# physics

• Aug 21st 2007, 07:45 PM
alderon
physics
consider a elevator weighing 3200 lbs. rising with a=4.0 ft/s^2
what is the tension in the supporting cable?
• Aug 21st 2007, 08:58 PM
CaptainBlack
Quote:

Originally Posted by alderon
consider a elevator weighing 3200 lbs. rising with a=4.0 ft/s^2
what is the tension in the supporting cable?

The net force on the elevator is 3200x4 Newtons. This comprises the sum of
a downward force due to gravity of 3200xg, and the upward force equal to
the tension in the cables, so:

3200x4 = T-3200xg,

so:

T = 3200(4 + g).

Opps.. lbs not kg, so the elevator mass is ~3200*.4536 ~=1451.5 kg

and ft/s not m/s, so ~4 ft/s 4*0.3048 ~=1.22 m/s

so we have:

T= 1451.5(1.22+g)~=16010 Newtons.

I would recomend to your instructor that they stop using customary units immeadiatly
the confusion in the system between the units of mass/weight at the very risky and
confusing.

RonL
• Aug 21st 2007, 10:18 PM
ticbol
Quote:

Originally Posted by alderon
consider a elevator weighing 3200 lbs. rising with a=4.0 ft/s^2
what is the tension in the supporting cable?

If the elevator is going up, the tensile force on the cable must be greater than the weight of the elevator. Then this upward net force is causing the elevator to accelerate upwards. This upward net force is equal to the (mass of the elevator)*(acceleration). Or, the old reliable F = ma.

Weight of elevator = 3200 lbs.
So mass of elevator = 3200/32 = 100 slugs -----if g = 32 ft/sec/sec

F = ma
T -3200 = 100(4)
T = 400 +3200