4.18. a)
We want to find the resulting force from the three vectors,
, 
, and 
. We find that
Therefore, we can find that the resultant force 
has
magnitude
at and angle
given
by 
below the negative x
axis.
Since m is a scalar and we know from Newton that 
, then 
must also be at the same angle of 
below the negative x axis.
b)
Now, to find the mass of the object we manipulate Newton's second law to obtain
and since and are in the same direction, we
find that
c)
To find the object's speed after 10 seconds, break the acceleration into x and y components.
d)
We found the components of the velocity after 10 s in our derivation of part c.
