## Physics 109n Homework #8

Due: 2:00, Tuesday November 5, 1996

1. A sky diver jumps out of a plane and is in free fall. Her downward velocity gradually approaches a limit of 80 meters per second (about 160 mph, terminal velocity). After falling at close to this speed for several seconds, she opens a parachute and her speed quickly drops to about 3 meters per second, and remains the same until she lands.

Plot a graph showing qualitatively her downward acceleration from the moment she leaves the plane until after she's landed on the ground. (Qualitatively means you should get the direction right, and some idea of where the acceleration is large, where it's very large, and where it's small).

State what forces were acting on her at each stage in the descent.

2. If you are sitting at rest on a stool (so your legs are just hanging) what forces are acting on you? What are the corresponding Newtonian reaction forces to these forces, and what are they acting on?

3. Sketch an elliptical orbit for a planet going around a sun. At the point in the orbit where the planet is closest to the sun, draw an arrow indicating the velocity of the planet as it goes around the orbit, and another arrow showing its acceleration. (The important point here is to get the directions right).

Now draw arrows indicating velocity and acceleration at the planet's furthest point from the sun.
Finally, draw velocity and acceleration arrows at a point in between.

(I want to see that you have some rough idea how the velocity and acceleration vary on going around the elliptical orbit).

4. Newton's Second law states that the acceleration of a body is proportional to the total external force acting on the body.
Suppose you are standing still, then begin to walk. What external force caused your acceleration?
Suppose you are standing still, then jump. What external force caused your acceleration?

5. See what connection you can find between Isaac Newton and the following: Voltaire, William Blake, John Locke, Thomas Jefferson, John Maynard Keynes, the Declaration of Independence. Write a couple of sentences or so for each connection you find. (Check the news page for relevant books for this question!)