Question

two cars are headed straight for each other, each at a constant speed of 40mph.

A. do the two cars have the same speed?
b. do the two cars have the same velocity?
c. what is the acceleration of each car while moving at a constant speed

Answer 1

They have the same speed since speed is a scalar quantity.
They do not have the same velocity since velocity is a vector quantity.
They are moving at a constant speed. What do you thing the acceleration is?

Answer 2

You can read speed on your speedometer. It is a scalar.
For velocity you need both the speedometer AND a compass. It is a vector.

If you move at constant velocity, no acceleration.
Acceleration = rate of change of velocity.