# Homework Help: Basic dynamics/acceleration question

1. Apr 3, 2014

### Mike_Stokes

1. The problem statement, all variables and given/known data

A 1000kg car is travelling at 27.0 m/s. Determine the force required to stop it in 70.0 m.

3. The attempt at a solution

I attempted to treat this question as if I knew how many seconds it took to stop, and did this:

70.0m / 27.0m/s = 2.59259s

2.59259s / 27.0m/s = 0.096

1000kg / 0.096 = 10414.29N

F = 10.4kN

But then I realized that this is all predicated on the idea that we are moving at 27.0 m/s throughout the whole 70.0m rather than accelerating. Now I have no idea what to do, and both of my textbooks are proving useless. Please help, and sorry if the question is messed up or hard to decipher, this is my first post.

2. Apr 3, 2014

### PhanthomJay

You should first understand and become familiar with the SUVAT equations of motion for constant acceleration which you can find in any text to solve for the acceleration, then correctly apply Newton's Laws.

3. Apr 4, 2014

### Simon Bridge

"suvat" equations are sometimes called "kinematic equations"
you can also work stuff out from the velocity-time diagrams.

- if you have not seen v-t diagrams, then you have probably had some work on average velocity.

Using average velocity to do this will make most people here cringe, but some courses do it that way at first.

4. Apr 4, 2014

### Mike_Stokes

Ahh okay, yeah we've done kinematic equations, but I was having a hard time remembering them, and we never referred to SUVAT at all as an acronym. Thanks so much for the help guys, I finally got the right formula for the question:

a = (Vf^2 - Vi^2)/2D

F = 5.26kN

Thanks for helping my (now seeming stupid) question!