physics please!
posted by jane .
A 0.21 kg block oscillates back and forth along a straight line on a frictionless horizontal surface. Its displacement from the origin is given by
x = (16 cm)cos[(16 rad/s)t + π/2 rad]
What force, applied to the block by the spring, results in the given oscillation?
why can you not just do F=ma?

An oscillating force is applied by the spring. You can use F = ma
The force required to make this motion happen is
F = ma = m * d^2x/dt^2
= 16^2 * (0.16m) m cos [(16 rad/s)t + π/2 rad]
= 256 m x Newtons
(If x is in meters)