Physics
posted by Bella .
A force sensor is used to measure a force applied to a 7.5 kg box on a hard, horizontal surface. It records that the box starts to move when the force is equal to 16.5 N. This force makes the box start to move with an acceleration of 0.5 m/s2. Find s and k respectively.
I keep getting .0510 or .0510 but it's wrong.
Ff=M*N Ff=Fnet=m*a N=m*g these are formula's i've used..

The force that makes it start moving and accelerating satisfies these two equations:
F  M g k = M a
F = M g s
Therefore
M g (sk) = M a
a/g = 0.051 = s  k
You can also use the second equation to write
16.5 = M g s = 7.5*9.8 s = 73.5 s
s = 16.5/73.5 = 0.224
k = s  .051 = 0.173
Those are the static and kinetic coefficients of friction, respectively.