Conservation of Momentum & Energy

Conservation of Momentum & Energy


Col. Stapp's Ride


Air Force Col. John Paul Stapp rode rocket sleds in the late 1940s and early 1950s. The rocket accelerated it to high speed and water was used to abruptly stop it.


The top three photos below show his face as the sled accelerated, and the bottom three show the deceleration from 421 mph to zero in one second. He lived, and conducted similar experiments many times. Murphy’s Law came from these tests, when an engineer named Murphy who worked with Stapp said essentially, when anything can go wrong, it will.

(US Govt. photos.)


The rocket sled was used to simulate deceleration as in a plane crash. Later his findings were used in designing safer seat belts for cars.


If the rocket sled with Col. Stapp in it weighs 104 kg and the gas jets on the back of the sled apply a force of 50,000 N for 10 seconds. What is the final speed of the sled?


Ft = p = mv


So, v = (Ft)/m

= 50,000 N x 10 s /104 kg

= 5 x 105 Ns / 1 x 104 kg = 50 m/s


How did all those complicated units reduce to just m/s? Remember that a N has units of kg*m/s2 so that when it is multiplied by seconds and then divided by kg everything cancels except m/s. Physics works.



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