1. Sep 8, 2007

### myelevatorbeat

1. The problem statement, all variables and given/known data
A 2.6 kg mass hangs at one end of a rope that is attached to a support on a railroad car. When the car accelerates to the right, the rope makes an angle of 8.5° with the vertical, as shown in Figure P4.68. Find the acceleration of the car.

2. Relevant equations
(Fnet)x=max
(Fnet)y=may

3. The attempt at a solution

(Fnet)y=may
Tcos8.5-mg=0
T=(2.6 x 9.8)/cos 8.5
T=25.76

(Fnet)x=max
Tsin8.5=max
Tsin8.5/2.6=ax
ax=25.76 sin 8.5/2.6
ax=1.5

NOW HERE'S MY PROBLEM:

I am doing homework on webassign.net and it is very picky about the significant digits in your answer. So far I've put in: 1.5, 1.4, 1.46, and 1.47.

I get 6 tries, which is frusturating because I know I solved it correctly, I just don't know how many significant digits. I assumed I would have 2 significant digits and the answer would be 1.5 because all the given values have 2 significant digits. If someone could help me figure it out, I would appreciate it.

Also (reason for edit): When I put in 1.5 and 1.4 I got this response:
Your answer is within 10% of the correct value. This may be due to roundoff error, or you could have a mistake in your calculation. Carry out all intermediate results to at least four-digit accuracy to minimize roundoff error. m/s2

However, when I bring it out to 1.46 or 1.47 I don't get that message.

Last edited: Sep 8, 2007
2. Sep 8, 2007

### myelevatorbeat

Nevermind! I put in 1.465 and it worked.

How frusturating though.