To avoid a collision what is your reaction time?

Click For Summary
SUMMARY

The discussion focuses on calculating the reaction time required to avoid a collision while driving at 75 km/h (20.8333 m/s) when a stalled vehicle is 48.0 meters ahead. The correct approach involves using the kinematic equation Vf^2 = Vi^2 + 2ad to determine the final velocity (Vf) and subsequently applying Vf = Vi + at to find the reaction time. The initial calculations led to an incorrect negative time of -1.89 seconds, which was rectified to the correct reaction time of 0.13 seconds by properly applying the acceleration as negative to indicate deceleration.

PREREQUISITES
  • Understanding of kinematic equations, specifically Vf^2 = Vi^2 + 2ad and Vf = Vi + at.
  • Basic knowledge of physics concepts such as velocity, acceleration, and deceleration.
  • Ability to convert units, particularly from kilometers per hour to meters per second.
  • Familiarity with the concept of reaction time in driving scenarios.
NEXT STEPS
  • Study the application of kinematic equations in real-world scenarios, particularly in collision avoidance.
  • Learn about the effects of different deceleration rates on stopping distances in driving.
  • Research the impact of reaction time on road safety and accident prevention.
  • Explore advanced topics in physics related to motion, such as friction and its effect on braking distance.
USEFUL FOR

Drivers, traffic safety educators, physics students, and anyone interested in understanding the dynamics of vehicle stopping distances and reaction times in emergency situations.

KaseyK
Messages
8
Reaction score
0

Homework Statement


As you are driving with a velocity of 75 km/h [N], you suddenly realize that there is a stalled vehicle in your lane 48.0m directly ahead. You react, applying the brakes to provide an acceleration of 4.80 m/s^2 . If you managed to avoid a collision what was your reaction time.


Homework Equations



What was your reaction time?

The Attempt at a Solution



First I wanted to find Vfinal so I used the kinematic equation Vf^2 = Vi^2 +2ad

I used:
Vi = 20.8333 m/s
d = 48.0 m
a = 4.80 m/s^2

This gave me Vf^2 = 29.9136 m/s

Then to find the time I used Vf = Vi + at

I got -1.89 s. The answer should be 0.13 s.

What did I do wrong?
 
Physics news on Phys.org
You applied the acceleration wrong.
You were looking to avoid the collision, did you accelerate towards the car, or did you want to stop (Vf=0) so as to avoid the collision?
 
I wanted to avoid the car so the acceleration should have been negative?
 
You want to know what distance the car travels before reaching a complete stop (where your final velocity is zero).

Consider the case in which your reaction time is instantaneous, and you hit the brakes when you are 48.0m away from the car. How close are you to the car before you stop? This distance is the same as the distance you could have waited before hitting the brakes.
 

Similar threads

Replies
20
Views
2K
  • · Replies 7 ·
Replies
7
Views
1K
  • · Replies 6 ·
Replies
6
Views
3K
  • · Replies 5 ·
Replies
5
Views
3K
  • · Replies 11 ·
Replies
11
Views
1K
Replies
15
Views
2K
Replies
2
Views
8K
  • · Replies 7 ·
Replies
7
Views
3K
  • · Replies 2 ·
Replies
2
Views
4K
Replies
3
Views
4K