A1.5-kilogram cart initially moves at 2.0 meters per second. it is brought to rest by a constant net force in 0.30 second. what is the magnitude of the net force?

Acceleration = (change in speed) / (time for the change)

Change in speed = (speed at the end) minus (speed at the beginning.

The cart's acceleration is

                               (0 - 2 m/s) / (0.3 sec)

                           = ( -2 / 0.3 ) (m/s²)  =  -(6 and 2/3) m/s² .

Newton's second law of motion says

                             Force = (mass) x (acceleration) .

For this cart:      Force = (1.5 kg) x ( - 6-2/3 m/s²)

                                       = ( - 1.5 x 20/3 ) (kg-m/s²)

                                       =      - 10 newtons .

The force is negative because it acts opposite to the direction 
in which the cart is moving, it causes a negative acceleration, 
and it eventually stops the cart.