1.13: A hypothesis is scientific if it makes predictions that can be tested. In other words, if it is possible to prove it wrong. Note that you cannot PROVE a hypothesis to be true. For example, the hypothesis that 'All crows are black' makes the prediction that you will never see a white crow. If after observing one million crows, they are all black, you are very confident that the hypothesis is correct. However, you have not PROVED it, because if you EVER see a white crow, then the hypothesis is wrong.
Exercises:
1.7: 'Theory' in day-to-day use means 'guess' or 'hypothesis'.
Theory in scientific usage refers to a synthesis of a large body of validated
information. The scientific theory has made a large number of predictions
that have all been verified.
2.6: Nothing keeps them moving. The sun's force deflects their paths but is not needed to keep them moving. ('Inertia' is an acceptable answer. So is 'absence of forces'.)
2.10: The concept of inertia, to resist change, is a scientific, measureable concept pertinent to matter. It serves as a metaphor for a bureaucracy since organizations resist change. However, we can measure motion, amount of inertia, and forces applied to matter. We cannot quantify 'change' or 'inertia' for a bureaucracy.
2.12: The inertia of the whole roll resists the brief force applied by a sharp jerk. There is not enough time for the roll to start moving so the sheet rips off. If you pull slowly, then there is time for the entire roll to start moving and unrolling.
2.22: It is in equilibrium so the sum of forces is zero. The upward forces add up to 400 N. The downward forces are 250 N plus the weight of the staging. Thus the weight of the staging = 400 N - 250 N = 150 N.
2.29: When you are standing on a floor, the floor pushes upward against your feet with a force equal to your weight. This upward force (called the normal force) and your weight are oppositely directed and, since they act on the same body (yours), they cancel to produce a net force on you of zero. Hence you are not accelerated. The push from the floor comes because your weight makes the floor sag a tiny bit and the floor pushes up to resist sagging. The floor cannot push you up in the air.
2.36: You and the wall are travelling at the same speed. Jumping into the air will not change your speed significantly (since a body in motion tends to remain in motion).
Extra Credit: This is an exercise in estimation, not in research. Looking up the answer will get you 1/2 credit this time only.
a) How many miles do all Americans drive each year: There are about 280 million Americans. Most Americans drive so let's say that there are 200 million = 2 * 10^8 drivers. I drive about 15,000 miles per year and new car warranties are good for 10 years/100,000 miles implying that people drive about 10,000 miles per year = 1 * 10^4 miles.
Thus the total mileage driven is the number of drivers times the miles per year for each driver = 2 * 10^8 drivers * 10^4 miles per driver per year = 2 * 10^12 miles (or two trillion miles).
b) How much gas do we burn doing this? At 20 miles per gallon,
we get: gallons burned = miles driven divided by the miles per gallon
so
gallons burned = 2 * 10^12 miles / 20 miles per gallon = 1 * 10^11
gallons = 100 billion gallons.
Note that these answers are accurate to about one digit. I wrote
the answer as 1 * 10^11, not as 1.034276652354238 * 10^11. The numbers
I am starting with are not very accurate, so the numbers I end up
with are not very accurate. If I need accuracy, then I will put in
a lot more work to get better numbers.