Interactive Lecture Demonstration 13 – Ballpoint Pen Launcher

Every group of 2 students will need a retractable ballpoint pen and a ruler or meter stick.   You will also need to use a digital balance at some point during this activity.

Dismantle your ballpoint pen into

a) ink holder

b) pen case bottom

c) pen case top

d) spring (initially in pen case bottom; the pen bottom is the part that your hand would hold if you were writing with the pen)

Only the holder, case bottom and spring will be used in this experiment.  Invert the ink holder and hold it vertically on the table.  Place the spring on top of the inverted holder, and place the inverted case bottom on top of that.  Press the case bottom down as far as it will go & release.  The case bottom should be launched vertically to some height h above its launch point.  Your goal in the following parts is to predict the height h.

1) Determine the spring constant  k  by pressing the bottom of the ink holder against the pan of a digital balance, pressing the pen case bottom down against the spring until the spring is  just  maximally compressed, and measuring the distance you compressed the spring.  Use an appropriate force diagram and net force equation to determine your value for k. Hints: a) what force does the scale tell you ? b) You should do a force diagram for the object on which that force acts.

2) Use your value for k to determine the maximum height that the case bottom will reach.  Use well-labeled diagrams to show initial position, final position, etc. of the case bottom.  Hint: you do not need to break this problem into parts ! Choose appropriate initial and final locations of your case bottom to make this problem as simple as possible.

3) Measure and record (with at least 5 perfect runs; "perfect" means the case bottom was launched entirely vertically with no horizontal motion and no vertical friction force acting) the maximum height reached of the case bottom (relative to its initial position).  

4) What things were ignored in this experiment that might have caused an incorrect prediction for the maximum height of the spring?  Would ignoring these things have led to a prediction of maximum height that was too large, or too small ?  Explain.

5) Another conservation of energy problem for practice: Find the speed with which the case bottom left the spring.