When the lift / elevator is going down, the person experiences a downward acceleration, a.

The resultant force acting on the person is also downward. The equation for the resultant force can be expressed using Newton's Second Law

which is

F = ma

where m = mass of the person.

From the forces (R and W) shown in the diagram above, the resultant force F can also be obtained as such:

W - R = F (resultant force)
mg - R = ma
-R = - mg + ma

R = mg - ma  or  R = m(g - a)

Hence, the reading of the weighing scale / machine which is the measurement for R decreases (has a reading smaller than the actual  weight W = mg of the person) when the lift / elevator is moving downwards.

An interesting point for consideration:
When the (downward) acceleration of the lift / person is equal to the acceleration due to gravity, g

the reading of the weighing scale / machine becomes

R = m (g - a)
R = m (0) because a = g

Hence, R = 0 N.

The person experiences weightlessness.
 
Click on / Touch the diagram above to continue.

Apabila lif bergerak ke bawah, penumpang lif mengalami  pecutan ke bawah, a .

Daya paduan yang bertindak pada penumpang juga bertindak ke bawah. Persamaan daya paduan boleh dihuraikan menggunakan Hukum Newton Kedua, iaitu

F = ma

di mana m = jisim

Selain daripada itu, menggunakan daya-daya (R dan W) yang ditunjukkan dalam rajah di atas, daya paduan boleh juga diperolehi daripada persamaan berikut

W - R = F
W - R = ma
mg - R= ma

R = mg - ma

Yakni, bacaan skala penimbang (ukuran bagi daya tindak balas R) menurun apabila lif bergerak ke bawah dengan pecutan. Dalam ertikata lain, bacaan penimbang adalah lebih rendah daripada  berat sebenar penumpang.

Sesuatu yang menarik untuk dipertimbangkan:
Apabila pecutan ke bawah lif dan penumpang itu bersamaan pecutan graviti

bacaan skala penimbang akan menjadi 0 N. Ini kerana

R = m (g - a)
R = m (0) kerana a = g

Yakni, R = 0 N.

Penumpang mengalami kenirberatan.

Klik / Sentuh pada gambar rajah di atas untuk nota selanjutnya.