A man of mass 60 kg is standing in a lift at rest. What will be the reaction force on the man when the lift is (i) stationary, (ii) moving up with an acceleration of 4.9 `m cdot s^(-2)` , (iii) moving up at a constant speed, and (iv) moving up with a retardation of 4.9 m`cdot s^(2)`? [ g = 9.8 m`cdot s^(2)`]

Explanation

Reaction of the lift floor when the lift is stationary,
R = mg = `60 xx 9.8 N ` = 588 N.
(ii) The lift is moving up with an acceleration,
a = 4.9 m`cdot s^(2)`.
Reaction, R = m(g + a) = 60(9.8 + 4.9) N = 882 N.
(iii) When the lift is moving up at a constant speed, a = 0 .
`therefore" " R = "mg" = 60 xx 9.8 N = 588` N.
(iv) When the lift moves up with a retardation of 4.9 m`cdot s^(-2)`,
R = m[g + (-a)]
= m(g - a) = 60(9.8 - 4.9) N = 294 N.