Question

A 1.5-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?
(1) 0.40 N (3) 10. N
(2) 0.90 N (4) 15 N

Answer 1

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.