Question

The net force on a car is zero in both the horizontal and vertical directions. Which two situations could be true about the motion of the car?

The car is speeding up onto the highway.
The car is parked.
The car is moving at a fixed speed and direction.
The car is braking (slowing down).
The car is being struck by another car.

Answer 1

No. `2` and `3`

Explanation:

Net force along vertical direction remains zero as because,normal reaction provided by the road,balances the weight of the car.

Now,when horizontally net force is zero,that means,either it is at rest (option 2) or moving with contant velocity ,( option no.3)

Simply you can prove with Newton's `2` nd law of motion,

for fixed mass, `F=ma=(m(v-u))/t`

If,`a=0`, then either `u=v=0` or `u=v`

That is the car is at rest or moving with initial velocity `u`

Well the term fixed direction is mentioned because,if it changes direction with constant speed,velocity being a vector,just because of change in dirction,it will have acceleration,so net force won't be zero,you can refer to circular motion of constant speed.