A car is travelling at a speed of 28 m/s at t = 0 s. Suddenly, the driver sees a child
crossing the road at 100m and decides to do an emergency stop. The driver’s
reaction time is 0.8 s. After the brakes are applied, the car comes to rest 5s later.
Use a graph to plot a speed – time graph for the above situation.
Paste the graph here
Answers
Given info : initial speed of car , u = 28 m/s.
the drivers sees a child crossing the road at 100m and decides to do an emergency stop. the driver's reaction time is 0.8 s. After the brakes are applied, the car comes to rest 5s later.
To plot : the graph between speed and time here.
solution : time of reaction is 0.8s , it means drivers take the time 0.8s to apply break. so car's speed remains same during 0.8s
case 1 : 0 ≤ t ≤ 0.8s
graph between speed and time will be a straight line parallel to time axis. [ means there is no change in speed in this duration. ]
case 2 : after applying brakes, car regarding its motion and after 5s , it comes to rest.
i.e., final speed , v = 0
using formula, v = u + at
⇒0 = 28 + a × 5
⇒a = -5.6 m/s²
after 0.8s , graph between speed and time will be straight line but its slope will be -5.6 m/s² [ as you know slope of velocity and time gives acceleration]
now you see plotted graph in attached figure.
