Physics of a Broken Swing Image
Surely by now, someone online has clearly shown this image to be fake. Instead, let’s use this as an example looking at the way people think about forces and motion.
- Forces and Circular Motion
Suppose something is moving around in a circle. Maybe it is a amusement park swing. Here is a force diagram for one of the riders:
There are only two forces on the swinger. There is of course the gravitational force pulling down. The only other force is the tension in the chain pulling on the swinger in the direction of the chain. Then why does the swinger move in a circular path? A component of the tension force pulls up to counter act the gravitational force. The other part of the tension from the chain pulls towards the center of the circle. It is this part of the tension force that makes the swinger move in a circle.
If you like, you can break all forces into two types. If a force is in the same (or opposite) direction as the motion (velocity) of an object, that force will cause the speed to either increase or decrease. If the force is perpendicular to the direction of the velocity, this force will cause the object to change directions. Of course you can have a force that both speeds up an object and causes it to turn.
Really, that is it. That is the only physics that you need to get this swinger to move around in a circle. Sure, there is a relationship between the angle the swing is at and the speed that the swinger moves, but for now we can leave that alone.
- What Would Really Happen?
If the chain suddenly breaks, what happens next? Well, the force diagram becomes a little bit simpler. It would just look like this:
This gravitational force would cause the velocity to change in the downward direction. So, clearly, it would fall down since before the chain broke it wasn’t moving in the vertical direction at all. But what else would it do? Here is a diagram of the swinger from the top view.
In this view (after the chain broke), you can’t see the only force pulling on the swinger – the gravitational force is pulling down. Since there aren’t any forces pushing the swinger to the left or right, from the top the swinger would just go in a straight line.
In the faked image, the falling swinger is clearly moving away from the swing in a path perpendicular to the way he was originally moving.
- Why Would a Faker Get This Wrong?
Many people seem to think that if an object is moving in a circle there is a force pushing outward from the center of the circle. This is the fabulous and mythical centrifugal force. Even though the centrifugal force is fake, it can still be useful in some ways. However, the point is that if you are in a stationary frame then there IS NO force pushing outward.