Physics Project Newton s First Law of Motion

Transcription

1 Physics Project Newton s First Law of Motion Submitted by: Shannon Carpenter Michigan State University Teacher Education Student Modified from Center of Excellence for Science and Mathematics Education at the University of Tennessee at Martin Date: November 16,2000 Grade Level: Appropriate for any general physics class in high school. (Grades 11 and 12) Requirement: Students should have completed some physics background before doing these demos. Overview: These demos have been placed together into order to help students see the concept of Newton s Third Law. Students must use some of their prior knowledge of the subject before seeing each demo. Included below: a short 10-minute lecture reviewing the concept of Newton s Third Law and four demonstrations which should be done within a one-hour class period. Each demonstration should last around 2 or 3 minutes with about a 5 to 10-minute class discussion following each demo. Student Objectives: Demonstrate ability of discussion and reasoning. Analyze demonstrations showing the concept of Newton s First Law of Motion. Teacher Objectives: Test student s familiarity and knowledge of previous learned physics information/concepts. (Examples: Forces on an object, gravity and friction.) Assess student s understanding of Newton s First Law of Motion. 10-minute lecture should discuss the following points of information: Newton's first law of motion is often stated as: An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force. Unbalanced forces cause accelerations. Example: A Book sliding from left to right across a tabletop. There are two parts to this statement - one that predicts the behavior of stationary objects and the other which predicts the behavior of moving objects. The property of any object keeping its motion unless disturbed by others is called inertia. Newton s first law is also termed as the law of inertia. The law is valid in an inertial frame, which does not accelerate with respect to the rest of the Universe.

2 Newton s thought was: if an object moves on a level floor, it stops because of friction. If the floor is smooth, the object goes straight further. In the case of no friction, it will move unchanged forever. Demonstration #1: Sugar Cubes and Hoop Demonstrates "objects at rest tend to stay at rest". Stack of Sugar Cubes Large 12-inch Embroidery Hoop Large Glass Narrow-Mouth Bottle 1. Very carefully balance the embroidery hook vertically on the mouth of the bottle. Stack one sugar cube on top of the hoop. With practice a stack of sugar cubes can be used in place of the single cube. 2. Face the hoop toward you so that you can see the full circle. 3. Swing your arm across your body horizontally, quickly grabbing the far inside edge of the hoop while your arm is moving, and flicking your wrist to move the hoop from beneath the sugar cubes. 4. The sugar cubes should drop into the bottle. 5. Next, try this again but this time grab the hoop on its near side. 1. Is inertia involved? 2. What kept the sugar cubes from scattering? Yes The hoop moves while the sugar cubes do not, other than to fall into the bottle. As long as the sugar cubes rested on the hoop there were equal forces pushing up and pushing down on them and they didn t move. When, by snatching the hoop away, the upward force exerted by the hoop on the sugar cubes was taken away the sugar cubes experienced an unbalanced force and started to move. When they started moving they continued to move until another force, the bottle of the bottle, acted on them and brought them to a halt.

3 3. What happened when I grabbed the hoop from its near side? If you grab the near side of the hoop first, you momentarily stretch the hoop vertically, so that the coins are tossed into the air - they may or may not land in the flask. 4. What part of Newton s First Law of Motion does this demonstration illustrate? "Objects at rest tend to stay at rest". Demonstration #2: Penny and Index Card Demonstrates "objects at rest tend to stay at rest". 3 x 5 Index Card One Penny 1. Cut the index card in half and place the penny in the center of the card and balance the card and penny on your left index finger. 2. With your right middle finger flick the card off of your index finger. This should be done so that the card moves in a horizontal plane and so the penny is able stay on the fingertip. 3. If the removal of the card is not done quickly both the card and the penny will move. 1. Have the students predict what will happen when you flick the card based on the previous experiment. 2. Why doesn t the penny fall on the ground with the index card? The inertia of the penny accounts for it remaining where it started while the card motion. is set in 3. What part of Newton s First Law of Motion does this demonstration illustrate? "Objects at rest tend to stay at rest".

