# Finding the distance in an acceleration problem (different)

## Homework Statement

You are travelling at 60km/h when all of a sudden you see a pink elephant which is 0.125km down the road. Your car can slow down at a rate of -5.0m/s^2. If it takes you 20 m of distance to react and hit the brake, how far will you be from the elephant when you stop?

D = (V1+V2/2)t
A = V2-V1/T

## The Attempt at a Solution

To be honest, I really don't even know where to begin with this problem. I don't even know how to visualize this.

Okay so the break is hit when it's 0.105km away. (0.125 - 20).

Time = Velocity / Acceleration
= 60/-0.03 (5m = 0.005km/s = 0.03km/h)
= 200 hours?

Just from this point I know it's wrong. I can't continue further because I'm already starting off on the wrong foot.

I need some guidance in the RIGHT direction, please & thank you!

Also, the answer is supposed to be 77m.

ehild
Homework Helper
Take care of the units and dimensions. The acceleration is not 5 m, neither 0.005 km/s, and 0.005 km/s is not 0.03 km/h.

The best thing first when you start to solve a problem to convert all units to Si ones: m, s, kg, and so on.

ehild

You need to use the equations for motion with constant acceleration.

Time is the variable you do not know, so use an equation that does not include time: v2=v02+2a(x-x0)

The final velocity (the v on the left side of the equation) will be zero (because you will be stopped). You want to solve for (x-x0) and then try to work it out from there using the distance you travel before braking and the distance you start from the elephant. Also, convert the 60km/hr to meters/second.

I worked the problem and came up with 77m.