesnt read this

4. Question is, when ur riding at 90km/hr in the city and some pedestrian starts to cross in a marked crosswalk 1/2 block ahead of you, can you stop in time? Or how about some cager running the stop sign 1/2 block ahead? Just curious how many feet you'd travel in the time of say...in the 1 1/2 seconds reaction time before you hit your brakes?

Kinetic energy, E = 1/2 m V V
(0.5 times mass times velocity squared)

Momentum, M = m V.

6. And very valid numbers they are. There's been more than one pic of accidents where a bike T boned a car and the car was all but split in two. In one case it was very obvious from the jeans sticking out of the wreckage that the rider added his mass to the equation.... (RIP) In another case the bike ripped up from the trunk lid (bike rear ended car) and the car was split right up the middle with the bike's front wheel stuck into the dash.

Nope, it's not always JUST the bike that comes out second best in these accidents.

7. Originally posted by fz1
Question is, when ur riding at 90km/hr in the city and some pedestrian starts to cross in a marked crosswalk 1/2 block ahead of you, can you stop in time? Or how about some cager running the stop sign 1/2 block ahead? Just curious how many feet you'd travel in the time of say...in the 1 1/2 seconds reaction time before you hit your brakes?
90km/h times 1.5 seconds = 37.5 metres = 123 feet

assuming coefficient of friction between your tire (only front tire really) and the pavement is 1.0 and you are on the verge of the back tire coming off the ground, it would take at least another 32 metres (105 feet) to stop. And that's optimistic! More likey it is 130 feet in real life.

so do the math, you need about 250 feet to stop from 90km/h.

Haha...ur avatar mentions geek in it...man...

you'd better hope Scheme doesnt read this

9. At the approximate top speed of my bike, 160mph, and assuming me and my bike weighs 675 lbs, I have 582451 ft-lbs of energy. A .50 bmg, the largest practical rifle calibre which was once widely used as an aircraft munition has approximately 12000 ft-lbs of energy at the muzzle. The .50 pales into insignifigance beside the bike figure. Amazing amount of energy, we ought to be careful with it. Neat thread jonesboy, makes you think, which can be fun. Angus.

10. Yes, I gave momentum numbers. One can derive Ek from momentum easily, but I should have noted it. Also, the fact that we're dealing with momentum rather than kinetic energy makes the comparison no less valid.

-Matt

11. What about the impact area? The impact area of a shell or a bullet is a lot smaller than a bike (with a rider). Since I took business instead of physics in school (stupid me)

Can someone comment on the amount of energy absorbed by the impacted area on each. Just curious.

12. divide total ft/lbs of energy by the square inches of the impact area

that not work?

13. its all about energy density. obviosly its alot more concentrated in the bullet...but enough about this go out a ride and let the physics take care of its self.

14. I can remember about 14 years ago a sportbike t-boned a car (a Chevette, or Civic I think) embeded halfway into the car and actually flipped it onto it's roof. This happened either on 152nd, Scott Road or King George (hard to remember) in the 70 ave- 90 ave area. I remember seeing the pic on the Province.

As for the physics, the one I go by is: It's not the speed that hurts, it's the sudden stop.

im jus givin ya'll a hard time

16. ## b.s. 250 feet

90 km/h to full stop in 250 feet is bullshit.Do all the math u want but any good racer or stunna will prove your math wrong.I can do it way shorter.Forget the math,ill do the riding.

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