Energetically Challenged
Maximize your mousetrap potential
Re-cap
Wcar
• Linear Forces: Friction
– Proportional to Normal force
– Static Friction opposes relative
F N F
motion of two surfaces
– Coefficients of friction found experimentally
s
• Rotational Forces: Torque
– Torque causes angular motion
– Moment of inertia found for
drive axle
T
Wmass
s
N
N
Work and Energy
• You (120 lb) and a friend (150
lb) go hiking (uphill) after
eating ½ of a pizza.
– Who will use more energy as they are
hiking?
– Who will be able to hike farther
(higher)?
– Why do you make these predictions?
• What is the unit of energy that people track?
• How is this hiking example similar to your
mousetrap car?
Work and Energy
• Your heavier ‘friend’ will use
more energy: his weight (a
force) is higher, so he will use
more energy over the same
distance.
• Because you and your friend
start with the same amount of
energy (calories from the ½
pizza), you will be able to go
farther than your friend.
Energy: Potential and Kinetic
• Potential energy: an object at rest
– Apple before falling from tree: E  mgh  Wh
– Mousetrap arm when trap is set: E  Fd
(calculated from transferred force)
• Kinetic energy: an object in motion
– Apple falling from tree
– Mousetrap car in motion
1 2
E  mv
2
Putting it together
Work out
Efficiency 
Work in
• Energy efficiency can be calculated, improved
by analyzing work done by forces
– Mousetrap arm
– Friction
– Torque
Em  Fm  darm
EFs  Fs  d
EFs  0.5 N  2 m  1 N  m
ET  T  θ
ET  2N  m  2π rad  4 π N  m
Max Your Ride
• Follow workbook to calculate efficiency of
current model car
• Use what you know: past experiments
– Propose improvements
• Troubleshoot, Redesign, Rebuild!