Earlier today, Steve and I went to see
Captain America. It was very entertaining, but as usual, I have a physics nitpick. In one scene, the Captain and his men ride a zip line down from a mountaintop and drop off onto a speeding train, landing with ease. It seems to me it would require an enormous height difference to achieve the necessary speed to land on the train without being thrown off.
We can find the height necessary to achieve a certain speed using energy. The kinetic energy gained by dropping a height
h is
where
m is the object's mass and
g is the acceleration due to gravity. Meanwhile, the energy involved in traveling a velocity
v is
Putting these together and solving for
h gives
Given the era, I'm guessing the train was a
diesel engine, so its top speed would be around 100 km/h. Plugging that into our equation gives 39 meters, or about 13 stories. Also note that this is the minimum height, since it assumes that all his downward momentum gets transferred to horizontal. If the zip line were at a 45° angle, it would require twice the height. Don't let my pedantry put you off though; it was a great movie, well worth seeing.
No comments:
Post a Comment