How Do You Solve These Challenging Physics B Problems?

[8parks11]

physics B problems :( hard hard

A single cable supports an 837-kg elevator car. What is the tension in the cable when the car is moving with a) constant speed, b) an upward acceleration of 3.21 m/s^2 and c) a downward acceleration of 3.21m/s^2

I'm not totally lost. Since the T is directed downward, I can derive from

F=ma
T+mg=ma
T=ma+mg (since g is negative)

so I thought T would be for
a)873(1)+837(9.8)
b) 873(3.21)+(873)(9.8)
c) 873(-3.21)+873(9.8)


is this correct?? if not someone please help me!





next question: A truck loaded with heavy cartons is forced to stop suddenly with a deceleration of 5.0m/s^2. Calculate the minimum coefficient of frcition between the cartons and the truck bed given that the cartons do not slide.


I thought it would just be 5. is this correct?





and last but not least


http://img216.imageshack.us/img216/5...caleid1.th.jpg
(ignore the mass of the scale btw)

what will the spring scale S in the middle read when
a) m1=1kg m2=1kg
b) m1=1.2kg m2=1kg

I thought it would be 1 for a) but that's not the answer.
b), I thought it would be 1.1

i did 1/2(m1+m2) but I don't think this is right
someone please confirm it!
thanks





thanks

 

[BobG]

A single cable supports an 837-kg elevator car. What is the tension in the cable when the car is moving with a) constant speed, b) an upward acceleration of 3.21 m/s^2 and c) a downward acceleration of 3.21m/s^2

I'm not totally lost. Since the T is directed downward, I can derive from

F=ma
T+mg=ma
T=ma+mg (since g is negative)

so I thought T would be for
a)873(1)+837(9.8)
b) 873(3.21)+(873)(9.8)
c) 873(-3.21)+873(9.8)


is this correct?? if not someone please help me!

a) If you're moving at a constant speed, then acceleration is 0, not 1.
b) and c) Correct.

 

[8parks11]

so a) is just mg and b) is also correct?

 

[8parks11]

answer

what will the spring scale S in the middle read when
a) m1=1kg m2=1kg

ok for this, the answer is 1.. kg? anyways its 1 something..


b) m1=1.2kg m2=1kg


ok how do I do this one?
I have no idea how

 

[8parks11]

ccan someone confirm that
b) 1.1?