Transcript Purpose
Physics 2015: Mechanical Energy Conservation
Purpose
Study energy conservation by looking at the relationship
between three different types of energy:
Gravitational potential energy (GPE)
Elastic potential energy (EPE)
Kinetic energy (KE)
Physics 2015: Mechanical Energy Conservation
Energy is “conserved” if we can add up all of the
different types of energy present in a closed
system and see that the total sum remains
constant.
A “closed system” is one where no energy is added to or
taken away from the system (in our case, a system with
negligible friction)
The types of energy will change depending on the setup.
Ours were listed on the previous slide.
Physics 2015: Mechanical Energy Conservation
Different Energy Types
• Gravitational Potential Energy (GPE)
(relative to a vertical level where h=0)
GPE m g h
Your choice where that is – be smart about it!
• Elastic Potential Energy (EPE)
(k=spring constant
x=distance stretched)
• Kinetic Energy (KE)
(m=mass
v= velocity)
1
EPE
k x2
2
1
2
KE
mv
2
• Heat Energy (Friction can turn KE into heat energy, for example)
Physics 2015: Mechanical Energy Conservation
Our Setup in the Lab
photogate
Note as the system is released, the quantity
GPE+EPE+KE should stay the same. We will be able to
measure all three energies (and thus their sum) at two
different points of the cart’s travel and compare.
Physics 2015: Mechanical Energy Conservation
Activity I - Find the spring constant k
Hook’s law:
F=-kx
Attach different masses.
Measure the change of length of the spring compared to it’s un-stretched
length.
You can plot your data in Excel and use linear regression to find the slope k.
measured
m
calculated
measured
mg
x
mg
slope = k
x
Physics 2015: Mechanical Energy Conservation
Activity II - Conserving Energy.
photogate
Decide where you want to make your GPE=0. A good choice
will make your calculations a lot easier.
Measure all masses (including the mass of the cart).
Set photogate such that the cart will pass through it (to measure
it’s velocity) as the hanging mass accelerates the cart to the left.
Physics 2015: Mechanical Energy Conservation
Hints
% Energy change
E final Einitial
mh g x
mh mass of hanging mass
x changeof thespring length
change of height of hanging mass
If you do find an energy change, ask whether your “system” was
totally “closed”.
Was some form of energy not accounted for?