Physics Lab Problem

MATERIALS

1 oak plank, 180 cm x 30 cm x 1 cm

5 oak blocks, 30 cm x 30 cm x 30 cm

one piece of string

1 pulley

SITUATION:

Three blocks are stacked underneath one end of the plank. The fourth block is placed on top of the plank, at the opposite end. A string is run from it, up the plank, over the pulley, and the final block is attached, hanging from the other end. When the two blocks are released, the fifth block falls due to gravity, and pulls block number four up the plank.

QUESTION:

How long does it take before the block hits the pulley?

RULES:

„ you may work in a SMALL group. Turn in one copy of your solution, with everyone's name on it.

„ write down all assumptions

„ document any reference materials utilized

„ draw a picture of the situation

„ show all work

„ neatness counts

SOLUTION

F_net = ma = T - F_f - Fg₁₁

F_net = ma = F_g - T

solving each for T: T = ma + F_f + Fg₁₁

T = F_g - ma

combining: ma + F_f + Fg₁₁ = Fg - ma

2ma = F_g - F_f - Fg₁₁

F_f = uF_n = uF_gcosŲ

a = (Fg -Ff-Fg)/2m

a = 1/2 g (1 - ucosŲ - sinŲ)

*CRC Handbook shows coefficient of friction for oak to be .48

a = (9.8 m/s²)(1-(.48) cos30 ^0_ sin30⁰)

a = 0.413 m/s²

d = v₁t - at² but v₁ t = 0

t = (2 d / a )^1/2

t = (2 (1.80 -.3) /.413 )^1/2

t = 2.7 sec