Physics Question

[Don Plumley]

Originally Posted by Dantilla

Does the 10-pound object have a "Type R" sticker on it? If so, you can't ignore it's massive torque.

LOL!!

[snowman]

KE=1/2 M*V^2, Inelastic collisions. Energy must be conserved. Figure it out.

Think billiard balls, no english.

Ball that is not moving has 0 KE as V=0

Energy is conserved, therefore all KE is transfered to 2nd ball leaving the initial ball with 0 KE, ie no longer moving.

The velocity of the lighter ball will be = 1.41 x the velocity of the heavy ball (the sqrt of 2 more precisely)

Want to check it out. Figure the KE of the 20 pound ball, then assume a velocity of 1.41 times that velocity for the 10 pound ball. The KE's are equal. Energy is conserved. QED

Now for a difficult question. Why do you launder a wash cloth? ITs clean, you use soap and water to use it. Why doesn't the soap and water clean the wash cloth? Just wondering.

Then theres the towel. Its clean, your clean, just water on you. You wipe down. Yet you still have to launder it!! WHY?

I am sitting here, waiting to wipe my butt, just wondering, just wondering.

[masraum]

I'll use caps for mass and Velocity, so the subscripts, i and f for initial and final and 1 and 2 for the two objects make more sense.

M1V1i + M2V2i = M1V1f + M2V2f
and
1/2 M1V1i^2 + 1/2 M2V2i^2 = 1/2 M1V1f^2 + 1/2 M2V2f^2

What it all comes down to is
V1i + V1f = V2i + V2f or
V1i - V2i = -(V1f - V2f)

excerpt from my college physics book concerning elastic collisions.
Physics for Scientists & Engineers 2nd ed, Serway

Let us consider some special cases: If m1 = m2, then we see that v1f = v2i and v2f = v1i. That is, the particles exchange velocities if they have equal masses. This is what one observes in billiard ball collisions.

...

If m1 is very large compared with m2, we see from Equations 9.22 and 9.23 that v1f~=v1i and v2f~=v2i. That is, when a very heavy particle collides with a very light on initially at rest, the heavy particle continues its motion unaltered after the collision, while the light particle rebounds with a velocity equal to about twice the initial velocity of the heavy particle.

Eq 9.22 says that V1f equals the difference of the masses divided by the sum of the masses times the initial velocity of that particle.

so V1f = 5/15 * 10 or 3.333mph

Eq 9.23 says that V2f equals double the mass of particle 1 divided by the sum of the masses times particle 1's initial velocity.

so V2f = 20/15 * 10 or 13.333mph

Based on the math in the first two equations, I get different answers. I'm guessing that the difference between 10# and 5# is not enough to use the second set of equations.

Here are the results with the first set of equations. The one thing that seems odd to me is the final speed of the second particle. I'd expect it to be higher.
10*10 + 5*0 = 10*V1f + 5*V2f
.5*10*10^2 + .5*5*0^2 = .5*10*V1f^2 + .5*5*V2f^2

so
V1f = 10 - .5*V2f -- substituting that into the second equation we get

.5*10*100 + 0 = .5*10*(10 - .5*V2f)^2 + .5*5*V2f^2

when I solve, I get that V2f = 0 or 1.333.... since the weights aren't equal, it's not 0, then plugging 1.333 back into the first equation I get V1f = 9.3333

So part of that seems correct. The speed of the first ball hasn't changed by much since the second ball is lighter. So it only slows from 10 -->9.333..., but I'd expect the second ball to be going faster. So I'm not sure what, if anything I did wrong.
so, after, the 10 ball would be moving at 9.333 and the 5 would be going 1.333

Maybe I got the answers backwards. Maybe the masses are close enough that the first ball almost stops and the second ball is going almost as fast as the first.

[snowman]

Only problem here is that the heavy particle is NOT much larger than the light particle. Much heavier imply s a factor of at least 10. In your case likely a factor of 1000 or more. Also the collision is not totally inelastic in your case, because energy MUST be conserved. The lighter ball MUST end up going faster than the heavy ball. Your calcs are probably what a real item would do (elastic}, but we are dealing with ideal (inelastic) balls here.

