A theoretical physics question on frictionless surfaces

I’d stick to a pocket fan. Cheap, easliy concealable, and, if in a mostly asian population, quite fashionable.

Angular momentum is not conserved in this situation, since the the frictionless surface can still exert a perpendicular force.

And, even if it were conserved, you could still turn your head. You are not a fixed body. Rotate your head left and your body will rotate right. Reverse and repeat.

That wouldn’t be University Physics, by Sears and Zemansky, would it? Those problems sound suspiciously familiar.

I think you’re right - it would work for the same reasons that pop-pop motors work - fluid is drawn in from a hemisphere of directions, but expelled in only one direction - generating a net thrust.

Air resistance would stll be an impediment to jet propulsion by mouth though, even if the ground surface was completely frictionless.

I was thinking about turning my head left to right. In order to turn my head to the left, the rest of my body needs to push right. I can’t turn because there is no friction to push against.

I see now that you could exhale outwards and inhale down or something, though. That would give you a net force in one direction.

No, your head needs something to push against. And it does: your body.

It’s your total angular momentum that’s conserved, not just that of your head. You turn your head to the right, your body turns to the left by a small amount. Angular momentum is conserved.

How did you think satellites adjusted their attitude? They don’t have to fire the rockets to change their pointing. They have reaction wheels inside. Spin the wheel clockwise and the rest of the satellite turns counter-clockwise.

You can rotate yourself on the spot by raising one arm straight above your head and waving it in rapid circles - you will rotate on the spot until you stop waving. (this is just a meatware version of scr4’s reaction wheels)

I understand about angular momentum. I know that in theory you can turn part of yourself clockwise by turning another part counterclockwise. I’m questioning that part; the turning the other part of myself counterclockwise.

Ultmately my body maneuvers itself by using muscles to push against things in my environment. If there is no friction, I can’t see myself even getting started.

Okay, I’ve been playing around with a rolling chair, and I guess I can see it. I just wasn’t believing my anatomy could utilize what I knew was possible from physics.

Forgive me, guys; it’s 3 am where I’m at. My brain just isn’t firing on all cylinders.

I did not know that! Awesome! Hooray for physics!

What matters is momentum. momentum = m*v so we have to know what velocity you can throw a brick vs. what velocity you can throw a golf ball. Let’s say you can impart the same amount of kinetic energy

E[sub]k[/sub]=1/2(mv[sup]2[/sup])

to either type of object (and I am not sure that’s true, BTW), then a brick at 2 kg thrown at a modest 1 m/sec has energy of 1/221 = 1 Joules. Its momentum is 2 * 1 = 1 kg*m/s.

A golf ball weighs about 0.046 kg. A golf ball with 1 Joule of kinetic energy is traveling at sqrt(1/((1/2)0.046)) = 43.5 m/s. That gives it momentum of 2.0 kgm/s. So *a single golf ball * really fired off using the same energy as lobbing a brick will give you double the momentum bump. But intuitively I don’t know if you could throw a golf ball at that speed (97.3 MPH).

I invite a review of my math since it’s not my strong suit. :o

A couple of martinis always works for me :smiley:

Sounds like problem 3-12 on page 45 of the 5th edition.

FWIW, I had Hugh Young as a Physics professor.

It actually sounds like the book we used at USC: A kind of dark blue book by Kleppner and Kolenkow. Either way, they are good problems.

Bingo. TJdude wins the prize. MIT used it for the Intro Physics for Masochists course, 8.012

I didn’t have either of them for freshman physics, but I later had Dan Kleppner for Advanced Mechanics.

I love that in the book all the space ships look like the Pan Am Space Clipper from 2001.

Hey Cal, when did you take 8.012? I took it in '85. It only took a further 3.5 years to decide I didn’t want to be a physicist. Of course, the live “shoot a pendulum with a .22 and measure the displacement” demo in 26-100 was worth the cost of admission itself - I’m guessing they don’t do that anymore.

Well, the good news is that if you accidentally walk on to a frictionless surface, you’ll keep sliding across it because of the momentum from the step that took you onto the surface (though you’ll slow down because of air resistance, but that will be pretty negligible at low speeds).
In fact, I’m having trouble imagining a way to get onto a frictionless surface that leaves you truly motionless relative to the surface, aside from being very carefully and precisely lowered onto it.

1973 – truly ancient history. I didn’t get to see that demo, but I saw lots of others, including the use of those wave-demonstrating ball things they always have hinging on either side of the stage at 26-100. But the electricity demonstrations for 8.022 were frequently better.

This is not required, google “pop-pop boat” for a number of explainations. My short version: Before you inhale it, the still air has zero velocity. after you exhale, it has a velocity. Therefore you must end up with a velocity that offsets the change in the air’s momentum.

This effect is strongly related to Feinman’s “underwater lawn sprinkler” thought experiment.

I assume that the frictionless surface is both perfectly flat and level. If it was just a little bit sloped you would slide right off.

Would gravitational forces that are not perpendicular to the surface move you around on this surface? The Earth orbits the sun and moves a bit closer to Jupiter and off you slide (however slight) towards Jupiter. Then Saturn pulls you back and you realize that for at least one time the alignment of the planets may actually mean something to you.

Why has no one said breat in through your nose and out through your mouth? Would that not work?