You're quite right, of course. The idea was to give a time-independent example of energy, but explaining the difference between mass and weight may not be clear to beginners, especially why acceleration has to be involved. So we should measure the block being pushed in terms of its weight, avoiding mass entirely. Beginners may not be happy with newtons, but my example may still be the simplest example. Steve ----- Original Message ----- From: "Eugene Salamin" <gene_salamin@yahoo.com> To: "math-fun" <math-fun@mailman.xmission.com> Sent: Tuesday, July 18, 2006 7:50 AM Subject: Re: [math-fun] Another physics question
--- Steve Gray <stevebg@adelphia.net> wrote:
Try this. Energy is the capacity to do work. Work is force*distance. Let the work not accelerate anything but overcome friction at low speed. Let a 10 kg block sit on a table and have a coefficient of friction of 0.1, so the constant force to move it slowly (without acceleration) is 1 kg. If the energy in question is 4 kg-meters (converted suitably to common units), the block can be pushed 4 meters. This avoids time, velocity, and specific heat, keeping the whole thing purely mechanical for intuitive appeal.
Steve Gray
There seems to be a bit of confusion here between mass (a quantity of matter) and weight (the force due to gravity). A mass m (located on Earth's surface where g = 9.8 m/s^2 is the acceleration of gravity) has weight mg. So the 10 kg mass weighs 98 N (Newton = SI unit of force), and the work done in your example is 39.2 N m.
Gene
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