The second real world example is speed . When the rate of speed of a car (acceleration) slows down , we say that it is negative. increasing , it is positive, but decreasing it is negative. so we might say it's acceleration is -25. that is difficult for some students to understand. because they think. hang on. if a cars acceleration reaches 0 , it's stopped. But what must be remembered is, we are not measuring speed, but rather, the rate of change, which can be negative.
