Question about newton's third law

Question

A student of mass 75kg investigates forces on him in a lift by standing on a set of bathroom scales. Taking g=10N/kg, what would the scales read if the lift has an upward acceleration of 2m/s^2.

Answer=900N.

How do i do it?

Answer 1

dont knw....but ive studied :P

Answer 2

N / kg is just a nice way of saying m/s². ^_^;;

Let's say that you're the student. The lift needs to accelerate you enough to overcome gravity's 10 N/kg, and give you 2 N/kg in the upward direction. That requires accelerating you at 12 N/kg, with a force of:

12 N/kg * 75 kg = 900 N.

So *you*  are being pushed *up* with a force of 900 N. But what are you touching, that's pushing you up? The scale: you're standing on the scale.

But if the scale is pushing you up, then you must be pushing the scale down! This then works exactly the same as if you simply weighed 900 N: the scale just reports the amount of force that you're pressing it with, so long as it's trapped between you and a hard place.

Answer 3

W = mg + ma

   = m(g+a)

substitute:

m = 75kg

g  = 10N/kg

a  = 2m/s^2

into the equation

W = 75(10+2)

   = 900

THE ABOVE EQUATION CAN BE APPLIED TO BOTH UPWARD AND DOWNWARD ACCELERATION.

HOWEVER, SI UNIT MUST BE USED FOR EACH SET OF DATA.

REMEMBER  ^^

Answer 4

Shouldn't you know that?

Answer 5

look....

             the force of the boy downwards is his weight that is

     w=mg

     w=75*10

     w=750N

  

now the as the lift applies an upward force on boy the boy will react according to newton 3rd law

force of lift is   f=ma

                     f=75*(-2)        taking downwards direction as positive upwards acceleration is negative:  so

                       f=-150N



this means the upwards force on boy is 150 N   so the downwards force will be equal and opposite  that is +150

as the boy has a weight of  750N  and 150N reaction the total force downwards is  750+150=900

howsthat??????so simple

Answer 6

I saw that in the Mastering Physics Online workbook. I dont have the answer off the top of my head but the link I am attaching has all of the solutions and explains them as well.

Mastering Physics Solutions

http://gtwebapps.googlepages.com/masteri...

Answer 7

WELL,easy!!!!!!!!!!!!!!!!!!!!!!!!!!

use  T-mg=ma

T-(75 x 10)=(75 x 2)

             T=150 + 750

             T =900 N  

BUT i see a lot of complicated formula,but actually correct la....