Question

a 68 kg man is standing on the floor of a lift .cal. the apparent weight of man when the lift is a)at rest .b)moving upward with acceleration of 1 m/s^2.c)moving downward with acceleration 2m/s^2.​

Answer 1

Answer:

680N , 860N, 544N

680N , 860N, 544NExplanation:

Weight = mass x acceleration = force

Weight = mass x acceleration = forcea) when the lift is at rest

F= ma = 68 x 10 = 680 N

b) when the lift is moving upwards,

we add the gravitational force and the acceleration so,

F = m(g+a) = 68(10+2) = 68 x 12= 816N

c) when the lift is moving downwards,

we subtract the gravitational force and the acceleration so,

F = m(g-a) = 68 (10-2) = 68 x 8 = 544N

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Answer 2

Given :

mass of man (m) = 68 Kg

To Find :

The apparent weight of man when the lift is

a)at rest

b)moving upward with acceleration of 1 m/s^2

c)moving downward with acceleration 2m/s^2.​

Solution :

(a) We Know, Normal reaction of the man in the lift gives its apparent weight

    As lift is not moving (rest), acceleration = 0

As per question,

Normal reaction of the man - mg = 0 (where g = acceleration due to gravity)

or,   Normal reaction of the man   = mg

or,  Normal reaction of the man    = 68×10 = 680 N

∴ The apparent weight of the man when the lift is at rest is 680 N.

(b) When the lift is moving upward with acceleration of 1 m/s², three forces is acting on the man -

Normal reaction (N) in the upward direction, Force due to gravity (mg) downwards and a force due to acceleration (a = 1m/s²) in the upward direction.

Hence,  Normal reaction - mg = ma

or,          Normal reaction         = ma + mg

or,          Normal reaction         = 68×1 + 68×10

∴            Normal reaction         = 748 N

∴ The apparent weight of the man when the lift is moving upward with acceleration 1 m/sec² is 748 N.

(c) When the lift is moving downward with acceleration 2m/s², three forces acts on the body -

Normal reaction (N) in the upward direction, Force due to gravity (mg) downwards and a force due to acceleration (a = 1m/s²) in the downward direction.

Hence,        mg - Normal reaction = ma

or,                        Normal reaction = mg - ma

or,                        Normal reaction = 68×10 - 68×2

∴                         Normal reaction  = 544 N

∴ The apparent weight of the body when the lift is moving downward with acceleration 2 m/sec² is 544 N