F = m • a
F = (15 kg) • (4 m/s^2)
F = 60 Newtons
NOTE:
That's NOT a "resultant force".
That's the force you have to EXERT on 15 kg to MAKE it accelerate at 4 m/s^2 .
Using Newton second law
[tex]\\ \sf \longmapsto Force=Mass\times Acceleration[/tex]
[tex]\\ \sf \longmapsto Force=15(4)[/tex]
[tex]\\ \sf \longmapsto Force=60N[/tex]
