Physics Help Forum Issues with Kinetic Energy Formulae

 Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum

 Aug 2nd 2017, 10:05 AM #1 Junior Member   Join Date: Aug 2017 Posts: 1 Issues with Kinetic Energy Formulae we know that energy=work done=F*s f=ma therefore, e=ma*s if initial velocity is 0,acceleration is final velocity divided by the time taken therefore,e=m*v/t*s velocity is distance pet unit time therefore,e=m*(s*s/t*t) e=m*(s^2/t^2) e=m*v^2 how is this possible when we know that kinetic energy of a body is 1/2mv^2 ??
Aug 2nd 2017, 10:19 AM   #2
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Join Date: Jun 2016
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 velocity is distance per unit time
yes but,
you are applying the final velocity to the entire time period
you should be using the average velocity!
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 Aug 2nd 2017, 10:37 AM #3 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,287 Just to be clear, you claim that s/t = v, but that's true only of velocity is constant. It's not in this case - and as Woody says you should use the average velocity, so s/t = v/2. You can see thus from the basic equation of motion - for initial velocity = 0: s = (1/2) a t^2, or s = (1/2) (v/t) t^2 = (1/2) (vt). Hence s/t = v/2.
Aug 2nd 2017, 12:14 PM   #4
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 Originally Posted by RishabhChhajer we know that energy=work done=F*s f=ma therefore, e=ma*s if initial velocity is 0,acceleration is final velocity divided by the time taken therefore,e=m*v/t*s velocity is distance pet unit time therefore,e=m*(s*s/t*t) e=m*(s^2/t^2) e=m*v^2 how is this possible when we know that kinetic energy of a body is 1/2mv^2 ??
The work done by a force exerted by, say, your hand, doesn't always change the speed and thus the kinetic energy. For example, let's say that you're moving a book across a table by pushing it from point A to point B where the speed is constant and in a straight line, i.e. force is in direction of motion. Then the work done by you is Force x distance. The energy that you expended goes into thermal energy of the block and the table. There is no kinetic energy change because that involves the total force and in this example we ignored the force of friction because we were only interested in the energy that you expended so we could determine how much of that energy went into thermal motion.

I just noticed that I never created a webpage to describe this. I've got my work cut out for me I see?

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