1. The Four Force(men) of the Apocalypse Carl Wozniak Northern Michigan University

2. What is a force?  Any influence (PUSH OR PULL) that gives energy to an object, sometimes causing a change in the motion of the object.  We recognize the forces that occur when one object touches another, contact forces, but there are forces that we can’t see, forces that work at a distance.

3. Contact forces  Contact forces include:  frictional forces,  tensional forces (pull a string tight),  normal forces (support force: book resting on table),  air resistance forces (special type of frictional force),  and applied forces (pushing a desk).

4. Forces at a distance  Gravitational force  Electrical force  Magnetic force

5. Mass vs. weight  Many students of physics confuse weight with mass.  The mass of an object refers to the amount of matter that is contained by the object.  The weight of an object is the force of gravity acting upon that object.  Mass is related to how much stuff is there and weight is related to the pull of the Earth.

6. Four forces of nature

7. Electromagnetic force  Electromagnetism causes like-charged objects to repel each other and oppositely charged objects to attract each other.  The electromagnetic force binds negative electrons to the positive nuclei in atoms and underlies the interactions between atoms.  Its force carrier particle is a photon.

8. Electromagnetism in history  Ben Franklin didn’t invent electricity (1752)  Lightning has been known for all of history

9. Electromagnetic force  James Clerk Maxwell did pioneering work in the mid- 1800s that expressed the basic principles of electricity and magnetism. James Clerk Maxwell  Maxwell’s work predicted that the energy of electromagnetism moved in waves.

10. Electromagnetic force  E-M force is an infinite-range attractive or repulsive force which acts between charged particles.  Coulomb's Law: Like charges repel, unlike charges attract.  The amount of force between two charges is directly proportional to amount of charge and inversely proportional to the square of the distance between the two.  It is observed as light or magnetism.

11. Gravity  Gravity is the brainchild of Isaac Newton, although it was also conceived much earlier by Hildegard von Bingen (1099-1179)Isaac Newton  Newton upset centuries of thought espoused by Aristotle.

12. Aristotle’s world  Aristotle taught that the everything was made of a combination of 4 elements and the “quintessence.” Aristotle  Air  Earth  Fire  Water

13. Aristotle’s world  In Aristotle’s view, heavier objects have a tendency to be at rest in their natural environment  Thus, earth and water were constantly striving to reach their “center,” the Earth’s center.

14. Aristotle’s world  A stone falls to the ground because the stone and the ground are similar in substance.  The natural place for water is to be just above earth and air’s natural place is just above that.  The natural place for the element fire is somewhere above us (but well below the Moon).

15. Aristotle’s world  Aristotle devised laws governing the motion of objects as they had outside forces imparted on them, just as Newton did.

16. Aristotle’s world  Aristotle’s Laws of Motion  An object’s velocity is proportional to the force applied to it.  The speed of a falling object is proportional to the weight of the object.  The speed at which an object falls is inversely proportional to the density of the medium through which it falls.

17. Galileo’s contribution  Galileo (1564-1642) studied the speed of falling bodies to see if they, as Aristotle said, fell at different rates depending on their mass.  Let’s examine some hypotheticals.

18. What do you think?  Setting the stage: You are holding a bullet in your hand, a foot above the ground, and your lab partner has an identical bullet in a gun, pointed exactly horizontally and also a foot above the ground.  The question: If you drop the bullet at the instant the gun is fired, which bullet hits the ground first?

19. Galileo’s thought experiment  Imagine two objects, one heavy and one light, falling together and tied together by a thin string.  Are the objects falling as one object, hence falling faster (because of their increased mass) than if the objects were untethered?  If you were falling alongside and you snipped the string, would the objects start to fall at differing rates?

20. Back to Newton  Newton probably really did get inspiration for his theory of gravity by watching an apple fall.  But it most likely didn’t fall on his head.

21. Newton’s gravity  Newton realized the apple was accelerating, starting at zero as it hung on the tree, picking up speed until it reached the ground.  Newton’ imagined a very tall tree, and what would happen when the apple fell from it.  Newton’s genius came from the next postulation: If the force of gravity reaches the top of the tree, might it not continue far beyond that, all the way to the moon?

22. Newton’s gravity If so, the orbit of the Earth and moon could be due to the gravity of the two bodies countered by the acceleration of the revolving system. The cannonball falls, but continually “misses” the Earth.

23. Newton’s gravity Newton came to the conclusion that any two objects in the universe exert a gravitational attraction on each other.

24. Newton’s gravity What the formula essentially says is that the force of gravity is dependent on the masses of the two bodies and the distance between them.

25. Newton’s gravity The greater the masses, the greater the attraction The farther away the two masses are, the less the attraction