## How does moving your legs propel you forward in swimming?

Ok, so I know why your arms move you forward in swimming. You are pulling the water back which pushes you forward a lot because there is a lot of resistance, and then you are moving your arm forward in the air which offers little resistance to push you back. So the amount you push back in the water is greater than the amount the air pushes you back when you move your arm forward.

But what about the legs? There are two things I am confused about:

1)How does moving your legs vertically propel you forward at all? I don't know how a downward kick can move you forward. If I were to kick the ground right now, it would not move me forward.
2) If kicking your legs down propels you forward, shouldnt bringing your leg back to the original position move you backward? For instance, if I were to floor the gas pedal in my car while it was in drive for 5 seconds, it would move me a distance forward. If I were to then throw my car in reverse and floor the pedal for 5 seconds, it would move me back to my starting point. So how come kicking down propels you forward, while kicking up does move you backward?

Sorry if I did not explain my question clearly enough.
 PhysOrg.com physics news on PhysOrg.com >> Kenneth Wilson, Nobel winner for physics, dies>> Two collider research teams find evidence of new particle Zc(3900)>> Scientists make first direct images of topological insulator's edge currents
 On a side note with my retarded thinking....how do lubricants work to make something easier to pull or move around? Like if something is stuck in something else, you use lube to free it up. But by adding lube, you are decreasing the space to pull the thing out. I know that was stated confusingly. But like lets say my finger is stuck in a bottle, and there is only like 1/100 of a inch of space between my finger and the bottle, making it hard to pull my finger out. By adding 1/200th of an inch of lube to make the bottle slippery, I am decreasing the free space by 50%....so how is it easier to pull out after that?

Recognitions:
 Quote by CuriousBanker Ok, so I know why your arms move you forward in swimming. You are pulling the water back which pushes you forward a lot because there is a lot of resistance, and then you are moving your arm forward in the air which offers little resistance to push you back. So the amount you push back in the water is greater than the amount the air pushes you back when you move your arm forward. But what about the legs? There are two things I am confused about: 1)How does moving your legs vertically propel you forward at all? I don't know how a downward kick can move you forward. If I were to kick the ground right now, it would not move me forward. 2) If kicking your legs down propels you forward, shouldnt bringing your leg back to the original position move you backward? For instance, if I were to floor the gas pedal in my car while it was in drive for 5 seconds, it would move me a distance forward. If I were to then throw my car in reverse and floor the pedal for 5 seconds, it would move me back to my starting point. So how come kicking down propels you forward, while kicking up does move you backward? Sorry if I did not explain my question clearly enough.
First a disclaimer--I'm an amateur swimmer, not a coach or sports physiologist, so I'm essentially guessing here. Having said that, IMO there are two effects. First, you swim on the surface so kicking up displaces little water compared to down. Second, your scenario might apply underwater if your leg swung like a stick on a hinge, but if you watch a good swimmer you see that they use a subtle undulating motion all the way down through the feet. Each leg mimics (with smaller amplitude) the motion of the dolphin kick used in the butterfly stroke. The undulating motion is essentially a traveling wave that moves down the length of the body, propelling you forward.

Recognitions:
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## How does moving your legs propel you forward in swimming?

Hi CuriousBanker. It is better to confine your thread to one topic, to keep responses focused and to attract the interest of those who may wish to follow it.

The swimming question has two parts to its answer, I'll address one of them: fluid friction.

A fact of fluid friction is that the faster your velocity through a Newtonian fluid, the greater the friction you must overcome. It's easy to slowly wade through a shallow swimming pool, but try sprinting and you'll very quickly tire. (For water, I think drag is proportional to v3 or v4, someone is sure to look it up and tell us in detail. ). So if you try to move your leg downwards very quickly, the water exerts a large force upwards against your leg opposing your effort. Then, if you more slowly retract your leg upwards, the water exerts a significantly lesser force downwards against your movement. On balance, you encounter a large force upwards for a short period of time then partially countered by a lesser force downwards for a longer period. Fortunately for pool owners, the mathematics and water viscosity consistently seems to work in favour of you being able to kick your way out of drowning.
 If a force is exerted on a body, it will move. Gravitational force in exerted on any mass. When you are on a solid ground, the ground will exert a force on you to counter the gravity. On water, bouyancy will keep a boat afloat. So the boat does not need additional force to counter the gravity. When you are swimming, the bouyancy does not guarantee you afloat. You need extra force, additional to bouyancy. Going forward also require applied force too. The resultant of forces produced by your legs and hands is forward motion and afloat.

Mentor
Considering the arms: You can use them for diving, too - while you can use the air here, it is not required. See NascentOxygen's post for an explanation.

 how do lubricants work to make something easier to pull or move around?
They reduce the coefficient of friction.
 Recognitions: Science Advisor Hi H2bro, thank you for the detailed lesson on kicking. I wasn't trying to say that the flutter and dolphin kicks are the same, but to make the point that the leg doesn't move stiffly back and forth. The leg moves in a coordinated way at the hip, knee and ankle. You can see it in this video of Michael Phelps, starting at minute 1:40 where it shows him from the side. He has a wave motion all the way down to his ankles and feet and even his toes flex; it is most visible in the dolphin, less so in the flutter kick partly because the camera is far away and there are a lot of bubbles. http://www.youtube.com/watch?v=ax77_hHq9Dc Maybe the downstroke is faster, but it certainly looks like he is exerting more force downwards than upwards. I looked around a bit for a slow motion shot of a swimmer kicking without great success, but there's a short clip here--starting at 3:50 to 4:15, she has what looks like a traveling wave down her leg. http://www.youtube.com/watch?v=YlhGqEy8MBc It seems like these motions must be important. EDIT: Actually, addressing NascentOxygen, up and down strokes must take be of the same duration since both legs complete one cycle at the same time but out of phase.

 Quote by CuriousBanker 1)How does moving your legs vertically propel you forward at all? I don't know how a downward kick can move you forward. If I were to kick the ground right now, it would not move me forward. 2) If kicking your legs down propels you forward, shouldnt bringing your leg back to the original position move you backward? For instance, if I were to floor the gas pedal in my car while it was in drive for 5 seconds, it would move me a distance forward. If I were to then throw my car in reverse and floor the pedal for 5 seconds, it would move me back to my starting point. So how come kicking down propels you forward, while kicking up does move you backward? Sorry if I did not explain my question clearly enough.
I'm totally guessing here remembering my childhood swimming days. After H2Bro's explanation this may sound unnecessary.

Even though kicking legs may propel a swimmer forward, but that forward motion, IMHO, is very small. The main purpose of kicking, again IMHO, is to keep balance in the water, to stay afloat in water. If you don't kick your feet, the feet will sink in water making works by hands more difficult.
Well, I don't have pool around to try it out.