Answer to Question #154105 in Mechanics | Relativity for Haines Ho

Question

An object is attached to a lift. The mass of the object is 3 kg. 

Two spring balances S1 and S2 are attached to the upper and lower string 

to measure their tensions. 

The strings are light and inextensible. The lift is initially at rest. 

Assume g=10m per sec sq.

(a) If the reading in S1 is 40N, Find the reading in S2.

(b) The lift starts to move and accelerate upwards with a magnitude of 5m per sec sq.

What is the readings of S1 and S2? 

(c) When the lift approaches the end of its journey, it decelerates at a magnitude 

of 5m per sec sq, what is the readings of S1 and S2 ? 

Answer 1

m = 3kg g = 10$\frac{m}{s^2}$

a) The lift is initially at rest that is to sat a = 0

$S_1 + S_2 = mg => S_2 = mg - S_1 = 3 * 10 - 40 = -10N$

b) $S_1 + S_2 = mg + ma => S_2 = mg + ma - S_1 = 3 * 10 + 10 * 5 - 40 = 40N$

c) $S_1 + S_2 = mg - ma => S_2 = mg - ma - S_1 = 3 * 10 - 10 * 5 - 40 = -60N$