Finding initial speed of sliding pen using energy conservation

[jbjohnybaker]

Homework Statement

A pen of mass 0.057 kg slides across a horizontal desk. In sliding 25 cm, its speed decreases to 5.7 cm/s. The force of kinetic friction exerted on the pen by the desk has a magnitude of 0.15N. Apply the law of conservation of energy to determine the initial speed of the pen.

Homework Equations

Eth = Fk deltaD
deltaEk = 1/2mv^2f - 1/2 mv^2i
Eth = delta Ek

The Attempt at a Solution

Eth = (.15)(0.25)
= 0.0375 J

DeltaEk = 1/2(0.057)(0.057) - 1/2(0.057)Vi^2
= 9.25965x10^-5 - 0.0285 Vi^2

Eth = DeltaEk
0.0375 J = 9.25965x10^-5 - 0.0285 Vi^2

Vi = sqrt ((0.0375 - 9.25965x10^9)/0.0285)
Vi = 1.1 m/s

I get the right answer but i am not sure if i used the right steps.. can you please check my steps.. thx

 

[Birkeland]

The notation is different than I am used to, but it looks like all the steps are correct, even the unit conversions. (Which is normally the part that people miss)

Good job!

 

[jbjohnybaker]

okay.. thank you