Car Dynamics Problem

1. Car B is travelling a distance d ahead of car A. Both cars are travelling at 60ft/s when the driver of B suddenly applies the brakes, causing his car to decelerate at 12ft/s^2. It takes the driver of car A 0.75 to react. When he applies his brakes he decelerates at 15ft/s^2. Determine the minimum distance d between the cars so as to avoid a collision.

Diagram
2. ______CAR A_(<----d---->_CAR B__________

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cristo
Staff Emeritus

i dont know how to post it on here

cristo
Staff Emeritus
Well, what equations do you know? Do you have any that will relate to this problem? If so, post them, along with your ideas for attacking the problem.

To determine how long it will take you to stop assuming a constant rate of deceleration, you need only divide your starting velocity ft/s by your rate of deceleration.

60/12 = 5 for car B

60/15 = 4 + 0.75 seconds to react = 4.75 seconds to stop

Can anyone help me?

berkeman
Mentor
Moved to Intro Physics forum.

s4orce, the problem asks you for the minimum starting separation between the two cars to avoid a collision. What is the kinematic equation for distance versus time, in terms of initial distance, initial velocity and acceleration? What is the kinematic equation for the velocity as a function of initial velocity and acceleration. Set up these equations for the two cars assuming some initial separation distance, and then figure out what you need to calculate to answer the initial question.

berkeman
Mentor
I did help you. Set up the equations that I mentioned, and that will either give you the answer outright, or if you have some math problems with them, we will be able to help you better by seeing where you are stumbling. We do not do your work for you here on the PF.