Topic 4 objectives

1. Know what explanations for planetary motions were proposed before Newton’s Principia.

2. Know how ideas concerning laws of motion developed before Newton’s Principia.

3. Understand Newton’s arguments for the existence of a universal gravitational attraction

operating between all bodies, including:

a) how he concluded that the attraction holding the Moon in its orbit around the Earth is

the same as the attraction operating on the Earth’s surface

b) how he showed using pendulums that a variety of materials have the same ratio

between weight and mass

c) how he demonstrated that a universal gravitational attraction would produce planetary

orbits corresponding to Kepler’s three laws of planetary motion

4. Know why Newton and the followers of Descartes each rejected the other’s explanations for

planetary motions, and understand how Newton used his four “rules for the study of natural

philosophy” to strengthen his arguments for an explanation based on universal gravitation.

5. Know the four fundamental forces in nature, the three approaches in describing forces, and the

principle of equivalence.

Reading for Monday, September 20

Chapter 9 in textbook

1. Describe Descartes’ theory of motion.

2. Describe Descartes’ explanation for planetary motion.

Chapter 10, pp. 109-119

1. Compare Newton’s ideas of motion with those of Descartes and Huygens.

2. How would you distinguish between weight and mass? (Refer to p. 113) How could you use

Newton’s law of universal gravitation to help explain the difference?

Chapter 10, pp. 119 to 127

1. Explain the difficulty that Newton’s contemporaries had with his law of universal gravitation.

How did Newton defend himself?

Reading for Monday, September 27 (Murphy Aud.)

Extra handout

1. Describe how particle exchange could explain a repulsive force. How could the force be made

an attraction?

2. Briefly discuss the relative merits of describing a force in terms of (a) action at a distance

(b) a field (c) particle exchange.