64. We take rightwards as the +x direction. Thus, 

F

1

= 20ˆi in Newtons. In each case, we use Newton’s

second law 

F

1

+ 

F

2

= ma where m = 2.0 kg.

(a)If a = +10ˆi in SI units, then the equation above gives 

F

2

= 0.

(b)If a = +20ˆi m/s

2

, then that equation gives 

F

2

= 20ˆi N.

(c)If a = 0, then the equation gives 

F

2

=

−20ˆi N.

(d)If a =

−10ˆi m/s

2

, the equation gives 

F

2

=

−40ˆi N.

(e)If a =

−20ˆi m/s

2

, the equation gives 

F

2

=

−60ˆi N.