We all know that the rope supporting a swing does not act on the girl at its top. It is like asking if the floor of an elevator acts on someone standing in it. This question, however, was posed to me by a student who had found this puzzle in his textbook: “A girl stands near one end of a light rope that passes over two supports. What will happen when she swings?” I went through my normal explanations about how motion only occurs when there are forces acting and tried to explain what forces were involved here. He said he understood that but still wanted me to answer his original question. So I asked him what he thought would happen? He suggested that maybe the force from her swinging might make something else move or change. So I asked him what he thought would happen to her as she swung? He said maybe the rope might tighten up or get shorter, but that it wouldn’t be like someone swinging on a swing from one side of the playground and acting on someone else who was just standing still out in the middle of the yard.
I guess this question is related to Newton’s First Law: “Every object either remains at rest or continues moving in a straight line unless acted upon by an outside force.” That force could come from friction (something rubbing against something else) or even gravity when you’re talking about motion caused by people jumping around. But here there are no forces involved with anything besides gravity, so we can say without any doubt that only gravity is acting on this girl.
What happens to things that are not being pushed, pulled, hit with something else, or otherwise made to move from their current position (objects that have no external forces exerted against them)? They stay put! Even if all of the forces on you this morning were to suddenly disappear, and we’re not talking about just your clothes falling off (not that I would want them to) but anything making you move from where you started out in the world, then all of those things will stay put.
Is it true? We can’t say for sure! It is possible that there are other external forces at work depending on what she’s wearing or if her shoes have a grip like an ice skate blade does. With nothing else going around here though besides gravity pulling down on everything, someone who’s interested in physics could make some conclusions based solely on our first law: “Every object either remains at rest or continues moving in a straight line unless acted upon by an outside force.”
In this case, the girl is going to stay at rest on her swing due to an outside force. The only other external forces are gravity and friction: they don’t act here because she’s in a constant state of motion. We can safely say that no one does anything to the girl while she swings back and forth so nothing will happen!
The only external forces that affect her are gravity and friction. Gravity pulls down on everything, but she is in constant motion so will stay at rest. And while there might be some air resistance as well (which could make it feel like something is happening upwards), this line of thought doesn’t apply because no outside force has acted to change what would have happened without any other influence than an object’s own inertia. So we can say with certainty that nothing happens to the girl when she swings!
no, because she’s in a constant state of motion. We can safely say that nothing happens to her while she swings back and forth so it won’t happen!
gravity only acts down which doesn’t change what was going to happen without any other influence than an object’s own inertia. So we can say with certainty that nothing happens to her when she swings! The only external forces that affect her are gravity and friction. Gravity pulls down on everything, but she is in constant motion so will stay at rest. And while there might be some air resistance as well (which could make it feel like something is happening upwards), this doesn’t change what the girl is going to do with her own inertia, which could be swinging back and forth.
An object moving uniformly does not need a net force acting upon it because its velocity remains unchanged from one moment to the next; likewise for an object at rest or undergoing simple uniform circular motion around a fixed point.
