06.28.2019: Science Notebook Ch. 11

Today’s soundtrack is Steven Sharp Nelson: Tender Mercies, a cello-centric album that features many covers of beautiful spiritual and classical songs.

Everything You Need to Ace Science in One Big Fat Notebook
Chapter 11: Gravity, Friction, and More Forces in Everyday Life

Gravity is the “pull” between objects with mass. The two factors that affect the strength of an object’s gravitational pull on another object are 1) the distance between the two objects and 2) the mass of the objects.

Note that I said mass, not weight, is a factor in gravity. Weight is just a relative value, because weight changes based on how close or far an object is to an object with large mass, and of course is also dependent on the object’s mass.

Friction is the “force that opposes movement between touching surfaces” (p. 111). Even air can count as a surface in this respect: if one were to go bungee-jumping on a planet with no atmosphere, he would fall much more quickly than he would on Earth, because the earth’s air resistance slows down falling objects in our atmosphere. This is most visible if we throw a styrofoam cup vs throwing a baseball, or if we drop a feather next to a marble. Every object that is dropped in an atmosphere will eventually reach terminal velocity, which is “the speed at which the force of gravity equals air resistance” (p. 113). Without external propulsion, an object will not fall any faster than its terminal velocity if it is falling in an atmosphere. If an object is falling with no atmosphere, its fall will accelerate more and more the closer it gets to the object with large mass.

Besides air resistance, friction can be classified in three ways:
1) Static friction: The friction that makes it harder to slide an item that is at rest than it is to slide an object that is already moving. It is generated by “molecules on one surface adhering to the other surface” (p. 112).
2) Sliding friction, or kinetic friction, is the friction that makes an object come to a stop once it’s in motion if it is in contact with another surface. Because the molecules of the two objects aren’t adhering to each other as they do with static friction, kinetic friction is weaker than static friction.
3) Rolling friction is the weakest kind of friction; we see this kind of friction bring a skateboard to rest. A skateboard will travel much further than a block of cheese before finally coming to rest.

Two other kinds of forces that are all around us are magnets and electricity. Magnets are kinds of materials that are “attracted to iron, steel, or other magnets” (p. 113). Every magnet has a positive side and a negative side. Positives will reject each other; negatives will reject each other; positives and negatives are attracted to their opposites. Similarly, electricity is “caused by positive and negative charges in […] matter” (p. 114), and like magnetic forces, the strength of electric forces depends on the distance and the amount of charge.

We can combine the forces of electricity and magnetism in the field of electromagnetism, which uses the charges of electricity to make a conductive piece of metal into a magnet.

We learned in Chapter 10 that an object going in a circle is technically constantly accelerating, because its direction is in a continuous state of change.

Remember learning about centrifugal force? Interestingly, it isn’t a “real” force; it feels like it is, but in actuality, it’s simply inertia: if an object is in motion but is being prevented from moving in a straight line, if the connection breaks, the object will continue in a straight line until some external factor (gravity, a dump truck, etc.) changes its path. There is a true force that is often confused with centrifugal force (which is fair enough, because the two are related). It is called centripetal force. Centripetal force counters centrifugal force; it is the force that makes an object change its direction continuously. An example is the Sun’s gravity on the orbits of its planets: without the Sun’s gravity, the planets would fly off into space in a straight line. We can also see centripetal force in action when a daredevil does a loop-the-loop.

Finally, buoyancy: the “upward force exerted by a fluid onto an object that is immersed in the fluid” (p. 116). This force is dependent on the fluid’s density, the shape of the object, and the mass of the object in the fluid, since that determines how much fluid will be displaced. According to the Archimedes Principle, we can determine how much buoyancy force is being exerted by measuring the weight of the displaced fluid. The weight of the displaced fluid is the buoyant force.


  1. The force that attracts masses together is gravity.
  2. A bowling ball has more gravity than a soccer ball because its mass is larger.
  3. When you push a shopping cart, the friction that stops the cart from rolling when you let go is “rolling friction”
  4. Centripetal force is the force that keeps an object in a constant state of direction-change
  5. Centripetal force always points toward the centre of the circular path of motion
  6. A feather drifts back and forth as it falls because its sides are alternately being affected by air resistance.
  7. The force of gravity decreases as the distance between masses increases
  8. In magnetic and electric forces, likes repel and opposites attract
  9. When the force of air resistance equals the force of gravity, a falling object reaches terminal velocity
  10. The name of the upward force exerted on a boat in the water is buoyancy
  11. The buoyant force of a dog that displaces 10 pounds of water is 10 pounds.

Nice, I got them all right! That’s it for today!

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