## NotTim 3 years ago Determine the magnitude of the apparent weight of a 56 kg student standing in an elevator when the elevator is experiencing an acceleration of (a) 3.2 m/s^2 downward and (b) 3.2 m/s^2 upward.

1. bahrom7893

W = m*a When it's going up, acceleration is: g+a, when it's going down acceleration is g-a

2. bahrom7893

a) total a = -9.8-3.2 = -13 W = 56*(-13)N

3. NotTim

You see, here's what I did for a and b.

4. bahrom7893

b) total a = -9.8 + 3.2 = -6.6 W = 56*(-6.6)N

5. NotTim

a) f=ma =(56)(3.2+9.81) =728.56N

6. NotTim

Why is the force negative in both cases?

7. bahrom7893

because weight is pushing down.

8. NotTim

Ok. Give me a second to redo this...

9. bahrom7893

same thing, just change the signs

10. bahrom7893

you found the normal force, which has the same magnitude, but opposite direction

11. NotTim

And its alright if my signs are negative? Because the answers are A) 3.7*10^2N and b) 7.3*10^2N

12. bahrom7893

ohh yea they asked for magnitude. If they just asked for weight, ur answers wld be negative

13. bahrom7893

since weight has magnitude and direction. In this case it's going down, hence the negative.

14. NotTim

k. ill work with this. if i need help, ill call out

15. NotTim

Hey @bahrom7893 ?

16. bahrom7893

hey

17. NotTim

the answer for A) was wrong. you probably explained this, but i forgot

18. NotTim

We got -728.56N. The book says 3.7*10^2N

19. bahrom7893

20. NotTim

No.

21. bahrom7893

A) 3.7*10^2N and b) 7.3*10^2N Aren't those the answers?

22. NotTim

yup

23. bahrom7893

oh yea.. obv. Hang on I know what we did wrong

24. NotTim

for A), i did this: "56)(-3.2-9.81)

25. bahrom7893

well it makes sense.. i just always confuse the signs of g. g has to be positive in this case.

26. NotTim

would that make my value of 3.2N positive too?

27. bahrom7893
28. bahrom7893

it's the sign of g, i always get confused with it..

29. NotTim

am I to consider 3.2N a positive or negative force?

30. bahrom7893

well is it pushing down or up?

31. NotTim

It's pushing down

32. NotTim

If it was -ve, it would make sense mathematically, but not in terms of real world physics(for me)

33. bahrom7893

then it's negative, but they're just asking for magnitude, or abs value of that force.

34. bahrom7893
35. NotTim

ok. give me a moment to readi guess