Question Video: Identifying Regions on a Velocity–Time Graph | Nagwa Question Video: Identifying Regions on a Velocity–Time Graph | Nagwa

# Question Video: Identifying Regions on a Velocity–Time Graph Physics • First Year of Secondary School

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The graph shows the change in the velocity of a vehicle over time. The driver notices an obstacle at time 0. 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?

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### 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.

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