- #1

docbrown

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## Homework Statement

I am about to start AS Level Physics and have been given some questions/problems to solve as introductory homework. However, I am stuck on one and cannot seem to get the answer specified as the correct one.

An electron of mass 9.11x10

^{-31}kg is accelerated by a positive charge such that the electron experiences a constant force of 8.5x10

^{-29}N. The electron, which was already moving, then travels 45mm in 35microseconds. By first determining the acceleration of the electron, calculate its initial velocity.

## Homework Equations

F = ma

s = ut + 1/2at

^{2}

F = Force (N)

m = mass (kg)

a = acceleration (ms

^{-2})

s = distance travelled (m)

u = initial velocity (ms

^{-1})

t = time (s)

## The Attempt at a Solution

*Converting Units*

45mm = 0.045m

35microseconds = 0.000035s

45mm = 0.045m

35microseconds = 0.000035s

__Working Out Acceleration__

F =ma

8.5x10

^{-29}= 9.11x10

^{-31}x a

8.5x10

^{-29}/ 9.11x10

^{-31}= a

a = 93 + 277/911

Check: 9.11x10

^{-31}x (93 + 277/911) = 8.5x10

^{-29}✓

__Working Out Initial Velocity__

Rearranging the Equation:

s = ut + 1/2at

^{2}

ut = s - 1/2at

^{2}

u = (s-1/2at

^{2}) / t

__Putting in the Values__

u = (0.045 - 1/2(93+277/911) x 0.000035

^{2}) / 0.000035

u = (0.045 - (1.142974755x10

^{-7}x 1/2)) / 0.000035

u = (0.045 - 5.714873765x10

^{-8}) / 0.000035

u = 0.04499994285 / 0.000035

u = 1285.712653

**According to the booklet, the answer should be 4290ms**

^{-1}and not the value I got.I have been working at this for hours and cannot seem to find where I am going wrong. Help me out?