Question

Calculate the average acceleration needed by a train travelling initially at 12m/s[E] to stop in 120s.

Answer 1

By definition, the acceleration is given as follows:

$$a = \dfrac{v_f-v_i}{t}$$

where $v_f = 0m/s$ is the final speed, $v_i = 12m/s$ is the initial speed, and $t = 120s$ is the time.

Thus, obtain:

$$a = \dfrac{0m/s-12m/s}{120s} = -0.1m/s^2$$

If [E] is positive direction, then $a$ is directed [W] because of "-" sign.

Answer. $a = 0.1m/s^2[W]$.