Solving Physics: Chris the Inventive - Acceleration & Force

[bahamut1111]

An inventive child named Chris wants to reach
an apple in a tree without climbing the tree.
Sitting in a chair connected to a rope that
passes over a frictionless pulley, Chris pulls
on the loose end of the rope with such a force
that the spring scale reads 272.5 N. Chris’s
true weight is 464 N, and the chair weighs
67 N.
The acceleration of gravity is g = 9.8 m/s2 .
A. Find the magnitude of the acceleration of
the system.
Answer in units of m/s2.

B.
Find the magnitude of the force Chris exerts
on the chair.
Answer in units of N.



i got the acelleration and it was marked correct (0.258380414312617702)

but when i look for the force which i used f=ma... it was marked wrong... i tried first in N than in kg and still nothing...

 

[rl.bhat]

Chair is going up with an acceleration a. Weight of Chris acts in down ward direction. So net reaction on the chair is F = m(a+g)

 

[bahamut1111]

ok would i use the mass of the chair only or include the kids too?