4 Demonstration #3 Inertia in an Automobile For students discover the importance of wearing a seatbelt. Demonstrates that a person in motion tends to stay in motion with the same speed and in the same direction. 2 meter piece of string 1 dynamics cart One 200-g hook mass One rubber band 1 small doll (to fit the dynamics cart) 1 pulley 1 block of wood 1. Attach one end of the string to the dynamics cart and the other end to the 200-g mass. 2. Attach the pulley to the end of the desk and hang the mass over the pulley so that the mass is on the floor and the cart is on the desk. 3. Place the block of wood on the table in front of the pulley (between the pulley and the cart). 4. Then place the doll on the cart. 5. Next, pull the cart back and release it so that it accelerates toward the edge of the desk. 6. Repeat this procedure, but this time, attach the doll to the cart with the rubber band or tape which acts as the seat belt. 1. Why does the doll fly out of the cart when the cart hits the block of wood? The force of the desk on the wheels provides the unbalanced force to change the car's state of motion, yet there is no unbalanced force to change the doll s own state of motion. Thus, the doll continues to stay in motion, flying in a forward motion. 2. What part of Newton s First Law describes this? A person in motion tends to stay in motion with the same speed and in the same direction. 3. What is happening when the seat belt is applied to the doll?

5 The seat belt provides the unbalanced force, which brings you from a state of motion to a state of rest. 4. What would be different if this collision had happened in zero-g (in a vacuum)? If this collision had happened in zero-g, in a vacuum, the dummy would theoretically keep on hurtling away from the car at the same speed the car was traveling. Demonstration #4 Concept Extension Illustrates multiple concepts surrounding Newton s First Law. Four pieces of thread about 3 meters long 2 Identical paperback books 1. Suspend from the ceiling or other horizontal support two pieces of thread that are 3 meters long. 2. Wrap the first piece of thread around a paperback book allowing it to hang down. 3. Wrap the second piece of thread around an identical paperback book so that it hangs down an identical distance from the ceiling. 4. Tie another piece of thread around the first book so that it hangs about one meter below the bottom of the book. Both strings tied to the book should be in the same vertical line. 5. Repeat step #4 on the second book so that there are two identical setups. 1. What will happen when you pull on the bottom thread attached to the first book? (If the students say the thread will snap above the book, jerk rapidly and the thread below the book will snap. If the students say that the thread below the book will snap, pull slowly on the thread and the thread above the book will snap.) By pulling the thread slowly, we are not only putting a strain in the thread, but in the thread above the book, the book s weight adds to this pull. Thus compared to the strain below the book, this is much larger and the thread snaps wherever the strain is the highest.

6 When a sharp jerk is exerted on the thread, the inertial of the book keeps the strain below the book. Although there is some strain in the thread above the book, compared to that below the book, the strain in the latter is still higher, and the thread snaps below the book. Overall Assessment: One type of assessment that you may want to do following the demonstrations could be a journal type activity for the students to do. This would be a set of analysis questions based around Newton s First Law of Motion the students could do either individually or in a small group. This homework activity should be due the next day following your demo presentations. Here is one such example of this: These questions were taken from newton %27s_ first_law_wks.htm: 1. An F18 Hornet aircraft sits motionless on a runway because of inertia. If that plane starts to move down the runway, it means that there must be a force acting on it. Of the four main forces that act on aircraft, which one is most likely to cause the forward motion down the runway? 2. Which of the four forces causes the airplane to become airborne? 3. Once the F18 Hornet is airborne, which force is equal to the force of thrust? 4. As the F18 Hornet travels at constant velocity, it burns fuel, which decreases its weight. In response to this decrease, which of the four forces is also likely to decrease? Consider the situations below to determine whether or not there is an external force acting on the airplane. Remember that according to Newton's First Law, a plane will resist a change in its state of rest or motion unless it is acted on by a force. Write the word force next to the situations where an external force acts. 1. The F18 is flying at 200 mph at an elevation of 30,000 ft. 2. The F18 turns northeast at 200 mph and an elevation of 30,000 ft. 3. The F18 increases its speed to 220 mph at the same elevation. 4. The F18 continues flying at 220 mph at 30,000 ft. 5. The F18 drops to an elevation of 28,000 ft. 6. The F18 heads due east maintaining its speed and elevation. Strengths of Exercise: These exercises are a good experience for students to see that science can be fun and interesting, if they see physics as being something that is all around them and very applicable such as Newton s First Law is. The exercise is also good because it depends somewhat on the prior knowledge of certain basic physics concepts and laws. These demonstrations also allows for student-student interaction/discussion when a class discussion is done after each demonstration.

7 What to Look Out For: As in all class discussion type situations, it is necessary to make sure that all of the students are included in the discussion and that one person does do all of the participating. Also, you make sure to give some time (more than 3 seconds) between asking questions and receiving answers so students have some time to think about the answer. One good way of doing this is to have the students write done their answers individually before any of the students even answer them.