physics
posted by sam .
A 6 kg box with initial speed 5 m/s slides across the floor and comes to a stop after 1.4 s.
a.What is the magnitude of the average net force on the box?
b.You put a 3 kg block in the box, so the total mass is now 9 kg, and you launch this heavier box with an initial speed of 5 m/s. How long does it take to stop?

physics 
Damon
a = change in v/ change in t = 5/1.4 = 3.57 m/s^2
so
F = m a = 6(3.57) =  21.4 Newtons
The friction force will be larger because proportional to the normal force (eight)
F = 21.4 (9/6) = 32.1 Newtons friction force
F = m a
a = 32.1 / 9 = 3.57 m/s^2
so same time, 1.4 seconds
(the mass and the retarding force go u in the same proportion) 
physics 
Bhf
DÃ¼sseldorf
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