Physics Involved- Normal Forces, Friction, Work and Some Gravitational Potential Energy
Push-ups are a simple but effective exercise that utilize only your body weight and gravitational force to develop your upper body muscles. In essence, your muscles counteract the force of gravity on your body by applying a normal force to the ground, resulting in you being pushed away from the floor. Free-fall is not a factor in this exercise, since controlled up and down movements are key. The normal force exerted by you must be greater than the gravitational force on you (F=ma). The amount of inertia you have is dependent on how much you weigh. Although you'd think that friction isn't really an issue here, since your acceleration on the x-axis is negligible, at best, it is. You need friction to keep your hands in the same position as you move your body up and down; if you tried to do push ups on ice, for example, you'd find it a lot harder than doing them on a hard surface with a higher coefficient of friction, so it's important to note where you'll be exercising. Action-reaction force pairs also play a big role. No matter what surface you're pushing up on, there'll be an equal and opposite force to match it, as per Newton's Third Law of Motion. The whole exercise is based on action reaction force pairs, and the assumption is that you'll be exercising on a surface that won't deflect enough to be unsafe to you or others.
Push-ups are a simple but effective exercise that utilize only your body weight and gravitational force to develop your upper body muscles. In essence, your muscles counteract the force of gravity on your body by applying a normal force to the ground, resulting in you being pushed away from the floor. Free-fall is not a factor in this exercise, since controlled up and down movements are key. The normal force exerted by you must be greater than the gravitational force on you (F=ma). The amount of inertia you have is dependent on how much you weigh. Although you'd think that friction isn't really an issue here, since your acceleration on the x-axis is negligible, at best, it is. You need friction to keep your hands in the same position as you move your body up and down; if you tried to do push ups on ice, for example, you'd find it a lot harder than doing them on a hard surface with a higher coefficient of friction, so it's important to note where you'll be exercising. Action-reaction force pairs also play a big role. No matter what surface you're pushing up on, there'll be an equal and opposite force to match it, as per Newton's Third Law of Motion. The whole exercise is based on action reaction force pairs, and the assumption is that you'll be exercising on a surface that won't deflect enough to be unsafe to you or others.
All the usual action-reaction force pairs are present in this exercise, but the most important ones are those corresponding to Normal Force and Gravitational Force. If you're doing the exercise right, you should feel like you're doing a lot of work. At the end of it, you should be huffing and puffing, tired but satisfied with the progress you've made. But you may not have done the amount of work you think you did. In physics, work is measured in terms of energy out vs. energy in. Lifting yourself takes the same amount of physics work each time you do it, even though your muscles need to exert themselves more. It's independent of them because of the law of conservation of energy, which states that the energy in a system must remain constant throughout any movement it may have. So let's say that the part of your body your arms are suspending has about 1000 Joules (that's a rough number and not indicative at all of how much your body actually has) of gravitational potential energy. That means your muscles need to take those Joules, and turn them into an equivalent amount of kinetic energy. But since your muscles are not flawless automatons that can work ceaselessly without tiring, this grows tiring after a while. That's why you feel the way you do after long bouts of exercise- your muscles can't keep up with your body's inherent and always present energy (at least that's the positive way to spin it).
Here's a Youtube video I whipped up about push-ups:
Here's a Youtube video I whipped up about push-ups: