- A small block starts from rest at height h on a frictionless track that includes a vertical loop of radius R. What is the minimum h (measured from the bottom of the loop) required for the block to just maintain contact at the top of the loop?
A
h = 2R
B
h = 5R/2
C
h = 3R
D
h = R
Answer and explanation
Correct Answer: B — h = 5R/2
At the top, just-maintained contact implies normal force = 0, so v^2 = gR. By energy: mgh = mg(2R) + (1/2)mv^2 = mg(2R) + (1/2)m(gR) ⇒ h = (5/2)R.