To find the missing variable, which is the force (F), we can use Newton's second law of motion:

F = m * a

where
F = force (in newtons)
m = mass (in kilograms)
a = acceleration (in meters per second squared)

Given:
m = 0.5 kg
t = 0.2 s
v = 10 m/s

Since acceleration (a) is the rate of change of velocity, we can find it using the formula:

a = (v - u) / t

where
v = final velocity
u = initial velocity
t = time

Given v = 10 m/s and u = 0 m/s (assuming the initial velocity is 0 since it is not specified), we can calculate the acceleration:

a = (10 - 0) / 0.2
a = 50 m/s^2

Now we can calculate the force:

F = 0.5 kg * 50 m/s^2
F = 25 N

Therefore, the force (F) acting on the object is 25 Newtons.