

Kinetic Energy
Name: N/A
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: 1991
Question:
I am a senior at Monta Vista High School in Cupertino, California
and I am currenlty enorlled in Physics. Right now we are dealing
with kinectic energy. My question is:
You were given a 2Kg mass and you applied a 9 Newtons of force
across a frictionless table for 4 sec. starting at rest. What
kind of kinetic energy does it have now?
Umm.. Think about it and let me know as soon possible. I understand
the equations in Physics, but the problem solving is most of the time
a problem for me.
Replies:
from F = m * a we have a = 4.5 m/s^2
then v = a * t = 18 m/s
then E = 1/2 * m * v^2 = 324 J
John Hawley
Kinetic energy = (1/2)*M*v^2 where M = mass, v = speed
We know M, but not v, so we need to find v at time t = 4 sec.
speed : v = v0 + A*t where v0 is the initial speed (it is 0 here),
A is the (constant) acceleration,
A = Force/Mass,
t = time
So, at t = 4 sec., v = 0 + (9/2)*4 = 18 m/sec
Thus, K.E. = .5 * 2 * 18^2 = 324 joules
rcwinther
So, how does one solve such problems in general? Start
from what you need to know and work backwards a bit,
and start from what you already know, and work forwards until
you can get the two sides to meet somewhere:
Here  you needed kinetic energy. Did you look up the
formula for kinetic energy? That is the first thing you
need to do  find out what it is you need to know in order to
know what the K.E. is. The formula tells you what you need:
mass and speed. Do you already know either of those? Yes! You
know what the mass is. But you still need speed. Is there
a formula for speed around somewhere? Well maybe it is a good
time to start at the other end  you are given a force, and
what does force tell you? There is Newton's law F = ma which
you should realize by now is the key to almost all force
problems. That tells you what a is (since you know the mass m).
And so you know a, you need v. Can you find a formula that
links the two? You also know initial speed (0) and the time
that the force was applied (4 seconds). Well, the others have
already given complete answers...
Arthur Smith
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Update: June 2012

