Will the car hit the barrier?

[adamlazz]

Hi there, I am a grade 11 student, learning physics.

Homework Statement

A driver of a car going 90km/h suddenly sees the lights of a barrier 40.0 m ahead. It takes the driver 0.75s before he applies the brakes and the average accelleration during braking is -10.0m/s2.
a) Determine whether or not the car hits tha barrier.
b) What is the maximim speed at which the car could be moving and not hit the barrier at 40.0 m ahead? Assume the accelleration does not change.

a) Treat toward the barrier as positive.
v2=0m/s delta(t)= 40
v1=25m/s
a=-10m/s2

I am not quite sure how to do b)

Homework Equations

v2^2 = v1^2 + 2a(delta(d)) <---- a)

The Attempt at a Solution

I have already determined that it would take the car 50 m to come to a complete stop, so it would hit the barrier.

I am having trouble with b)


thanks in advance.

 

[rmorelan]

I could be wrong but you just need to use the same formula again:

v2^2 = v1^2 + 2a(delta(d)) <---- a)

You know d, v2 is 0, and your given acceleration from before

There is only one unknown left, V1.

 

[hage567]

If you assume the car will stop just before it hits the barrier, you now have a new piece of information you can use in your equations to work out the new v1.