# Sit-up exercises - Newton's Third Law **URL:** \ **Category:** Factual Questions\ **Created:** [August 23, 2015, 11:26pm UTC](https://boards.straightdope.com/t/sit-up-exercises-newtons-third-law/728659 "2015-08-23T23:26:02Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![Velocity](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/velocity/32/18006_2.png) [@Velocity](https://boards.straightdope.com/u/Velocity)\ **Post date:** [August 23, 2015, 11:26pm UTC](https://boards.straightdope.com/t/sit-up-exercises-newtons-third-law/728659/1 "2015-08-23T23:26:02Z")
When you do a sit-up exercise, your feet tend to rise as you are lifting your head up. This is why people doing sit-ups will often have someone holding their feet down, so that they can do the sit-ups more effectively. Is the feet rising the result of Newton’s Third Law; that for every action there is an equal and opposite reaction, or am I barking up the wrong tree entirely? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 24, 2015, 12:34am UTC](https://boards.straightdope.com/t/sit-up-exercises-newtons-third-law/728659/2 "2015-08-24T00:34:47Z")
It’s the rotational version of Newton’s Third Law: If your lower body is putting a counterclockwise torque on your upper body, then your upper body is putting a clockwise torque on your lower body. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [August 24, 2015, 12:38am UTC](https://boards.straightdope.com/t/sit-up-exercises-newtons-third-law/728659/3 "2015-08-24T00:38:27Z")
It’s a little clearer if you think about what your muscles are doing in a situp. Your abs are contracting, which tends to make your head get closer to your feet. Which one moves (head or feet) depends on weight and anchoring. ---
**Author:** ![Baracus](https://avatars.discourse-cdn.com/v4/letter/b/2bfe46/32.png) [@Baracus](https://boards.straightdope.com/u/Baracus)\ **Post date:** [August 24, 2015, 12:39am UTC](https://boards.straightdope.com/t/sit-up-exercises-newtons-third-law/728659/4 "2015-08-24T00:39:27Z")
Part of it is just balance too though. If you start at the top with your knees bent and slowly lower your head back, unless you are rather thick-legged, chances are your feet will lift of the ground before you get your head to the ground. ---
**Author:** ![Quercus](https://avatars.discourse-cdn.com/v4/letter/q/7ab992/32.png) [@Quercus](https://boards.straightdope.com/u/Quercus)\ **Post date:** [August 24, 2015, 1:43pm UTC](https://boards.straightdope.com/t/sit-up-exercises-newtons-third-law/728659/5 "2015-08-24T13:43:05Z")
I’d say, yeah, 3rd law is a good explanation. Your ab muscles are pulling; and the 3rd law says that any pull has to be pulling equally in opposite directions. So your ab muscles are pulling your chest, but also must be pulling your legs/hips, too. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 24, 2015, 1:50pm UTC](https://boards.straightdope.com/t/sit-up-exercises-newtons-third-law/728659/6 "2015-08-24T13:50:08Z")
> [@](#): > > Quoth **Baracus** : > > Part of it is just balance too though. If you start at the top with your knees bent and slowly lower your head back, unless you are rather thick-legged, chances are your feet will lift of the ground before you get your head to the ground. That’s not “part of it”. It is often the case in physics that any given situation can be described completely in multiple ways. It’s still the same phenomenon; all that changes is the description. The OP’s situation is 100% rotational Newton’s laws. It is also 100% linear Newton’s laws, though the description is less straightforward there. And it is also 100% balance. They’re just different ways of describing the exact same phenomenon. ---
**Author:** ![Isilder](https://avatars.discourse-cdn.com/v4/letter/i/cc9497/32.png) [@Isilder](https://boards.straightdope.com/u/Isilder)\ **Post date:** [August 25, 2015, 12:53pm UTC](https://boards.straightdope.com/t/sit-up-exercises-newtons-third-law/728659/7 "2015-08-25T12:53:01Z")
> [@Quercus](#): > > I’d say, yeah, 3rd law is a good explanation. No… because thats saying his body weighs 190, so the floor pushes up on his buttucks 90, the buttocks push down 190… The feet are held down because there’s an equivalent of the third law, effectively a fourth law… for turning… Basically, the sum of the turning moments on a body should be zero… or else it turns. For purpose of working out the weight required on his feet, you work out the turning moment when he first starts his situp, the part with maximum effort requirements. ---
**Author:** ![Quercus](https://avatars.discourse-cdn.com/v4/letter/q/7ab992/32.png) [@Quercus](https://boards.straightdope.com/u/Quercus)\ **Post date:** [August 25, 2015, 1:54pm UTC](https://boards.straightdope.com/t/sit-up-exercises-newtons-third-law/728659/8 "2015-08-25T13:54:40Z")
> [@Isilder](#): > > No… because thats saying his body weighs 190, so the floor pushes up on his buttucks 90, the buttocks push down 190… > > The feet are held down because there’s an equivalent of the third law, effectively a fourth law… for turning… Basically, the sum of the turning moments on a body should be zero… or else it turns. For purpose of working out the weight required on his feet, you work out the turning moment when he first starts his situp, the part with maximum effort requirements. Well, you can get as complicated as you want, carefully mapping out the forces and torques on each muscle or ligament attachment point, building a model of mass distribution and calculating centers of gravity and moments of inertia, adding in compressibility and bending moments for ligaments and bones and so forth, then come up with a Hamiltonian function for the system, etc. But, as the very simplest explanation, I think it’s a good demonstration of the third law: you can’t pull your head towards your knees without at the same time pulling your knees towards your head. (You can of course add another force to your knees, by having someone hold down your feet, which will keep your knees from moving when they’re pulled towards your head. And when you do add that force holding down your feet, there will be an equal and opposite force pushing up on whoever is holding them down, just as the third law says) ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 25, 2015, 1:56pm UTC](https://boards.straightdope.com/t/sit-up-exercises-newtons-third-law/728659/9 "2015-08-25T13:56:18Z")
There’s a rotational equivalent of each of Newton’s laws. If one insists on giving them new numbers, then the relevant one here would be the sixth law. It makes the relationship clearer to just call it “rotational third law”, though.