# Magnitude of Acceleration

delfam

## Homework Statement

electron starting from rest and moving with constant acceleration travels 4.7cm in 8.2ms. What is the magnitude of this acceleration? Answer in km/s^2

## Homework Equations

X=Xi + (Vi)t + 1/2at^2

## The Attempt at a Solution

Vi = 0 so the equation becomes X = Xi + 1/2at^2
t = 8.2ms and X = 4.7 so 4.7 = Xi + 1/2a(8.2)^2

I feel like I did everything right up to this point but all I need is acceleration to plug in but I'm not sure how to get that.

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## Answers and Replies

Homework Helper
Gold Member
You do not need to plug in acceleration. You are trying to solve for a.

Thus, what you need to plug in is xi, the starting position of the particle.

delfam
4.7cm = .000007km and 8.2ms = .0082 seconds

electron is from rest so Vi=0 so the equation is 2(X-Xi)/t^2, I have time and I have X which equals .000047km. How to I find Xi, or is that also zero?

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housemartin
Hello!
problem statement says that: "starting from rest", which means that initial velocity is zero. For initial position Xi you can chose whichever value you like. What is given - the distance moved, that is X-Xi. You can chose X = 4.7 cm only if your Xi = 0.

delfam
thanks guys, yeah I got it, it was 1.398 km/s^2

housemartin