- Good morning.
Because students often confuse
the mechanical energy equations with the momentum equations,
let's take a few moments to review those equations
and when to use them.
♫ Flipping Physics ♫
- But first, it's tacky sweater day.
- [All] Tacky sweater day.
Bo please give me a mechanical energy equation
and when to use it.
- Uh let's start with conservation of mechanical energy.
Mechanical energy initial equals mechanical energy final,
and we can use it when there is no friction.
- It is true to say we can use it
when there is no friction;
however, it is better to say we can use it
when the work done by friction equals zero.
- The work to due the force applied also has to be zero.
- Yeah, that's right.
- And Bobby another mechanical energy equation please.
- Work done by friction equals the change
in the mechanical energy,
and we can use it when the work done
by the force applied is zero.
- And the work due to friction is not zero.
- Actually even if the work done by friction is zero,
we can still use this equation,
it just works out to be conservation of mechanical energy.
- But it is easiest to think about it that we use work
due to friction equals the change in mechanical energy
when the work due to friction is not zero
and the work due to the force applied is zero.
- Right. - Right.
- And Billy, one more mechanical energy equation please.
- Net work equals change in kinetic energy.
This is the mechanical energy equation which is always true.
- I always confuse this one with the work
due to friction equals change in mechanical energy.
- Me too, they just look so the same.
- Yeah when we have these two equations
students often mix them together
and tell me like the work due to friction
equals change in kinetic energy which is not true.
So please be careful.
Class, what must we identify before we use any
of these mechanical energy equations?
- Initial point.
- Final point.
- Zero line.
- Yes, whenever you use any
of the mechanical energy equations
you need to identify the initial point,
the final point, and the horizontal zero line.
Now, what I would like to do is I would like
to juxtapose these mechanical energy equations
with the equations for momentum, impulse, and impact force.
And we've actually reviewed all of those items before,
so I'm going to now write down everything
from that video on the board.
If you need to watch that video to review,
please feel free to.
Now that we have all the equations for mechanical energy
and momentum on the board, I have three things to point out.
First...
students often tell me that the work
due to friction must be equal to zero
for conservation of momentum to be true,
but this is not correct.
And it's probably because students are confusing
conservation of energy with conservation of momentum.
Conservation of momentum is true
when all the forces are internal or balanced.
And we translate this to mean
during all collisions and explosions.
Second, class when using the momentum equations,
have we ever identified where the initial
and final points are?
- [All] No.
- That is because they are always assumed
to be identified like this.
The initial point is assumed to be right
before the collision or explosion and the final point
is assumed to be right after the collision or explosion.
So we don't need to identify those initial and final points.
Third,
impulse does not equal impact force.
Students often confuse the two.
I'm not sure why.
Maybe it's because they both start with the letter I.
I don't know.
But please do not confuse impulse with impact force.
Thank you very much for learning with me today.
I enjoyed learning with you.