Make sure the skater and the stopwatch start at the same time. Stop the stopwatch when the person crosses the 20-meter mark. Do this for as many students as you want, keeping careful records. This means that she is adding one meter per second to her speed every second. Now you're ready to figure the force generated by each skater.
Force is measured in newtons.
Remember that these are very rough calculations. Compare this to your weight in pounds. Spin a raw egg on its side in a bowl. Stop it, then let go of it quickly. Make a number of odd shapes out of cardboard.