Math Is Fun Forum

paulb203

Member

Registered: 2023-02-24

Posts: 317

Newton’s 3rd Law of Motion

I do a push-up. I push down on the ground; the ground pushes up on me.

I’m trying to relate this to water bottle rockets, where you part fill a plastic bottle with water, cork it, then pump air through a hole in the cork until the air pressure inside is great enough to push the water out and the bottle takes off like a rocket.

Relating this to me doing a push-up; am I the air and the water is the ground?

The air pushes down on the water like me pushing down on the ground (until the water forces the cork out and the water empties from the bottle) and the water pushes back up on the air like the ground pushing back up on me (causing it -and the bottle that contains it - to take off in the opposite direction)?

---

Prioritise. Persevere. No pain, no gain.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,623

Re: Newton’s 3rd Law of Motion

That sounds about right. The air pressure is producing the force and when the cork pops , the rocket moves up as the water moves down.

What you haven't asked is what happens to the Earth when you are doing push ups.  I'm thinking that as you move up the Earth must move down. But the amount is fortunately small compared with your movement.  Just don't get too large a group to join you and definitely don't all push up in synch.

It helps that you come back down again as that recombines you with the Earth as a single system. If you were able to push hard enough to achieve escape velocity (roughly 25000 mph) then you would leave the Earth for ever and the rest of us would be stuck on on  planet whose orbit is permanently affected

Bob

---

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

KerimF

Member

From: Aleppo-Syria

Registered: 2018-08-10

Posts: 240

Re: Newton’s 3rd Law of Motion

Hmm... What about NASA rockets and big volcanos

---

Every living thing has no choice but to execute its pre-programmed instructions embedded in it (known as instincts).
But only a human may have the freedom and ability to oppose his natural robotic nature.
But, by opposing it, such a human becomes no more of this world.

paulb203

Member

Registered: 2023-02-24

Posts: 317

Re: Newton’s 3rd Law of Motion

Thanks, guys.

And Lol!, Bob.

*

The ground pushing me up is a curious one, for me. When I push on the ground it's conscious effort, my muscles do their thing, I use up energy, I might sweat, etc, etc. Does the ground 'do' anything? Or does it push up on me merely by virtue of its being there?

---

Prioritise. Persevere. No pain, no gain.

Bob

Administrator

Registered: 2010-06-20

Posts: 10,623

Re: Newton’s 3rd Law of Motion

Material from volcanos doesn't usually reach escape velocity so things probably sort themselves out as stuff falls back to Earth.  Rockets that go into orbit haven't left either and the direction of push is pretty random as the Earth rotates so that seems to sort that out.

If too many rockets go on one way trips to, say, the Moon, then this might widen the Earth Moon distance, especially as the landing would use the Moon's surface for breaking. But if a regular trade in Moon material started up the other way then there's a bigger problem. The increase in the overall weight of the Earth would affect it's rotation. Ho hum for the simple life.

Bob

---

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob