«Техніка написання» анотацій і реферувань

First Stage READ AN ARTICLE ABOUT AUTOMOBILE STOPPING DISTANCES BY ROBERT GRIMM.

A classroom demonstration of automobile stopping distances

By Robert D. Grimm

One almost universal complaint of high school students as well as college undergraduates is that physics course material is often ir­relevant. Recognizing this problem, we need to identify things that are important to students and that are explained by physics concepts. One area that comes to mind almost immediately is the motion of automobiles. High school students are especially preoccupied with cars since they are at the age when they are beginning to drive. Here's a question on automobiles that can teach some physics.

Consider two cars, a 1500-lb economy model and a 3000-lb gas-guzzler both moving with the same velocity. Which car will stop first if both drivers lock up the wheels at the same instant?

After the students make predictions, open a driver's manual to the page where the stopping distances are listed according to the initial velocity of the car. The figures in such tables are based on smooth stops, without the wheels being locked and skidded. Note that the data are independent of type of car. This lack of depen­dence on mass would also be true with locked wheels.

If the coefficient of friction is the same for two cars on the same surface (usually a good approximation), then the friction force is µmg and the deceleration is jig, independent of mass. Two cars of different masses will stop in the same distance. This idea is not always obvious to students and is also an idea that has not received a great deal of attention as a classroom demonstration.

One rather simple demonstration of this concept is shown schematically in Fig. 1. The two ramps may be constructed from pieces of wood or whatever materials are available.

Fig. 1. The experimental set-up with two bathroom tiles loaded on the cart.

For the first trial one bathroom tile is placed on the top cart, smooth side down. The loaded cart is then released and travels down the ramp until it collides with the end of the shorter ramp.

At this time the tile slides off the cart and skids to a stop just as our economy car would in the "real life" example. If the ramps are built in such a way that the angles of inclination are the same, and the separation distance d is equal to the height of the car, the tile will slide onto the second ramp with very little interference.

A second run is now made with two tiles. Place the tiles on the cart with rough surfaces facing each other so they act as one body. These represent the gas-guzzler car in our example. If the experi­ment is done carefully, it will be seen that both cars stop in the same distance.

In both trials visibility of the stopping position can be increased by putting a flag in the centre of the tile and recording where the flag stops on the lower ramp with a second marker.

For the velocity of the car to be the same for both trials, the relational kinetic energy of the wheels must be negligible compared to the total kinetic energy of the cart. A fairly massive cart with light wheels is therefore suggested. A number of scientific supply houses have carts that closely approximate this requirement.

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