Newton's Laws of Motion in Motion

Transcription

1 Newton's Laws of Motion in Motion Objectives: Students will use simple techniques to demonstrate Newton's 1 st and 3 rd Laws of Motion. Students will demonstrate their understanding of thrust, drag, lift, and gravity by designing and launching water rockets. Suggested Grade Level: Ninth through tenth grade Subject Area: Science Math Timeline: Three class periods of 50 minutes each. National Education Standards: NS SCIENCE AS INQUIRY NM-ALG NM-DATA Materials: Plastic bottles (of various sizes) with standard sized opening. Card stock, paper, foam, and card board. Putty or similar substance to change center of gravity of rocket. Bottle rocket launcher (priced between approximately $35 - $200) 3 or 5 16 oz. Plastic cups. 10 round balloons 2 altitude trackers (link to instructions to make your own are in the resources section or purchase for approximately $5-$30) 2 stop watches Background: Newton's First Law of Motion: 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. This law can seem counterintuitive because on Earth an object in motion (ex. a ball flying through the air) will not stay in motion. This is because all objects on Earth are acted on by gravitational forces. Head rests in cars are an everyday example of this law. They are in place to prevent whiplash injuries from rear-end collisions.

2 Newton's Second Law of Motion: The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object. This is simplified in equation form: as net force = mass * acceleration (F = m*a). Newton's Third Law of Motion: For every action there is an equal and opposite reaction. Human made objects in space can move around by applying this law. Using short bursts directed in the opposite direction of the desired move (called OMS burns, or Orbital Maneuvering System burns) orbiters can attach to the ISS (International Space Station) docking bays with an accuracy of less than 1 meter. Be warned that while water bottle rockets are relatively safe they rely on air pressure to launch, so any rocket hitting a student upon lift-off could cause serious injury. Students should stay at least 10 feet away from the rockets they are currently launching. While launching their own rockets, students should wear protective eye wear. While others are launching their rockets, students should stay at least 30 feet away from the launcher. Lesson: Day One (50 minutes): T minus 48 hours 1. Introduce Newton's First Law of Motion: 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. Demonstrate this law with this 20 minute activity: In groups of 5 students will perform a short relay race. Using 16 oz. plastic cups filled with water as their batons, students will travel 10 feet and then hand the cup off to the next student until all 5 have traveled the 10 foot distance with the cup. Next have a brief discussion about this. When did the water spill? The water will spill when students attempt to change its direction or its state. When the water is at rest it will spill when they start to walk/run with it. Once they are walking/running it will spill again when they stop at the end. The water will also spill when they change direction with it. These effects are due to Newton's First Law of Motion. 2. Introduce Newton's Third Law of Motion: For every action there is an equal (in size) and opposite (in direction) reaction. Gravity is a force. So, if there is a gravitational force pulling you toward the Earth and you are not at the center of the Earth, that must be because the ground beneath your feet is pushing back up on you (normal force). Students can give examples of Newton's Third Law in nature: Fish swim by pushing back on water with their fins to propel themselves forward. Birds fly by pushing down on the air, which in turn pushes up on them. Bicycle tires push back on the sidewalk, which in turn propels the cyclist forward.

3 Demonstrate this law with this 15 minute activity: In groups of 3 students should use a balloon and the air in their lungs to demonstrate Newton's Third Law of Motion. Encourage students to find and describe the two forces equaling each other in size and opposing each other in direction in at least two different experiments with the balloon. Students might show how when a balloon is inflated the air pushes out on the balloon while the balloon pushes back toward the center, explaining why the balloon doesn't burst. Students might also show that when they let go of the opening of an inflated balloon it will move in the opposite direction. By inflating the balloon to different sizes, students can show that the action is equal in size by showing that a more inflated balloon flies fast and farther. Day Two (50 minutes): T minus 24 hours Discuss the four forces acting on a rocket: thrust, lift, drag, and gravity. Start with thrust so that when you discuss lift students won't assume that lift is a vertical force. Thrust is a reaction force described quantitatively by Newton's Second and Third Laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a proportional but opposite force on that system. In shuttle launches the thrust is created by the three main engines. Note that thrust is related to mass, so as a shuttle loses mass, its thrust changes as well. To get the simplified net force equation: F= m*a we assume that mass is constant. Is that true during rocket launches? No! Mass changes with the loss of fuel. Lift is generated when an object turns a fluid (in this case air) away from its direction of flow. When the object and fluid move relative to each other, the object turns the fluid flow in a direction perpendicular to that flow, and the force required to do this creates an equal and opposite force that is lift. The image below shows the effect that the wings of this craft have on the foggy (and therefore visible) air. Drag (sometimes called resistance) is the force that resists the movement of a solid object through a fluid (air). Drag is basically created by friction and opposes thrust. Drag can be

