# Simple Harmonic Motion Physics Question

• Mar 10th 2014, 03:06 PM
sakonpure6
Simple Harmonic Motion Physics Question
Quote:

A 50.0 g object connected to a spring with a force constant of 35.0 N/m oscillates on a horizontal, friction less surface with amplitude of 4.00 cm. Find the total energy of the system, the speed when x=1.0cm, and calculate the kinetic and potential energies when x =3.0cm
My question is that when solving for the kinetic and potential energies at x=3.0cm, why when finding the kinetic energy why do we use
Ek + Ee = Ee' + Ek' (which is the correct method and gives an answer of 12.2 mJ) and not Ek + Ee + Eg= Ee' + Ek​' + Eg', why can we exclude the gravitational energy? (I

and for the potential energy, how do we go about solving for it. I tried using Ek + Ee + Eg= Ee' + Ek​' + Eg' and Ee +Eg= Ee' + Eg' to solve but in either case I got the wrong answer (The correct answer is 15.8 mJ)
• Mar 10th 2014, 03:24 PM
romsek
Re: Simple Harmonic Motion Physics Question
the system is horizontal and frictionless. The only effect gravity would have is to produce a frictional force if the surface were not frictionless.

Is there a downward force on the surface due to gravity and the mass of the object? Yes.

Does it have any effect on the mechanics of the system. No.
• Mar 10th 2014, 03:28 PM
sakonpure6
Re: Simple Harmonic Motion Physics Question
Then how is an answer of 15.58 mJ is derrived, would Eg be 0 then?