Jump to content

questions


Recommended Posts

Suppose an object is moving along a straight line with position given by x= t^2- 8t where x is the directed distance in meters from some fixed point and t is the time in seconds.  At t= 4 seconds its speed is 0.  Is it "at rest"?  Is it "in equilibrium"?

Link to comment
Share on other sites

I am wondering if you are not confusing "equilibrium" with "stable equlibrium".

First imagine a ball rolling up a hill.  If it does not have enough kinetic energy to get to the top of the hill, it will slow to a stop, then roll back down the hill.  At the instant it stops, it is momentarily "at rest" but it is not "at equilibrium" because there is a net downward force.

Second imagine a ball sitting in a valley between two hills.  There is no net force because the downward force of gravity is offset by the upward force of the ground on the ball.  The ball is "at rest" and "at equilibrium".  If it were disturbed slightly, so that it moved a bit, it would roll back to the bottom.  That is a "stable equilibrium".

Finally imagine a ball sitting at the top of a smooth hill.  Again the only forces on the ball are gravity downward and the ground upward so there is no net force so the ball is "at rest" and "at equilibrium".  If it were disturbed slightly, so that it moved a bit, it would roll to the bottom of the hill.  That is an "unstable equilibrium".

Link to comment
Share on other sites

21 minutes ago, Country Boy said:

I am wondering if you are not confusing "equilibrium" with "stable equlibrium".

First imagine a ball rolling up a hill.  If it does not have enough kinetic energy to get to the top of the hill, it will slow to a stop, then roll back down the hill.  At the instant it stops, it is momentarily "at rest" but it is not "at equilibrium" because there is a net downward force.

Second imagine a ball sitting in a valley between two hills.  There is no net force because the downward force of gravity is offset by the upward force of the ground on the ball.  The ball is "at rest" and "at equilibrium".  If it were disturbed slightly, so that it moved a bit, it would roll back to the bottom.  That is a "stable equilibrium".

Finally imagine a ball sitting at the top of a smooth hill.  Again the only forces on the ball are gravity downward and the ground upward so there is no net force so the ball is "at rest" and "at equilibrium".  If it were disturbed slightly, so that it moved a bit, it would roll to the bottom of the hill.  That is an "unstable equilibrium".

I think you are kicking a dead donkey.

Though you are working very hard,

The OP hasn't been back since the day he joined and posted.

Link to comment
Share on other sites

5 hours ago, Phi for All said:

The donkey will kick you back, and that's what we call "stable" equilibrium.

I thought so it was Newton's 3rd law action-reaction.. but what I know..

Link to comment
Share on other sites

13 hours ago, Sensei said:

I thought so it was Newton's 3rd law action-reaction.. but what I know..

Newton kept his donkeys in the house because their knowledge of the law was unstable. 

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.