On/off magnet for purely mechanical (no power source) railgun-like device Here are the physics of the conventional "railgun" experiment. I suppose it depends on the voltage of the battery and force of the magnets, but most household experiments can power a light projectile a few centimeters. Here is the underlying concept:

This experiment is cool. However, although they are modest in size, they are still relatively complicated in terms of materials. In order to generate the Lorentz force as seen above, one needs a power supply. That means we will also need things like a battery, alligator clips, ect.
I was wondering if, with the help of a certain gadget, could we simplify the design and perhaps improve the strength of the device? The gadget I had in mind is an on/off magnet which is pictured here:
 
This particular on/off magnet is rated at 150lb of force. I would like to explore the physics behind using an on/off magnet to make a very simple railgun-like device. Let me try to frame this in an answerable question:
Question: If one was to attach some type of tube of the correct diameter to the bottom of the on/off magnet and activated the magnet while an oppositely charged magnet faces it, would that be all you need to have a crude railgun-like experiment? If so, would the force really be 150lbs as per the specs, or is the repulsion force calculated differently?
 A: It would be very, very difficult to turn that mechanically-switched magnet on and off quickly enough to make an acceptable railgun demonstration. I use that type of magnet in my hobbies, so know them well. 
You would do better to make a railgun with electromagnets.  See for example rail gun
Edit: The 150 lb force in the specs refers to how heavy an object the magnet can lift.  But the force exerted by the magnet on a piece of iron decreases dramatically as the distance between the magnet and the iron increases. Yes, conceivably, the switchable magnet could pull on a projectile with a force of 150 lb, but only over a very short distance like five hundredths of an inch.  Let's say the projectile weighs a tenth of a pound, and the force is 150 lb.  The acceleration would then be roughly 1500 times the acceleration of gravity.  The projectile would move the five hundredths of an inch hit the magnet in about 0.01 second. You wouldn't have time to switch the magnet off.
If you set up a long horizontal air table so that you could have essentially frictionless linear motion, it might be possible to illustrate the principle of a rail gun using mechanically switched magnets, but your timing would need to be superhumanly good because the effective range of the magnets is so short. The magnets would need to be turned on while the projectile is within the range of, say, a centimeter from the magnet, then turned off before the projectile passes the center of the magnet. Human reaction time is about 1/3 second.  At 60 miles per hour (VERY slow for a rail gun), the projectile would move nearly 30 feet in 1/3 second.  It's basically impossible for a human being to react quickly enough, even if the magnet could be switched rapidly enough.  If your goal is to make a demonstration of the principle of a railgun using inexpensive parts, it is actually very easy, but you'll need to use electromagnets.
A: Here is a video of how such magnets can work: https://www.youtube.com/watch?v=zsrxzkbKnkc
It is just a rotatable cylindrical magnet in a slotted tube made of soft iron.
I doubt that one could switch this device fast enough to induce sufficiently strong Lenz Law currents in a piece of copper or aluminium on the slot in the slotted iron tube. It would not jump, not like the classic jumping ring physics demo.
I suppose you could use two of these, one in the on position and the other in the off position. They should attract. When the second magnet is then also turned to the on position, they will move apart when same poles face eachother. 
It won't make a gun. A "Gauss gun" should be more fun.
A: So instead of Lorentz force, you are just using the repulsion of the poles, if I am understanding correctly. If so, then I see no reason why you couldn't do this, just make sure the magnet projectile has a strong enough magnetic field, and the mass is small enough so that you get the desired acceleration. Of course, all your projectiles will have to be magnets, and probably pretty strong, so I am not sure how viable this design is as a useful projectile launcher.
