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
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.
