IIT JEE , AIEEE Forum - Physics , Mechanics

Conjurer

IN one HC Verma question of work energy,there is an inclined plane with a spring connected at its base along the incline.Then a block of mass m is set into motion on that inclined plane.Now when the block reaches the spring it compresses it by 20cm.What is spring constant,k, if m,

and initial seperation are given.

My question is that though I found k using energy equations , why coudnt we get k by using force equation that is ,when the block is momentarily at rest after compressing the spring by 20cm, kx = mgsin

-umgcos

and hence find k.But its coming totally wrong.Though kx is variable force but when the block is momentarily at rest shouldnt k(20cm) = mgsin

-umgcos

???

anchitsaini

u can't use force eqns cos still it has acceleration, that is the system is not at equilibrium , hence u can't balance forces.
it doesn't have velocity but it does have acceleration.!!!

Conjurer

How does it have acceleration? The forces are in opposite direction,shouldnt they tend to cancel out?

karthik2007

I don't have the question, but can vaguely make out what you are trying to say. You want to find the elongation (or something else) when the spring comes to rest right? See, if you use the force equation that you mentioned above, then it implies that the object has zero acceleration, but IT MAY MOVE WITH CONSTANT VELOCITY. Hence, it does not take care of the static equllibrium part. If we use energy equation, it implies that all the PE stored in the spring is transferred onto some other body in the system, and hence confirms static equillibrium.

Cheers!

anchitsaini

the forces being in opposite direction doesn't mean that they are equal and would cancel each other.

it has accn otherwise why would it come only momentarily to rest , otherwise it would come to rest forever.

Ask & Discuss Questions with Community & Experts