- #1

James2911

- 13

- 0

## Homework Statement

https://imgur.com/a/bvcxjNy

## Homework Equations

Since the elevator is accelerating upwards,

N - mg = ma

or N = m(g+a) ------------ (1)

## The Attempt at a Solution

In order to get the accelerations at that instant, I drew a tangent at that point and used v2 - v1/t2 - t1 = a

and got;

for (i) t = 1s

v2 - v1/t2 - t1 = a;

20-10/2-1 = a

10 m/s^2 = a

Applying this to equation (1)

N = m(10+10) = 20m

(ii) Following the same steps as in (i)

20-20/8-2 = 0 m/s^2

N = m(10+0) = 10m

(iii) 10-20/12-10 = -10/5

a = -5m/s^2

N = m(10-5) = 5m

**The answer matches the solution but I have a doubt about the process of arriving at instantaneous acceleration.**

The tangent gives the instantaneous acceleration and we can use any points on the tangent to calculate to the acceleration.So for eg,

In (ii) above I used; v2 = 20, v1 = 20, t2 = 8 and t1 = 2 secs

But if instead of those I took v2 = 20, v1 = 0 t2 = 8 and t1 = 0 why does my answer comes out wrong?