Why is it one sec?,

[AyoubEd]

can you help me with this, please?

 

[CWatters]

Firstly it doesn't say "one second" it says "per second" which is a rate.

Momentum = mass * velocity
so
Momentum per second would be equal to mass * velocity / time

Velocity/time = acceleration.

can you do the rest?

 

[NihalRi]

Force is the change in momentum per unit time.
F=ma (m=mass a=acceleration)
a= (vf-vi)/t
Replacing a into the force equation:

F=m (vf-vi)/t
Which is change in momentum per unit time.

 

[AyoubEd]

thanks a lot NihalRi @CWatters :