Newton's Second Law vs. Energy Equations

[JJBladester]

Homework Statement

I'm taking a test next week on kinetics and energy of moving particles. Problems include things like sliding blocks, friction, braking automobiles, collars sliding on poles, springs, max/min heights/velocities, tensions, accelerations, etc...

We learned two ways of solving problems in the kinetics realm... Newton's second law and energy equations. When do I use one vs. the other?

Homework Equations

Newton's 2nd Law
F=ma

PE=Potential Energy, KE=Kinetic Energy, SE=Spring Energy, Work could be from friction, drag, etc.

Energy Eqn
PE₁+KE₁+SE₁+Work=PE₂+KE₂+SE₂

The Attempt at a Solution

My gut says that if the problem is asking for an acceleration or a normal force, F=ma needs to get involved. Otherwise, I may or may not need to use F=ma + the energy eqn.

We're using chapter 12/13 in Vector Mechanics for Engineers by Beer/Johnston 8th Ed.

 

[aim1732]

Energy equations are extremely helpful when dealing with problems involving velocities and displacements. They however do not have an explicit time factor.
Energy methods are more useful when dealing with position varying forces,as in a two body system. I remember I tried to solve a two body problem using the force method here on pf and ended up scratching my head for a real long time. Gravitation problems are also easier to do using energy methods.
However energy conservation has its limitations-Newton's Laws do not have that problem.Being the more basic they are applicable even when energy conservation is flouted.
I think you would like to check the thread "Force method for two body systems" to see my point.

 

[JJBladester]

Thanks aim1732... I couldn't find the thread you referenced. Perhaps you could post the link here?

The "do not have an explicit time factor" part makes sense. My tutor said something about not being able to use the energy equations with "non-conservative" forces, like the normal force. What, besides the normal force, air/water drag, and frictional forces are non-conservative in everyday life?

 

[aim1732]

Here's the link:
https://www.physicsforums.com/showthread.php?t=386683"
You would encounter non-conservative forces that you mentioned plus some lesser known ones as the battery force that is responsible for maintaining potential difference across the terminals of a battery,and non elastic material stress. However it is worth noting that energy equations(as opposed to conservation of mechanical energy) are still valid and can be applied meaningfully for even non-conservative forces.