OT: Physics question

peeplj · Post by **peeplj** » Wed Jun 18, 2003 12:55 pm

This is way off-topic, but...a question came up at lunch today and as of yet no one has proposed a serious (well, semi-serious) answer.

Here's the question: suppose you could have an entirely frictionless, completely level surface. You are standing beside it trying to move an object which is sitting on the frictionless surface. The question is how do you figure out what weight of object you'd no longer be able to move due to inertia?

All thoughts welcomed.

--James

meir · Post by **meir** » Wed Jun 18, 2003 1:06 pm

the object has no weight for your purposes, since weight is related to gravity and you are pushing it along a vector which has no gravity componenat. force=massxacceleration. the object has a mass. there is no mass at which you will not be able to move it with enough force.

meir

momerath · Post by **momerath** » Wed Jun 18, 2003 1:25 pm

Yeah, except for that little thing about equal and opposite reactions. Any force applied to an object would result in an equal and opposite force applied to you--up to the limit of the object's inertia. So p'raps you would be able to move any object with a mass less than your own?

Tyghress · Post by **Tyghress** » Wed Jun 18, 2003 1:33 pm

I can move an object with far more mass than me even on rough surface. Also, the problem did NOT state that you were on the surface...it said you're standing BESIDE it.

My take on this is that even with a modest effort you can move any mass... just not very quickly.

Ridseard · Post by **Ridseard** » Wed Jun 18, 2003 2:02 pm

All you have to do to make it move is to touch it with your hand. Suppose M is the mass of the object, m is the mass of your hand, and you move your hand sideways with a velocity v until it makes contact with M. For simplicity, suppose your hand remains in contact with M. Then according to the conservation of momentum, since M is originally at rest and therefore has 0 momentum,

mv + 0 = (m + M)V

where V is the final velocity of the system consisting of the hand stuck to the mass M. Solving this equation for V,

V = mv/(m+M)

which obviously is not 0. Therefore the mass M is moving. (Note that the larger the mass M, the slower its velocity V.)

Martin Milner · Post by **Martin Milner** » Thu Jun 19, 2003 3:38 am

I don't think the mass of your hand matters so much as the mass of your whole body, because you are going to move your whole body in an equal and opposite direction to the force you apply to the object assuming that you are also standing on the frictionless level surface.

Suppose you have half the mass of the object you are pushing, you will move in one direction at velocity V while the object moves in the opposite direction at the velocity V/2. If you had one tenth the mass of the object, you would move with ten times its velocity. The amount of force you use will determine the actual velocities.

Although the surface may be frictionless, there will still be air resistance, so the object will eventually stop moving, assuming it hasn't slid right off the (finite) surface first.

If as Tyghress pointed out you are really standing beside the surface, you could move the object no matter what its mass. This is akin to a sailor standing on a quay pushing an ocean liner. While the sea is not completely frictionless, she can still move the ship move.

1) Back to Peep's original question, the problem is how hard to push the object to move it at a safe velocity. The answer may be to use good old trial & error (easy does it), or as JF might have it, use just too much force and then ease off a tad.

2) As a secondary part to this problem, if you were alone and naked on a finite frictionless level surface, could you get yourself off it?

3) Third question. If you had a weight (say a shoe) on the end of a length of string, could you use this to move yourself off the surface, or would the act of reeling in the shoe for repeated throws bring you back to your original starting point?

Andreas · Post by **Andreas** » Thu Jun 19, 2003 4:04 am

Martin Milner wrote:

2) As a secondary part to this problem, if you were alone and naked on a finite frictionless level surface, could you get yourself off it?

3) Third question. If you had a weight (say a shoe) on the end of a length of string, could you use this to move yourself off the surface, or would the act of reeling in the shoe for repeated throws bring you back to your original starting point?

2) Yes. By removing parts of your body and throwing them in the opposite direction of where you want to go.

3) See 2).

Zubivka · Post by **Zubivka** » Thu Jun 19, 2003 4:53 am

Martin Milner wrote:3) Third question. If you had a weight (say a shoe) on the end of a length of string, could you use this to move yourself off the surface, or would the act of reeling in the shoe for repeated throws bring you back to your original starting point?

You'd move in the right direction. The trick is the shoe has to be thrown fast, on a long enough string (to avoid an end shock when the string gets taut), and reeled back slowly:
Reaction propulsion is proportional to the ejected mass, but squared by the velocity of ejection.

However, you'll have more efficiency by throwing the first shoe unattached:
1) The mass to be moved decreases on second thrust. You just invented the multi-stage rocket.
2) The neighbour downstairs can sleep once second stage (shoe) separates.

BTW, a related trick is how you can propel yourself on a bicycle (or skateboard if you lack balance) just by body impulsions. To make it work, the motion-inducing jerk is faster than the movement back.

Jerry Freeman · Post by **Jerry Freeman** » Thu Jun 19, 2003 4:55 am

It seems to me that a good sneeze would send you on your way. Or spitting. In either case, some liquid would be expelled in one direction while you slide happily in the other.

Zubivka · Post by **Zubivka** » Thu Jun 19, 2003 5:07 am

Jerry Freeman wrote:It seems to me that a good sneeze would send you on your way. Or spitting. In either case, some liquid would be expelled in one direction while you slide happily in the other.

This will help mainly out of the atmosphere.
Inside it, you'll save fuel by repeatedly blowing 3rd octave D's, aka stratospheric diatonic pulse jet.

Flyingcursor · Post by **Flyingcursor** » Thu Jun 19, 2003 5:11 am

This sounds like a Larry Niven type issue.

Frictionless would imply absolutely no adhesive properties due to friction. Therefore it seems to me that controlled movement would be nearly impossible. About the closest I've ever seen to frictionless is a patch of ice with a film of water on top. Try getting footing on that sometime and imagine the same effect multiplied.

Thus I would think that as long as you were not on the surface yourself and pushed any object, itself on the surface, it would slide for a long way. The only thing to stop it would be air resistance.

Jerry Freeman · Post by **Jerry Freeman** » Thu Jun 19, 2003 5:16 am

Zubivka wrote:... stratospheric diatonic pulse jet.

Gesundheit.

Walden · Post by **Walden** » Thu Jun 19, 2003 6:29 am

While, several angels could conceivably dance on the point of a needle (somewhat fewer in the case of seraphim, due to the whole six wings thing), it is my contention that, being celestial entities, and thus not mindful of bodily amusements, they would choose not to dance on the point of a needle. . . oh, wrong question.

blackhawk · Post by **blackhawk** » Thu Jun 19, 2003 7:09 am

I don't know. There, I said it.

Martin Milner · Post by **Martin Milner** » Thu Jun 19, 2003 7:20 am

Jerry Freeman wrote:It seems to me that a good sneeze would send you on your way. Or spitting. In either case, some liquid would be expelled in one direction while you slide happily in the other.

It's the massxvelocity balance here that worries me. The actual contents of a sneeze or spit won't have much mass.

If a 70kg adult sneezed a 0.01 gramme sneeze at 100mph parallel to the floor, s/he would move in the opposite direction at Xmph, where

Xmphx70,000g = 0.01gx100mph

= or .00001428571 mph, which is slow in anyone's book, and I'm not sure a sneeze really weights even that much. The spit would weigh more, but how fast can you spit?

I appreciate Andreas's mutilation idea, but I doubt I could even tear off a finger with my bare hands, let alone throw it fast enough afterwards.