So I’m teaching my third graders some basics of motion and forces, including the idea that all forces either exert a push or a pull on the object of the force. I understand that things like gravity and (under certain circumstances) electromagnetism exert a pull, and that (under certain circumstances) electromagnetism exerts a push. I also understand that if I hit a baseball with a bat, I’m exerting a push on the baseball.
My question is, are there circumstances under which it’s accurate to say that you’re exerting a pull in a sport? For example, swimming: am I pulling the water toward me, or am I technically pushing it with my hands? In gymnastics, and I’m pulling myself up on a bar, or am I pushing against the bar in such a way that my body goes up?
I don’t want to overwhelm students with high-level phyics, but I also don’t want to describe the physics inaccurately, and my own high-level physics training is practically nonexistent. Can someone help me untangle these ideas?
I wouldn’t use the terms “push” and “pull” at all; they just confuse the issue. A force is a force, and it has a direction, a line of action, and a magnitude. That’s really all you need to know.
I don’t know if this is what you are after; the clean and jerk powerlift. It combines pulling during the first stage of the lift then pushing during the second stage.
The distinction between pushes and pulls is not particularly useful, and not all forces even fit into one of those two categories. For instance, a charged particle moving through a magnetic field experiences a force at right angles to both the direction of motion and to the field.
Wrestling and gymnastics were the best examples I could think of. I think maybe I’m confused about what exactly defines a push and a pull. A push would pretty clearly increase the speed of something away from your body. Obviously throwing a ball involves a push–but does catching a ball involve a push, since you push on the ball until its speed toward your body goes to zero, i.e., until its negative speed away from your body increases to zero?
If that’s correct, a pull would probably involve increasing the speed of something away from your body. In most sports, you exert a force on an object by touching it, which means that the distance from your body is zero. If it stays in contact with your body, its distance from your body remains at zero, so where’s the pull?
Maybe I should avoid these terms entirely, in which case I blame Ms. Frizzle for making me think they’re the right ones to use :D.
You could say that you’re pulling the bar when it’s above you, and pushing it when it’s below you. But this distinction isn’t of much value.
I’m with TimRM here: It’s more useful to say that you’re exerting a downward force on the bar which is necessarily accompanied by an upward force on your body.
Look at an element of what is being acted on. Is that element experiencing mostly tensile or compressive stress?
Rope in tug of war? Practically 100% tension in the rope. That’s a pull. Swimming? How does a liquid act in tension? How about compression? That should be a clue. What your muscles do? They exert force by contraction which increases tension in the fibers and pulls on the bones.
My question is, are there circumstances under which it’s accurate to say that you’re exerting a pull in a sport? For example, swimming: am I pulling the water toward me, or am I technically pushing it with my hands? /QUOTE]
Ah you noticed that pedantically you must PUSH stuff with you hands. If you pull on a door handle, you have actually used your arm in such a way that your finger PUSHES on the door handle ? Not neccessarily .
However what if you squeeze the door handle between your fingers, you can pull on the door handle with no “push” - because the amount of friction that your fingers can hold is F= mu N , where N is the normal force (perpindicular to the friction ! in this case, how firm the fingers are squeezing… ), mu is a (constant) coefficient of friction… …
So for that reason , ignore the GRIP the hand provides, as the grip can be quite different and get the same result… describe what the arm is going.
Now as to vacuum cleaners, they do not suck. By pushing air out the back, they create the condition for the atmosphere to push air into the nozzle and through the dust collection system.
I have a Physics degree, and I endorse this statement. Well, I endorse the first clause, that it’s not useful to really make the distinction, because it’s just too arbitrary at times (if you’re hanging underneath a tightrope, are you pulling it? Does it change to a push if you climb up and stand on it? If you’re pushing it down by standing on it, how does that change to a pulling on the ends of the rope? )
I have some quibbles with saying magnetic force is a sideways force (it’s not, we just by arbitrary convention decided define the field as sideways), but that’s WAYYY too much for third graders.
I think that individual muscles can do nothing but pull by contracting – pushing just means you’re using the opposite muscle to straighten your arm (by pulling on the opposite side).
OK, the direction of magnetic field is to some extent just an arbitrary convention, but there’s no way you can define magnetic field in such a way that the force is always along the field. Or just don’t even talk about the field at all, but just the objects: Put a bar magnet on a table with one pole pointing up, and shoot an electron through the space above the top of the magnet. The bar magnet is clearly exerting a force on the electron, but is it a push or a pull?
My first thought would be that a contracting muscle would be pulling and an extending muscle would be pushing.
You can’t “extend” a muscle, you can only contract them. You extend your fingers by contracting the extensor digitorum muscles. You extend your lower arm by contracting your triceps.
In weight training, moving the weight toward you is generally called pulling and moving it away from you is generally called pushing.
Whichever direction you are moving the weight, the muscles doing that work are contracting.
It’s maybe more a semantics issue than a physics one (nobody who has ever been in a tug-of-war would describe the activity as “pushing” the rope towards their side).
I agree. Push and pull are more useful when analyzing the properties of different materials. A brick wall is great under compression but lousy under tension; a steel cable is great under tension but sucks under compression. Then there’s shear, and twist, and probably a few others.
If you’re just talking about good old Newtonian Physics, it doesn’t matter much. F=ma, no matter where the F is coming from.
That said, if you’re really dedicated to the push/pull dichotomy as it applies to sports, tug-o-war is about as good as you’re gonna get.