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.
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
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
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...
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Update: June 2012