Velocity vs. Speed
- On Interstate 90 heading for NYC
- speed is constant 60 mph
- velocity is constant 60 mph in constant direction East
- since velocity is totally constant, the acceleration is zero
- therefore by N2 -- the net force on the car is zero
- On race car at Indianapolis
- speed is constant 200 mph
- velocity is this speed, but in an ever-changing direction
- the velocity is not constant, since it’s direction changes
- a force must act on the car (the force of friction between the tires and the track does it)
- Acceleration is the rate of change of velocity (direction and speed) per unit time.