Shulou(Shulou.com)11/24 Report--

There is a rope with one end tied to a fixed point and a small ball at the other end moving uniformly in a circle on the horizontal plane. In what direction will the ball continue to move when we suddenly release the fixed point? (a) continue to do a short circular motion; (b) fly out in the tangent direction; (c) fly out in the radial direction.

Many people may think, isn't this question very simple? I learned in high school that when moving in a circle, the speed direction of the ball is along the tangent direction of the circle, so when we release the fixed point, the ball will definitely fly out along the tangent direction, so the answer is b. To be honest, my first reaction was also answer b, but in fact the answer was wrong.

The University of Highport in North Carolina has demonstrated this experiment. They use a special device to create uniform circular motion, and then press a switch to make the fixed point fall off. In the whole process, it will be filmed with a high-speed camera, and we can get the trajectory of the ball.

We can see the experimental results from the picture above. When they release the fixed point, the ball will continue to do a small circular motion. So, what's going on? Why is it different from our high school? To answer this question, let's start with the rainbow circle.

We often see bloggers shoot such experiments with rainbow circles: grasp one end of the rainbow circle with their hands, let the other end droop naturally, and then we suddenly let go of the rainbow circle. Originally, we expected the rainbow circle to fall completely, but the result was that the bottom remained motionless until the top had fallen to its limit.

What causes it? This is because when we grasp one end of the rainbow circle, every point on the rainbow circle is balanced by the tension caused by gravity and deformation. When we release the rainbow circle, the deformation of the rainbow circle still exists, so all the points are balanced except at the top. After that, they begin to contract near the top, and they lose the support of tension and begin to fall. After a while, the contraction wave travels from the top to the bottom, and finally the bottom begins to fall.

So what does this have to do with the experiment we mentioned? We know that when the rope binds the ball to make a uniform circular motion, the tension of the rope acts as the centripetal force of the ball. So how does the tension of the rope come from? Yes, as you might think, the tension caused by the deformation of the rope is so small that it is not easy to observe in life.

So, when we release the fixed point, the deformation at the end of the rope that binds the ball continues to exist, so the tension it produces continues to act as the centripetal force of the ball's circumferential motion. So, at that moment, the ball will continue to move in a circle. Therefore, the answer should be a.

If we replace the rope with a rainbow circle, the effect will be more obvious: after releasing the fixed point, the arc length of the ball continuing its circular motion will obviously be longer than that of the rope, because the contraction wave of the rainbow circle will travel more slowly than the rope.

This article comes from the official account of Wechat: Vientiane experience (ID:UR4351), author: Eugene Wang

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