EK physics 1001 number 242

[2010premed]

---

Members don't see this ad.

A 100 kg man dangles a 50 kg mass from the end of a rope. If he stands on a frictionless surface and hangs the mass over a cliff with a pulley as shown in the question above, the tension in the rope will be: (Note: ignore any frictional forces.)

A. 250N
B. 333N
C. 500N
D. 667N

Answer B... don't see how they got that

 

[fizzgig]

+x -->
+y down

mass sits here --> rope to pulley, mass hangs down from there.

if the mass on the cliff moves with acceleration a, you know the hanging mass is also moving with that acceleration. pick your coordinate system so that a is positive or negative for BOTH moving masses.

block on cliff: Fx = 100a
block hanging: -Fy + mg = 50a
Fx = Fy = T

-100a + 50g = 50a
50g = 150a
500 = 150a
a = 3.33

go back to the 100kg block and get T
Fx = T = 100(3.33) = 333

this seems to come up A LOT. i had better learn to do these faster...