Jim is playing with a 1-meter-long pendulum that is hooked
2 meters above the ground. he lifts the pendulum's bob to a height of 2 meters above the ground and then releases it. potential energy is assigned zero value at the ground level. which statement is true about this problem?
a. at the lowest position of the bob, the ke is equal to zero.
b. at the highest position of bob, pe is equal to zero.
c. at the highest position of the bob, the ke is equal to zero.
d. at all the positions except the highest positions the pe is zero.

Your starship, the aimless wanderer, lands on the mysterious planet mongo. as chief scientist-engineer, you make the following measurements: a 2.50-kg stone thrown upward from the ground at 14.0 m/s returns to the ground in 5.70 s ; the circumference of mongo at the equator is 2.20×10^5 km ; and there is no appreciable atmosphere on mongo. (a) the starship commander, captain confusion, asks for the following information: what is the mass of mongo? (b) if the aimless wanderer goes into a circular orbit 2.20×10^4 km above the surface of mongo, how many hours will it take the ship to complete one orbit? (c) an unmanned spacecraft is in a circular orbit around the moon, observing the lunar surface from an altitude of 44.0 km . to the dismay of scientists on earth, an electrical fault causes an on-board thruster to fire, decreasing the speed of the spacecraft by 23.0 m/s . if nothing is done to correct its orbit, with what speed (in km/h) will the spacecraft crash into the lunar surface?