I think the question meant to say net force on the box. Since force is a vector, the direction matters. 20N left is negated completely by the 50N right, which means the net force is 50N-20N to the right, 30N.
I think you're saying that once you start pushing on the cars, you want to be able to stop each one in the same time.
This is sneaky. At first, I thought it must be both 'c' and 'd'. But it's not
kinetic energy, for reasons I'm not ambitious enough to go into.
(And besides, there's no great honor awarded around here for explaining
why any given choice is NOT the answer.)
The answer is momentum.
Momentum is (mass x speed). Change in momentum is (force x time).
No matter the weight (mass) or speed of the car, the one with the greater
momentum is always the one that will require the greater (force x time)
to stop it. If the time is the same for any car, then more momentum
will always require more force.
Answer: 
Explanation:
Given
Mass of the elevator is 
Time period of ascension 
cruising speed 
Distance moved by elevator during this time
Suppose Elevator starts from rest
Distance moved
Gain in Potential Energy is
Average power during this period is
Answer: 5.79 s
Explanation:
Vs=0 m/s starting speed(from rest)
Vf=325 km/h= 325*1000/3600= 90.28 m/s
a=15.6m/s²
Using equation for acceleration we can find out time :
a=(Vf-Vs)/t
t=(Vf-Vs)/a
t=(90.28 m/s-0m/s)/15.6 m/s²
t=5.79 s
Stephen A. Douglas was elected senator of Illinois in 1858. He bested Abraham Lincoln in the election for senate.
