急！Form 4 Physics about motion and force

Question

A 50 kg man is standing on a balance in a lift. The balance reads 350N when the lift accelerates downwards. What is the acceleration of ther lift?　詳細好d!

Answer 1

Take downward direction as positive.

W=mg~~~~~~~~~~the fomula to caculate the force of gravity(地心吸力)
W=50(10)
W=500N~~~~~~~orginal force(when the lift does not move)

F=500-350=150N~~~~~~~~~~net force(淨力)
m=50kg~~~~~~~~mass of the man
a=?~~~~~~~~the thing you want to know


By F=ma,~~~~~~~~~~Newton's second law
150=50a~~~~~~~~sub the information into the fomula
150/50=a~~~~~~~calculate
3=a
a=3(ms^-2)~~~~~~~you get the answer

So, the acceleration of the lift is 3ms^-2

Answer 2

Since the man is accelerating downward, his weight (50g) must be greater than the normal reaction of 350 N

Thus, the net downward force = (50g - 350) N

Let a be the acceleration
using net force = mass x acceleration
50g - 350 = 50a
thus a = (50x10-350)/50 m/s2 = 3 m/s2