Why Does the Scale Reading Change in an Elevator?

[joe215]

Homework Statement

A person with a weight of 500N stands in an elevator on a scale.
from 0-5s his weight reads 500N.
from 5-10s his weight reads 700N.
from 10-15s his weight reads 500N.
from 15-20s his weight reads 300N.

Find his acceleration (in each time interval) and his velocity at 5s, 10s, 15s, and 20s.

The Attempt at a Solution

I think my accelerations are correct:
(Fn-mg) /m=a

0-5s: (500N-500N) / 51kg= 0m/s^2
5-10: (700-500)/51=3.92
10-15: (500-500)/51=0
15-20: 300-500/51= -3.92

Now, I think my velocities seem a little unreasonable.
vf=vi+at

at 5: v=0
10: v=0m/s + (3.92)(10)=39.2m/s <- that is really fast!
15: v=39.2m/s + (0)(15)= 39.2m/s
20: 39.2m/s+(-3.92)(20)= -39.2m/s

thanks for any help

 

[Doc Al]

Now, I think my velocities seem a little unreasonable.
vf=vi+at

That time is the time in the interval for which the acceleration is "a", not the time from the beginning. I'd write it as: Vf = Vi + aΔt.

 

[baba89]

or suggestions

I would approach this problem by first checking if the given information is physically possible. In this case, it seems unlikely that a person's weight would suddenly increase by 200N and then decrease by 200N in a matter of seconds. This could be due to a malfunction in the scale or perhaps the person is performing some kind of activity that is affecting their weight.

Assuming that the given information is accurate, I would suggest looking into the forces acting on the person in the elevator. The only forces that should be present are the person's weight (acting downwards) and the normal force from the scale (acting upwards). The acceleration of the person can then be calculated using Newton's second law, which states that the net force acting on an object is equal to its mass multiplied by its acceleration.

In the first time interval (0-5s), the person's weight and the normal force are equal, so the acceleration is 0m/s^2. In the second time interval (5-10s), the person's weight is 700N and the normal force is 500N, resulting in a net force of 200N. Using Newton's second law, we can calculate the acceleration to be 3.92m/s^2.

For the remaining time intervals, we can use the same approach to calculate the acceleration. However, the velocities at each time interval will depend on the person's initial velocity, which is not given in the problem statement. Therefore, it is not possible to accurately determine the person's velocity at each time interval without additional information.

In conclusion, I would suggest further investigation into the forces acting on the person in the elevator and the accuracy of the given information before attempting to calculate the person's velocity at each time interval.