# Lesson Worksheet: Reaction Time Physics

In this worksheet, we will practice describing the effects of a person’s reaction time on the motion of objects that they interact with.

Q1:

The thinking distance and braking distance for a car at different initial speeds are shown by the lengths of the two-colored bars in the diagram. The longer a bar, the greater the initial speed the car stops from.

Which of the following quantities is shown by the length of the gray part of the bar?

1. Stopping distance
2. Thinking distance
3. Braking distance • Ac
• Bb
• Ca

Q2:

The thinking distance and braking distance for a car at different initial speeds are shown in the graph. The thinking distance is in blue and the braking distance is in orange. What is the lowest speed, to the nearest kilometer per hour, where the braking distance is greater than the thinking distance?

What is the stopping distance for an initial speed of 50 km/h?

What is the stopping distance for an initial speed of 80 km/h?

How much greater is the braking distance than the thinking distance when the initial speed is 90 km/h?

How much lower is the braking distance than the thinking distance when the initial speed is 40 km/h?

Q3:

The velocity–time graph shows the change in the velocity of a car that suddenly brakes to come to a stop on a dry concrete surface. Which of the other graphs shown, (a), (b), (c), (d), and (e), best matches the velocity–time graph for the same car stopping, driven by the same driver, but where the car travels on a wet concrete surface? • A(c)
• B(b)
• C(e)
• D(a)
• E(d)

Q4:

A car drives along a road at 15 m/s toward a bridge, as shown in the diagram. When the front wheels of the car are 50 m from the bridge, the driver sees a sign warning that the bridge has a collapsed section. The car can decelerate at 5 m/s2. What is the maximum reaction time that the driver can have and still stop the car before it reaches the bridge? Answer to one decimal place. Q5:

The driver of a car traveling at 30 m/s has a reaction time of 1.5 s. The car’s brakes decelerate the car at 3.75 m/s2 once they are activated. How much time does the car take to stop, including the driver’s thinking time?

Q6:

The driver of a car traveling at 20 m/s has a reaction time of 1.2 s. The car’s brakes decelerate the car at 4.5 m/s2 once they are activated. What is the car’s stopping distance to the nearest meter?

Q7:

A person measures their reaction time by catching a ruler that is dropped by their friend, as shown in the diagram. The smaller the length of the part of the ruler below the catch point, the shorter the reaction time. Which of the following graphs most correctly shows how the length of ruler that passes the catch point would change as the distance above the catch point that the ruler drops from is increased, assuming that the ruler length is always the same? • A(d)
• B(c)
• C(a)
• D(b)

Q8:

The following velocity-time graph shows the change in the velocity of a car that suddenly brakes to come to a stop. Which of the other graphs shown best matches the velocity-time graph for the same car stopping on the same road conditions, where the driver has a longer reaction time? • A(a)
• B(b)
• C(c)
• D(e)
• E(d)

Q9:

A person measures their reaction time by catching a ruler that is dropped by their friend, as shown in the diagram. The smaller the length of the part of the ruler below the catch point, the shorter the reaction time. Which of the following factors would not affect the length below the catch point? • AThe reaction time of the person dropping the ruler
• BThe stability of the ruler while falling
• CThe height of the base of the ruler above the catcher’s hand when it is released
• DThe smoothness of the material that the ruler is made of

Q10:

The graph shows how the stopping distances of two cars change depending on the speeds at which the cars are moving at the moment they start to decelerate. What is the difference in the stopping distances of the cars when they both stop at the speed of 15 m/s?

What is the difference in the stopping distances of the cars when they both stop at the speed of 20 m/s?