4 useful on rockets to create stability. Gravity specifically refers to a force which all objects with mass are theorized to exert on each other to cause gravitation. The strength of Earth's gravitational pull is approximately 9.8 m/s². This force, in effect, pulls the rockets toward the Earth. Gravity is the force that creates weight on Earth. So, the rocket's center of gravity is the point at which its weight is evenly balanced. Discuss the parts of a rocket and what function they perform. What would a launch be like without a nose cone counteracting drag? What would a launch be like without tail fins lending drag to the bottom of the rocket to give it stability? Students will now build their water bottle rockets in pairs. Students may have a variety of designs. Students can have different water bottle sizes and different fin and nose cone designs. Students will decorate their water bottle rockets as well, as this will make it easier to identify which designs performed the best. Day Three (50 minutes): First Launch Day Start this day off with a launch of the rockets. For each rocket launched, students should record the height reached by averaging the two altitude tracker readings (instructional link for building an altitude tracker are included in the references below) and the length of time spent airborne (an average of the two stop watch readings). Back in the classroom after the launch: Students should record their altitudes and time spent airborne on the board. Students should hypothesize about why some rockets did better than others. What factors might have made a difference in the launches? Likely responses include: 1. Rocket size (16 oz, 20 oz, 1 liter, 2 liter bottles, etc) 2. Rocket weight 3. Nose cone shape 4. Fin shape 5. Fin number 6. quantity of water in the bottle at launch (1/3 rd full, ½ full, etc) Pick the two or three factors that the students think are the most likely to have influenced the launch. Discuss which forces these relate to (thrust, drag, lift, and gravity). Divide the students into two or three groups. Each group will build two or three more rockets to test their theory of what might have influenced the launch. It is important to stress that within each group the rockets should be approximately identical in every way aside from the variable being tested. For example: A group that is testing fin number should make three rockets of the same size, with the same nose cone and fin shape where one rocket has 2 fins, one rocket has 4 fins, and one rocket has 6 fins. Note: extra time for building the second set of rockets can be allotted at the beginning of Day Four.

5 Day Four (50 minutes): Second Launch Day Repeat the launch with the 4 to 9 variable test rockets the students have created and measurement height and airborne duration again. Back in the classroom after the launch the groups should record their new findings on the board. What changes made the biggest difference? Did the same rockets create the highest launches and longest flights? What factors might have influenced the launch that we did not control (example: variable wind speed, quality of the bottles used). Extension: 1. Use free body diagrams to go into detail about Newton's Laws of Motion. Show that even if the only two forces on an object are vertical, the object can still be moving horizontally. Example diagrams available here: 2. Discuss the Center of Pressure of the rockets. This is where all four forces acting on a rocket are said to balance. This is based on the shape and surface area of the object. The center of pressure must be behind the center of gravity for the rocket to be stable. NASA site on rocket Center of Pressure: 3. Teach students about international rockets and the (non-nasa) space programs that launch them. There are over 40 international space agencies, including some with the ability to launch 2,000 kg satellites: European Space Agency (ESA), Chinese National Space Administration (CNSA), and Indian Space Research Organization (ISRO). Evaluation: Students descriptions of the forces acting on their balloons should relate directly to

6 direction or size of the reaction. The students redesigns of their rockets should relate directly to the forces acting on the rockets (mainly thrust, drag, and lift) Resources: High end water bottle rocket launcher ($199): Low end water bottle rocket launcher ($35): Make your own altitude tracker: nmwg.cap.gov/santafe/activities/pdf/altitude%20tracker%20assembly%20instructions.pdf Information gained from the education team at the Space Foundation's Summer Institute course: Rocketry and the Biology of Living in Space, Space History and Space Law.