[填空题]
An elastic bungee cord of natural length $20\,m$ is being tested. A spherical object, originally at rest, is released from a platform. Air resistance and the mass of the cord are insignificant.
1.State the principle of conservation of energy.
2.Describe the eneryg changes that take place in the spherical object from the time it is released until it comes to rest for the first time.
3.The speed of the object just before the cord starts to extend is approximately $m\,s^{-1}$.
4.The magnitude of the average net force acting on the object by the cord between the point the cord starts to extend and the point where it reaches maximum extension is $360\,N$ . The time taken during the fall of the object between these points is $0.28\,s$.
1.The mass of the object is approximately $kg$
2.On the graph below, sketch how the the elastic potential energy $E_E$ of the spring changes with the vertical displacement $s$ as the cord stretches. There is no need to add values to the axes.
3.The extension of the cord at the point in time when the object has zero acceleration is $11\,m$. Determine the spring constant of the cord. $N\;m^{-1}$
