Physicists - what's wrong here?

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Joined
5 Apr 2008
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Hi guys,

What's wrong with this logic?

Force = mass x acceleration [f=ma]

Work Done / energy = Force x Distance [E=fd]

Therefore... E = m x (v/t) x vt .... (Where v/t is acceleration a and vt is distance d)

So .. Kinetic Energy E = mv^2



So what's happened to the 1/2 in KE=1/2 mv^2? If you were to derive the kinetic energy in a specific case using relationships like this or others like power=energy/time then how to you ensure that the 1/2 isn't lost?
 
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F=ma so f=e/s so ma=e/s e=mas thats algebra but i havent done anything like this, so by rearranging the equation you get this e=mas
 
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Your second vt won't actually be vt - it will be 1/2 vt as the average velocity rather than final velocity.


edit: so, you are saying E = fd = mad, but getting confused about how far your object has actually travelled.
the distance will be based on average velocity, which over a period of linear acceleration is 1/2 v, where v is the final velocity.

so, E = m * a * vt/2 = m * v/t * vt/2 = (mv^2)/2.

edit 2: Glad I could help :).
 
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Your second vt won't actually be vt - it will be 1/2 vt as the average velocity rather than final velocity.


edit: so, you are saying E = fd = mad, but getting confused about how far your object has actually travelled.
the distance will be based on average velocity, which over a period of linear acceleration is 1/2 v, where v is the final velocity.

so, E = m * a * vt/2 = m * v/t * vt/2 = (mv^2)/2.

Thanks! That makes perfect sense.
 
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