# Intro kinematics problem - is this ok?

Tyrannosaurus_

## Homework Statement

On a North-South highway, a Car drives 80 km/h [N]. A SUV, that is north of the Car, is moving 70 km/h [South]. The car is to the south of the SUV. The two are separated by 20 km. Where are the two cars when they pass each other.

## Homework Equations

Both are in uniform motion so V = d/t -> d = v*t

## The Attempt at a Solution

dc + dsuv = 20 km <--- this is my question... this part okay?
vc*t + vsuv*t = 20 km
t*(vc + vsuv) = 20 km
t*(160 km/h) = 20 km
t = 20 km / 160 km/h
t = 1/8 h

Next
dc = vc*t
dc = 90 km/h* (1/8) h
dc = 90/8 km [N]

So, the two meet at a place 90/8 km to the north of where the car started the problem from

Homework Helper
Gold Member
You originally stated that the car's speed was 80 km/h. Was that a mistake? Is it really supposed to be 90?

Tyrannosaurus_
Sorry, that's a typo when I subbed the speed for the car. It should be as stated, the 80 km/h.

Do you think it's okay that I set the sum of their displacements to 20 km?
Also, should I subtract their velocities (they're act in opposite directions) when common factoring the time, t?

Homework Helper
Gold Member
You add, not subtract. Think like this: each car is covering a distance. Both distances contribute to make the 20 km total. You could instead write it as 2 equations, with one car covering distance x, and the other car distance is (20 - x)

Staff Emeritus