A rectangular block is moving along a frictionless path when it encounters the circular loop as shown.The block passes points 1,2,3,4,1 before returning to the horizontal track.At point 3:
A)its mechanical energy is a minimum
B)the forces on it are balanced
C)it is not accelerating
D)its speed is a minimum
E)it experiences a net upward force

Question

A rectangular block is moving along a frictionless path when it encounters the circular loop as shown.The block passes points 1,2,3,4,1 before returning to the horizontal track.At point 3: 
A)its mechanical energy is a minimum 
B)the forces on it are balanced 
C)it is not accelerating 
D)its speed is a minimum 
E)it experiences a net upward force

Quizplus · Accepted Answer

At point 3, the block is at its highest point on the circular loop. This means that all its gravitational potential energy has been converted to kinetic energy. Therefore, it has its maximum kinetic energy (and speed) at point 3. As it moves up the loop, its potential energy increases, causing its kinetic energy (and speed) to decrease. Therefore, at point 3, the block has its minimum potential energy, which means it has its minimum total mechanical energy. Since the block is still moving, it must be accelerating, and it is experiencing a net downward force due to gravity. Therefore, choices A, B, and E are incorrect.

A Rectangular Block Is Moving Along a Frictionless Path When