Anyone can do the math, its the setup that separates the men from the boys.

[tcar]

For this to work, the balls have to be the same size, like pool balls, even though the weights are different.

So the contact is square on.

[The Gaijin]

I thought the balls were in space?? But yes, they have to hit square..

[rammstein]

A few things here-

#1- I just got back to this thread, so I am going to try out the math and make it work.

#2- Yes, in my scenario, the objects are hitting perfectly square.

#3- An answer to the question about washing towels. In college, I decided to not wash my towels for this very reason- I was clean from the shower, so what would the point be in washing the towel? This proved to be fine for awhile. But about 2 months in, the towels start to have an odor, and it eventually gets pretty foul. They do in fact get dirty, and its from dead skin cells, slow growing mold, and little amounts of dirt that you miss when you shower. Washing towels after every use is stupid, but once a week or so is reasonable. Never washing them makes them smell bad. I know this for a fact.

[masraum]

Originally Posted by rammstein

A few things here-

#1- I just got back to this thread, so I am going to try out the math and make it work.

#2- Yes, in my scenario, the objects are hitting perfectly square.

#3- An answer to the question about washing towels. In college, I decided to not wash my towels for this very reason- I was clean from the shower, so what would the point be in washing the towel? This proved to be fine for awhile. But about 2 months in, the towels start to have an odor, and it eventually gets pretty foul. They do in fact get dirty, and its from dead skin cells, slow growing mold, and little amounts of dirt that you miss when you shower. Washing towels after every use is stupid, but once a week or so is reasonable. Never washing them makes them smell bad. I know this for a fact.

Exactly on the towels. You put the towels up wet, they are in the bathroom with the shower, so the environment is probably warm and humid, and then when you leave, it's probably dark. Even without the dead skin, the things are dark, warm and wet which is ideal for mold/mildew.

Want to test the worst case scenario. Wash a load of laundry, but leave it in the washing machine for several days without drying the wet stuff. It'll smell horrible, and it's just been washed, hell, it's still sitting in the washing machine.

[cgarr]

You take off to a distant planet at twice the speed of light, you land, get out and set up a telescope, can you see yourself coming?

[Burnin' oil]

My cat's name is Mittens.

[Burnin' oil]

Originally Posted by The Gaijin

I thought the balls were in space?? ..

[Don Plumley]

This is my all time favorite answer to a physics question that I wish I used back in school:

[scottmandue]

How much is a Zerio?


If lesbian #1 is traveling at 10 Mph and hits lesbian #2 who is traveling in the same direction at 15 Mph what speed are lesbian #1 and #2 traveling after the collision?

[Porsche-O-Phile]

You're ignoring the effects of gravitational attraction between the two masses.

Also relativity and some ***** to do with quarks. And elves.

[masraum]

Originally Posted by snowman

Only problem here is that the heavy particle is NOT much larger than the light particle. Much heavier imply s a factor of at least 10. In your case likely a factor of 1000 or more. Also the collision is not totally inelastic in your case, because energy MUST be conserved. The lighter ball MUST end up going faster than the heavy ball. Your calcs are probably what a real item would do (elastic}, but we are dealing with ideal (inelastic) balls here.

Anyone can do the math, its the setup that separates the men from the boys.

The equations that I used were correct and were setup correctly. I just did the math wrong. I guess that's what I get for working on this when I was tired.

When I re-did the math I got the same thing that Steve/sjf911 got in his simple, short post

Originally Posted by sjf911

IIRC, it should be 1/3 of V1 for M1 and 4/3 of V1 for M2. So 3.3333mph and 13.3333mph.

My equations were for an elastic collision and did take conservation of energy and momentum into account.

before collision -- mv + mv = mv + mv -- after collision -- momentum
and
before collision -- 1/2 mv^2 + 1/2 mv^2 = 1/2 mv^2 + 1/2 mv^2 -- after collision -- KE

so, the big ball would be going 3.333 mph after the collision and the small ball would be going 13.333 mph after the collision. Not really quite double the speed of the big one.

[snowman]

I don't think so.