Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 10/12/2013 19:30:01

Title: What net force is acting on a body at rest?
Post by: thedoc on 10/12/2013 19:30:01
Nilesh asked the Naked Scientists:
   
If a block is resting on a table, what is the net force acting on it?

What do you think?
Title: Re: What net force is acting on a body at rest?
Post by: Pmb on 10/12/2013 19:51:26
Quote from: thedoc on 10/12/2013 19:30:01
Nilesh asked the Naked Scientists:
   
If a block is resting on a table, what is the net force acting on it?

What do you think?
The Newtonian answer is zero since the object is not accelerating. Same in GR. Howver in GR there is a non-zero 4-force acting on it. You've have to tell me the mass and the acceleration do to gravity. Then I can calculate the 4-force. I've never done it for such a simple example but would be a wonderfully simple exercise to do.
Title: Re: What net force is acting on a body at rest?
Post by: Bill S on 11/12/2013 18:44:19
Quote from: Pete
would be a wonderfully simple exercise to do.

Please Pete, keep the answer simple.  :)
Title: Re: What net force is acting on a body at rest?
Post by: evan_au on 12/12/2013 17:05:20
How about mass=1kg, and g=9.8 m/s2?
Title: Re: What net force is acting on a body at rest?
Post by: CliffordK on 12/12/2013 17:11:16
Quote from: evan_au on 12/12/2013 17:05:20
How about mass=1kg, and g=9.8 m/s2?

So, F=ma

Your block exerts a force of 1kg * 9.8 m/s2 = 9.8 Newtons downward on the table.
Your table exerts a force of 9.8 Newtons upward on the block.
And the net is zero.

Unless, of course, the table is sitting in quicksand, in which case the table may actually exert less than 9.8 newtons upward.
Title: Re: What net force is acting on a body at rest?
Post by: Pmb on 12/12/2013 17:17:41
Quote from: Bill S on 11/12/2013 18:44:19
Quote from: Pete
would be a wonderfully simple exercise to do.

Please Pete, keep the answer simple.  :)
I wish I could. :D

I've had so many people nit pick over so many answers that I've given in the past that I try to be thorough. I had a feeling that if I simply said zero then some wise guy would come along and say that if the net force on a body is zero only in free-fall otherwise there's a 4-force acting on it. That'd be the answer to give from a general relativity point of view.  The Newtonian answer would be zero. So it's not so easy.

Then again, those people aren't in this forum. You folks are a very special kind of science enthusiasts and that's why I respect the majority of you. :)