Scale/elevator question

Tags:
1. Apr 6, 2015

Prashant91

1. The problem statement, all variables and given/known data

A person stands on a scale in a elevator at rest. The scale reads 900N.
1) what is the persons mass
2) the elevator accelerates up at 2.5m/s^2. What does the scale read now ?
3)The elevator then continues to move upwards with a steady speed of 4m/s for 5 seconds. What does the scale read during this period
4)the elevator now decelerates at 1.8m/s^2 until it reaches zero velocity. What does the scale read during this period

2. Relevant equations : F=ma and W=mg

3. The attempt at a solution
1) rearrange w=mg for m = 900/9.81 = 91.7kg

2) a= 9.81+2.5 = 12.31 so f=ma .... f=91.7*12.31 = 1128.8N

3) I AM COMPLETELY STUCK ON THIS PART ? -if it is moving upwards at 4m/s for 5 seconds. Then there would be no acceleration as a=0
so the scale would read the same 1128 N
would i need to show any working out instead of w=mg = 91.7* 9.8 ?

4) COMPLETELY STUCK - I UNDERSTAND A= -1.8M/S^2 as it is decelerating

Last edited: Apr 6, 2015
2. Apr 6, 2015

Prashant91

Question 3) if it is moving upwards at 4m/s for 5 seconds. Then there would be no acceleration as a=0

so the scale would read the same 1128 N

would i need to show any working out instead of w=mg = 91.7* 9.8

Last edited: Apr 6, 2015
3. Apr 6, 2015

AlephNumbers

Do you draw a free body diagram and sum your forces? That would make it considerably simpler. Your solutions for 1 and 2 are indeed correct. However, I cannot think of a physics teacher that would not take off points on a test for the lack of free body diagram and summing of forces.

Last edited: Apr 6, 2015
4. Apr 6, 2015

Prashant91

how would i draw a free body diagram and sum forces ? is there any tutorial or website i could reference off ?

5. Apr 6, 2015

AlephNumbers

http://www.physicsclassroom.com/Class/newtlaws/U2l2d.cfm might help. And here is a decent video . Make sure you understand Newton's three laws. The mathematics are resultant from his postulations about motion. I would recommend taking an introductory physics course from a certified teacher, honestly. There are some free textbooks here on PF, but most of them seem to deal with more advanced topics. It's worth a look though.

6. Apr 6, 2015

AlephNumbers

Really, that video is hardly a substitute for a good lecture.

7. Apr 6, 2015

Prashant91

thanks for your help. i am understanding the concepts slowly. Where do i search on this website for textbooks. Would you know of any textbook that explains these concepts but are not to technical to understand.