Video Transcript
The graph below shows the change in
the velocity of a vehicle over time. The driver notices an obstacle at
time zero. They then react by applying the
brakes. The vehicle then decelerates and
eventually comes to a complete stop. Initially, the car decelerates on a
dry concrete surface before traveling on a wet concrete surface. Which time labeled on the diagram
represents the moment the vehicle passes from traveling on the dry concrete surface
to traveling on the wet concrete surface?
In this question, we are presented
with a velocity–time graph and we want to identify the time that the vehicle passes
from a dry concrete surface to a wet concrete surface. Let’s consider each time labeled on
the graph in order.
We are told in the question that
the driver notices an obstacle at time zero. This corresponds with time A on the
velocity–time graph. So time A is not the answer we are
looking for.
Afterwards, we are told that the
driver reacts by applying the brakes. At the moment that the brakes are
applied, the velocity of the car will decrease. Looking at the velocity–time graph,
we can see that the velocity remains constant between times A and B. And then velocity begins to
decrease after time B. This means that time B is the
instant at which the driver applied the brakes. So time B is not the correct
answer.
We are told that the car initially
decelerates on a dry concrete surface before traveling on a wet concrete surface and
eventually coming to a complete stop. When the surface changes from a dry
concrete surface to a wet concrete surface, the surface will become more
slippery. This means there’s less friction
between the tires of the car and the surface of the road. This will increase the braking
distance and decrease the deceleration. We can see on the velocity–time
graph that after time B, the car decelerates. After time C, the car begins to
decelerate at a lower rate. So we can see that time C seems to
represent the moment when the vehicle passes from traveling on the dry concrete
surface to traveling on the wet concrete surface.
Let’s consider the final time to
make sure of this. If we read off the value of the
velocity at time D, we can see that the velocity of the vehicle is equal to zero at
this time. If the velocity is equal to zero,
then this means that the vehicle is not moving. Therefore, time D represents the
moment that the vehicle comes to a complete stop. So time D is not the correct
answer.
So the correct answer is time
C. Time C represents the moment the
vehicle passes from traveling on the dry concrete surface to traveling on the wet
concrete surface.
