The Block on a Spring Demo

Theory

When a body falls in a gravitational field, it loses potential energy. If it falls freely, it gains an equal amount of kinetic energy (See "FALBAL" for a video method of measuring the speed of a falling ball). If the body is attached to a spring, the decrease in gravitaional PE is offset by an increas in the sum of kinetic and elastic potential energy. The body will oscillate about a new equilibrium point, which, except for frictional losses, will be halfway between its release point (zero KE) and its point of extreme travel.

Apparatus

The equipment needed for this demo is:

* A Pasco bearing track
* A cart
* A spring
* a Pasco photogate

There is a 2-meter bearing track in Room 302, over the door to 302A. Bearing carts, springs, and 1.2 meter Pasco tracks can be obtained from the P115/P116 Labs (Room 404, cabinet to right of door).
If you are satisfied with having a weight fall vertically, springs, masses, and photogates can be obtained from Room 403A or Room 404.

Procedure

Set of the bearing track on a level, stable surface. The spring should be attched to one end of the track and attached at the other end to a bearing cart.The bearing cart should have a flag inserted into the top to signal the photogate. A photogate should be put up just past where the spring's point of zero displacement is.

The cart is struck by a hammer with an arbitrary force. Upon impact, the energy from the hammer will be completely transfered to the other cart. That cart will displace the spring as it moves, transferring its kinetic energy to the potential energy of the spring

This demonstration happens in five stages: The first stage is at the time of impact just before the spring begins to move. The closer the photogate is placed to the zero point, the better estimation of the energy at this state will be. The second stage is when the second cart displaces the spring. The third stage is when the spring and cart are both at rest (when all of the energy is in the form of potential energy). The fourth stage is when the spring is moving the block away. The fifth state is when the cart moves past the photogate to the zero point

Helpful Hints

(1) Choose the right spring. The spring must be light enough so that the class can see it stretch out.

(2) If the spring attaching the cart and the end of the track lies along the track, it will scrape. This can be reduced by having it scrape over a smooth piece of paper or laying down some scotch tape on the track where it comes in contact. Another method is to suspend the spring, either by balancing it on a rolling device or attaching it pendulum-like by attaching strings to the coils from an overhead attachment such as a crossbar on a ring stand.

(3) Any such method in (2) should be tested before class to ensure that the particular arrangement doesn't result in large error in any calculations the class might conceivably do. Improper suspension of the spring or an excessively massive spring will affect the quantitative results.