Hands-On-Physics
MECHANICS

Messing
Around

- Introduction -

Background

The measurement of speed is central to the understanding of motion. We are all familiar with speed and many people enjoy high speed so much that they race. You know what speed is even if you don't know how to quantify it. Average speed is defined as the ratio of the distance travelled to the time it took to travel that distance; symbolically dist / time. While messing around, you will calculate several average speeds by measuring the distance and time.

Questions

Tools

To determine the speed of a very fast moving object, either long distances or short times must be measured. For relatively slow motion, a stop watch and measuring tape are sufficient to measure average velocities. To analyze motion in which the speed may be changing, a stopwatch which can measure several times in rapid succession. HOP uses a "lap" timer with a memory for nine separate lap times. For the third activity you build a tool for measuring force. This construction of this "Thruster" requires a hot glue gun and a sharp knife.

Materials

Two of these messing around activities require only a soft object to drop and throw and a human runner. Making nested boxes is good practice with cardboard construction before you build a Thruster. The Thruster is made from corrugated cardboard, some springs, and a wooden rod.

Activities

Reporting

A record of your messing around in mechanics might include a short written description of each motion measurement. Diagrams are also helpful for explanation. Whenever measurements are made this data must be recorded carefully and accurately, with units included for clarity. Tables are useful for repeated similar measurements. To make the logic of your calculations clear, first record the formula you used, then show the numbers used to evaluate the formula, and finally record your answer.
Example: An apple was dropped and its average falling speed was determined.
Height of drop (h) = 6 feet
Time for drop (t) = 0.5 seconds
The average speed of the apple = distance/time = h/t = 6 ft/0.5 sec = 12 ft/sec
Notice that the measurements in this example are given with only one significant digit of accuracy. The result of the calculation is therefore a rough approximation. Reporting measurements to three significant digits of accuracy gives a much more useful result. A more careful measurement might give; h=5.37ft and t=0.347sec. The average speed would then be calculated to be 15.5 ft/sec. To measure changing speed, multiple measurements must be made. In this case, a table is helpful for keeping order.


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