Answer to Question #236322 in Physics for eshiii

Question #236322

2. A car starts from rest and accelerates 6 m/s2 for 5 s, after which it travels with a constant velocity for 9 s. The brakes are then applied so that it decelerates at 4 m/s2 . Find the total distance traveled by car.


1
Expert's answer
2021-09-14T11:02:36-0400

Let's divide total distance dd into three parts:

1. d1d_1 with accelration a1=6m/s2a_1 = 6m/s^2 travelled during t1=5st_1 = 5s

2. d2d_2 with constant speed v2v_2 travelled during t2=9st_2 = 9s

3. d3d_3 with accelration a3=4m/s2a_3 = -4m/s^2.


The distance d1d_1 is given by the following kinematic equation


d1=a1t122d_1 = \dfrac{a_1t_1^2}{2}

The speed of the car at the end of this distance (wich will be the constant speed for the second part) can be found from the definition of acceleration:


v2=a1t1v_2 = a_1t_1

The distance d2d_2 travelled with the constant speed is given as follows:


d2=v2t2=a1t1t2d_2 = v_2t_2 = a_1t_1t_2

The distance d3d_3 can be found from another cinematic equation:


d3=02v222a3=v222a3=(a1t1t2)22a3d_3 = \dfrac{0^2-v_2^2}{2a_3} = -\dfrac{v_2^2}{2a_3} = -\dfrac{(a_1t_1t_2)^2}{2a_3}

where represents the final speed of the car at the end of d3d_3 (car stops).

Thus, the total distance is:


d=d1+d2+d3d=a1t122+v2t2(a1t1t2)22a3d=6522+659(659)2249458md = d_1 + d_2 + d_3\\ d = \dfrac{a_1t_1^2}{2} + v_2t_2 - -\dfrac{(a_1t_1t_2)^2}{2a_3}\\ d = \dfrac{6\cdot 5^2}{2} + 6\cdot 5\cdot 9 - \dfrac{(6\cdot 5\cdot 9) ^2}{-2\cdot 4} \approx 9458m

Answer. 9458m.


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