Inspiration from the siege
The ancients broke the city gate, put the big tree trunk on the pulley, and crashed into the city gate.
The kinetic energy of the soldiers was amplified ten times because the rolling friction coefficient was 1/10.
Suppose the soldiers were carrying the wood and ramming it against the city gate, using the same amount of force, the effect would be ten times worse than using a pulley.
The calculation is as follows: The work done by the soldiers W=FS
When there is a pulley, F=1/10ma (m represents the trunk mass, a=10 meters/second^2), and the work distance S=20 meters. W1=1/10m×10×20=20m
Without a pulley, the tree was lifted directly into the city gate. W2=m×10×20=200m
It is very intuitive to see: under the same acceleration, the tree trunk rushes out the same distance, and the kinetic energy of the tree trunk hitting the door is the same, E=1/2mv^2=1/2m(at)^2=1/2m×( 10×2)^2=200m (According to the final velocity formula v=at; according to S=1/2at^2, find t=2)
The work done by the soldiers was 10 times different.
It can be seen that the use of pulleys is not only labor-saving but also energy-saving. Soldiers only need to put in 1/10 of the work to get 10 times the kinetic energy. According to this principle, wouldn’t the door to the perpetual motion machine
be knocked open? !
