Al Tobias (wat4y)  Office: Gibson S123 & Physics 218, (434) 9240538
Demonstrations are cataloged according to PIRA Bibliography
PLEASE REQUEST DEMOS WELL IN ADVANCE
Due to Physics Building renovations, the lead time to set up demo requests has increased due to the need to transport equipment across campus. Please be kind and let me know well ahead of time what you need.
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Ballistic PendulumPurpose:To illustrate conservation of momentum in an inelastic collision by accurately measuring the velocity of a bullet.Procedure:PROPER SAFETY SHOULD BE OBSERVED AT ALL TIMES! RIFLE or PELLET GUN WILL NOT BE LOADED UNTIL YOU LOAD IT YOURSELF & DO DEMO.Rifle Setup for Rm. 203: With the pendulum hanging from two steel wires via the ceiling, place the "displacement" stand rod against the paper marker noting the original position via the affixed ruler. Load rifle and fire into end of tube. As the pendulum swings, the stand will move the paper such that the maximum displacement can be measured. As of Spring 2020, we can now add our new Caldwell Precision Chronograph G2 between the rifle and the ballistic pendulum to obtain a 2nd independent measurement of the bullet speed. Click here for the Caldwell Chronograph G2 User Manual. If the displacement is "X" then the velocity "v" is found by: v = X*sqrt(g/L)*(M+m)/m DATA:
For other classrooms there is a free standing shorter pendulum available. Use the pellet airgun with this version, NOT the rifle. Don't forget to load the pellet into the airgun. DATA:
Hints:Many initially argue that our expression above is missing a factor of 2. That is incorrect. Actually, the above expression is based on an approximation relating the height h our pendulum rises to the displacement X and pendulum string length L:h=X^{2}/2Lwhere we find this by one of the following methods: 1) Find the solution to the quadratic: L^{2}=(Lh)^{2}+X^{2} L^{2}=L^{2}2Lh+h^{2}+X^{2} use the "realistic" solution with the "negative" sign in front of the radical (ie. h is not larger than L or imaginary), and assume: h << X << L 2) By more of a "hand waving" geometry approximation arguement, assume the total distance the pendulum travels (horizontally and vertically) is approximately equal to the horizontal displacement X. Then, sinθ=X/2L and sinθ=h/X Rifle Setup for Rm. 203: Place rifle barrel more than 17cm away from freely hanging pendulum, so that once it starts swinging, it won't hit rifle. The displacement will be around 17cm with a bullet speed of about 500 m/s. Pellet Air Gun Setup: We have measured as good as 100 m/s with 10 pumps and a displacement over 4.5cm. Equipment:
