1、 2004 Penn State University Physics 211R: Lab Newtons First LawNewtons First LawReading Assignment:Chapter 5, Sections 2-4Chapter 6, Section 4Photo by Keith Larretthttp:/ common misconception is that astronauts are not subject to the force of gravity during space flight. In reality, for missions orb
2、iting the earth, astronauts are subject to a force of gravity that is only slightly less than what they would experience on the earths surface. Why, then, do they appear to be “weightless?” The answer is simple. While in orbit around the earth, they experience free fall motion in the same way that s
3、kydivers do. (There is simply no air pushing against them.) Free fall motion provides the sensation called “weightlessness.” Both orbiting astronauts and skydivers undergo free fall motion.There is a difference, however. The difference between skydivers and astronauts can be illustrated by looking a
4、t the forces acting upon them. An astronaut orbiting the earth is subject to only one force, the force of gravitational attraction to the earth. A skydiver, on the other hand, is subject to two forces: gravitational attraction to the earth and air resistance. Different net forces cause different typ
5、es of motion. Since an astronaut is subject to only one force, the net force cannot be equal to zero. Newtons Second Law states that this type of situation causes accelerated motion. In this case, the astronaut is accelerating towards the center of the earth with centripetal acceleration. The result
6、 is circular, or orbital, motion. Because a skydiver is subject to two forces, the net force may or may not be zero. The value of the net force depends upon the magnitude and direction of each of these forces. Newtons First Law, the Law of Inertia, states that a net force of zero causes an object to
7、 be at rest or in motion with constant velocity. The purpose of this lab activity is to investigate each of these cases.Air resistance is not a constant force. As you fall through air (or water or oil) a viscous force, air resistance, is generated as you push the molecules out of the way. This force
8、 increases with your velocity, the density of the medium and your cross-sectional area. A skydiver, after jumping out of a plane, eventually comes to be in equilibrium when air resistance balances the force of gravity pulling downwards. At this point, the net force is zero and the velocity becomes c
9、onstant. This constant velocity is known as terminal velocity. Since our insurance was cancelled after the last class “jump,“ you will explore this phenomenon using coffee filters. What does it mean for the Net Force on an object to equal zero? Many students think that it means there are no forces a
10、cting on the object. This is incorrect. The term Net Force is used to symbolize the sum of all of the individual forces.Fnet Fii 1n 2004 Penn State University Physics 211R: Lab Newtons First LawSince force is a vector quantity, both magnitude and direction must be considered when forces are added. E
11、quilibrium results when these forces effectively “cancel each other out.” In other words:What happens to the motion of an object if the Net Force on it equals zero? Many students think that the object must be at rest, but this is only true if the object was at rest originally. If the object was in m
12、otion originally, then the object remains at constant speed in one direction for as long as the forces are in equilibrium. Newtons First Law of Motion, although simple to describe, is not intuitive. Understanding it and applying it to different situations can be difficult. Remember that Newtons Laws
13、 are the foundation of dynamics, the study of why objects move the way that they do. Relationships between force and motion are one of the central themes of Mechanics.F n e t 0 2004 Penn State University Physics 211R: Lab Newtons First LawNewtons First LawGoals: Verify that Newtons First Law applies
14、 to objects at rest. Verify that air resistance causes an object to reach a state of equilibrium. Investigate the relationship between air resistance and velocity.Equipment List:Force table w/ Force sensor 2) before reaching terminal velocity; 3) after obtaining terminal velocity. In your sketches e
15、stimate the magnitude of the various forces at each of the times. 3. According to theory, what is causing the force of air resistance on the coffee-filter to change? Is the magnitude of the force of air resistance increasing or decreasing during this time interval (from the moment of release until t
16、he terminal velocity has been reached)? Why?4. Describe how the magnitude and direction of the Net Force on the coffee-filter is changing over this time period (from the moment of release until the terminal velocity has been reached).5. Explain why it is inappropriate to apply Newtons First Law to t
17、he coffee-filter during this time period.Activity 4: The Relationship between Air Resistance and Velocity1. Repeat the data taking procedure used in Activity 2 for at least 6 different masses of coffee-filters. (Different masses of coffee-filters can be obtained by stacking filters together.) After each run, calculate the value of the terminal velocity and the force of air resistance acting on the filter while experiencing terminal velocity.
