Suppose you want to use your knowledge of electric charge and potential to build a
homemade launcher for the ball you chose in Full Solution 1. The launcher would essentially
be a parallel-plate capacitor with a hole in the middle of one plate, connected to a stack of
batteries to provide whatever potential difference is needed to launch the ball at the desired
speed. Released from a position near the positive plate, the ball accelerates towards the
negative plate and is launched through the hole.
Could such a launcher be built, in principle? Decide by calculating how many batteries (of
your choice) would be required to make the launcher work.
1. The ball carries the maximum possible charge (found in Sol 1)
2. The ball is launched from rest
3. Air resistance and gravity can be ignored
4. The total mechanical energy of the ball is conserved while being launched
5. The voltages of the batteries powering the launcher add up; e.g., three 1.5-V batteries
supply a total potential difference of 3(1.5 V) = 4.5 V
You'll need to find the mass of the ball (measure it for yourself, or consult 2+ sources online),
choose the type of battery (household, car, etc.) you want the launcher to use, and choose a
The launch speed is totally up to you, so long as you briefly explain why you chose it.
Suggested: choose a speed typical for the sport the ball comes from. For example, what
should the speed be for a ping-pong ball launcher used to train someone to return gentle
serves (or fast serves)? This kind of information can be looked up online, and should be
checked against common sense.
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