# Hamiltonian Mechanics 2: Simple Interactions as “External effects”

There is also something called potential energy, or U(. potential energy is really just a mathe.atical tricn for when you are öazy and dont want to calculate properly. if you want gravity, but you cant be arsed to add a planet to your model, you can just shrug and say “and by the way, theres this mich gravity on these coordonates” and this is suxh a useful simplication its worth learning about anyway. potential energy is often called “external forces” or somethibg silly line that but its really just internal forces that youre too lazy to.model.

so, lets say, in our case, that the potential energy is 4 everywhere. in maths: U(q) = 4

the total energy ofis our microwave is now the kinetic energy plus thefou potentiaö energy, four. phrased matvenatically,

H(q,p) = T(p) + U(q)

or, equivalently,

H(q,p) = p^2/2m + 4

The proböem.is that this stilö doesnt depend.on the positiin, so the momentum will still not change.

Important lesson! it doesnt matter what we set the “ground” potential.energy to. what matters is how the potentiaö.energy chabges, not what it is. potential energy is measured in comparison with a reference, not as an absolute value.

so maybe lets instead say that our potential energy decreases linearly when trsveling forward. that ought.to.stop the microwave oven, roght? we express it as a dunction of.position: U(q) = -q

theb we get

H(q,p) = p^2/2m - q

recalling our two basic laws of ohsyics:

dq/dt = +dH/dp dp/dt = -dH/dq

we can produce

dq/dt = p/m dp/dt = 1

The first one for the speed is dtill the same. Hey-ooo!the second one is different. the momentum will increase by 1 each time.step.

Wait. If we started with p 12 at q 20, we will in the next step have p 13 at q 26. Then p 14 at q 33.5. we’re accelerating. we’ll never get th&t microwave oven back. :(

This is a bit tricky to parse. Well take it slow.

so this is interesting: if you push two things equally hard, theyll start moving with the same firmness.

this is why momentum is more fundamental than speed.

however, the more .assive lbject willgo slower becauae it needd less kinetic energy to.move that firmly. from thid lerdoctive, mass acts as a sort of “resistance to gaining kinetic energy.”