Energy and Human Ambitions on a Finite Planet, 2021a

5 <strong>Energy</strong> <strong>and</strong> Power Units 80
1. It could short-circuit your effort to learn the material;
2. Problems will ask you to do some of this;
3. This would be a fantastic opportunity for you to design <strong>and</strong>
populate your own master conversion table. Then you’ll really own
it.
Great idea! Go for it!
5.12 Problems
1. A typical textbook may have a mass of 1 kg, <strong>and</strong> thus a weight of
about 10 N. How high could the textbook be lifted (against the Weight is mg, where g ≈ 10 m/s 2 .
force of gravity) by supplying one Joule of energy?
2. If you look in your “energy wallet” <strong>and</strong> only have 24 J of energy
available to spend, how far can you expect to slide an empty box
across the floor if it takes6Nofforce to move it along?
3. A 50 kg crate might require 200 N to slide across a concrete floor.
If we must slide it 10 m along the floor <strong>and</strong> then lift it 2 m into a
truck, how much energy goes into each action, <strong>and</strong> what fraction
of the total energy expenditure is each?
4. Come up with your own scenario (a force <strong>and</strong> a distance) that
would result in 100 J of energy expenditure.
5. The numbers in Table 5.1 are reasonable but should not be thought
of as right. 40 You can make your own table by using mgh for lifting
<strong>and</strong> 1 2 mv2 for kinetic energy. For this exercise, pick three familiar
activities or situations that allow you to estimate an energy scale
in Joules <strong>and</strong> compute/estimate the results.
6. Just for fun, compute the energy associated with the mass of a tiny
bit of shaving stubble having a mass of 0.01 mg 41 using E mc 2 .
Make sure you use the correct units to put the result in Joules. The
speed of light, c, is approximately 3 × 10 8 m/s.
40: Every nerf toss is not 15 J; the bookbag
lift depends on how heavy <strong>and</strong> how high
the lift; every example would have a range
of reasonable numbers.
41: . . . based on 0.1 mm diameter <strong>and</strong> 1 mm
long
7. What exchanges of energy (between what forms) happens when a
h<strong>and</strong> grenade explodes <strong>and</strong> sends pieces of its casing flying away
from the explosion at high velocity? You may wish to describe
more than one step/exchange.
8. Follow the evolution of energy exchanges for a wad of clay that
you throw high into the air. Describe what is happening as the
clay moves upward, as it reaches its apex, as it falls back down,
<strong>and</strong> finally hits the ground with a thud. Where does the initial
energy you put into the clay end up?
9. A couch might take 100 N to slide across a floor. If someone slides i The equivalent force to lifting 10 kg, or
the couch 4 meters <strong>and</strong> does it in 8 seconds, how much power did 22 lb of pushing force.
they expend?
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.