Materials Included In Kit

Additional Materials Required

Prelab Preparation

Part B. Newton’s Second Law: Hanging Mass

1. Obtain the materials to construct the carts as shown in Figure 6. Note: The thick screw in the body of the cart is to allow for ease in attaching a 50-g mass. It is an optional component, and may be left off.
   {12144_Preparation_Figure_6}
2. Attach a 50-g mass to each cart. Secure the mass with masking tape so it will not shift as the cart accelerates. Note: The exact weight is unimportant; this step is merely to slow the acceleration of the cart to a measurable laboratory time scale. If a 50-g mass is not available, substitute another object of similar size and roughly comparable weight.
3. Set up two lab stations on separate tables with at least 1.5 meters of room on each.
4. Clamp one table pulley onto the edge of each table.
5. Set up barriers for the carts to run into, so the carts will not be pulled off the table by the hanging weight (see Figure 7). Possible barriers include small blocks or books. Fasten the barrier to the table using masking tape. A C-clamp may also be used if available.
   {12144_Preparation_Figure_7}
6. Using masking tape and a ruler, measure and mark distances of 0.1 m and 1.1 m from the edge of the table. Note: The distance between the marks may be less than 1 m depending on the height of the table.
7. Cut out two 1.3-m lengths of string.
8. Tie loop knots on both ends of each string (see Figure 8).
   {12144_Preparation_Figure_8_Loop knot}

Part C. Newton’s Third Law: Balloon Rockets

1. Unroll enough fishing line to extend the entire length of the classroom. Leave approximately a meter of fishing line slack and then cut the fishing line with scissors.
2. Tape or tie one end of the fishing line to one wall. Make sure the fishing line is high enough to extend parallel to the floor across the classroom without interfering with objects in the room.
3. Extend the fishing line to the opposite wall. Loosely tape the end of the fishing line to the wall so that the fishing line is taut and parallel to the floor. Alternatively, use a slip knot or clamp so the string can be undone and balloons can be launched on the fishing line.

Safety Precautions

Latex balloons may be an allergen. Use caution when launching the balloons. Be sure no one is in the path of the balloon rocket on the string before launching the balloon. The fishing line may be difficult to see. Be aware of your surroundings as you walk through the classroom. Inform students not to over-inflate the balloons and cause them to pop. Flying balloon pieces may injure eyes. Wear safety glasses. Remind students that horseplay is not permitted, and to follow all normal laboratory safety guidelines.

Disposal

The air pucks, table pulleys, carts and fishing line should be saved and stored for future use. Leftover straws, balloons and string may be saved for future experiments. Used balloons may be thrown away with the normal trash when the lab is complete.

Lab Hints

• Enough materials are provided in this kit for two complete activity stations for each of Newton’s Laws. All parts of this laboratory activity can reasonably be completed in one 50-minute class period. The prelaboratory assignment may be completed before coming to lab, and the Post-Lab Questions may be completed the day after the lab.Prelab Preparation is an essential component of lab safety, and it is also critical for success in the lab. (Standing in front of the lab station is not a good time for students to be reading the activity for the first time.) Having students complete the written prelab assignment and reviewing the safety precautions for each activity will help teachers ensure that students are prepared for and can work safely in the lab.
• Deriving Equation 2 from Newton’s Second Law activity station: Equation 3 shows how far an object will travel in a given amount of time with a constant acceleration.
  {12144_Hints_Equation_3}

where

d is the distance the object moves (in m)
a is the acceleration of the object (in m/s²)
t is the time (in s)
v_i is the initiail velocity of the object

The initial velocity of the object should be zero (as the carts are not being pushed, but rather released), allowing that term to drop out. Solving the equation for a leads to Equation 2.

• The Newton’s Hanging Mass activity station only has students repeating the measurement once for each data point. Usually measurements should be repeated twice, to average at least three data points, but time constraints may require a shorter lab.
• To ensure the smooth running of the lab, you may wish to set a time limit for each activity station, allowing students to rotate through all three stations.

Teacher Tips

• This activity may be used either as an introduction to Newton’s Laws of Motion or as a basic review, depending on the needs of your curriculum.
• The Post-Lab Questions in Newton’s Second Law Activity Station marked “advanced” are geared toward more advanced high school physics students. Depending on the level of your classroom, have your students omit these.

Further Extensions

Internal Reference Frames

• For more advanced classes, you may wish to discuss the importance of Newton’s First Law and inertial reference frames. Galileo first postulated that all laws were the same in non-accelerating frames, which Newton was able to codify in terms of his law of inertia. More specifically, an inertial reference frame is now defined as any reference frame in which Newton’s laws hold—it must not be speeding up, slowing down, or changing directions. An inertial reference frame therefore is one that does not have a net force on it. A train stopped at a train station and a train traveling at a constant velocity of 60 m/s will both show the same laws of motion—in either case, a ball thrown up in the air will land right back in the hands that threw it. However, in an accelerating reference frame, that is, a noninertial reference frame, the same laws of motion have to be modified to take this extra acceleration into account. As a discussion question, you might ask if the Earth is an inertial reference frame. Although our daily experience seems to indicate that it is, because the Earth is rotating, it is not an inertial reference frame. This is the reason for forces such as the Coriolis Effect. For more discussion on the Coriolis force, consider Flinn Scientific’s Coriolis Effect Student Activity Kit, Catalog No. AP7346.

Answers to Prelab Questions

Activity A. Newton’s First Law: Air Pucks

1. A truck driver places a crate on his perfectly slick flatbed, but forgets to secure it to the truck. As he drives off, the crate slides and falls off the back. Explain why this occurred in terms of Newton’s first law.

   Because there is very little friction between the flatbed and the crate, when the truck began moving, it was exerting very little force onto the crate. Because the crate had no external forces on it, it did not move because of its inertia, and the truck moved out from underneath it, causing the crate to fall.

2. Why are balloons used with the pucks for this activity? What advantage do the balloons provide?

   The balloons provide a pocket of air for the puck to float on, reducing the friction between the air puck and the table. Using the air pucks in a nearly frictionless environment makes it much easier to see Newton’s first law in action, when there is no hidden frictional force slowing the pucks down.

Activity B. Newton’s Second Law: Hanging Mass

1. Sketch and label the following free body diagram, filling in all force vector arrows.
   {12144_Answers_Figure_9}
2. Only the mass of the hanging weight will be varied during this activity. Does the mass of the cart make a difference? Why or why not?

   Yes, the mass of the cart makes a difference, as the mass of the cart means there is more mass for the hanging weight to accelerate.

Activity C. Newton’s Third Law: Balloon Rockets

1. Define Newton’s third law of motion in terms of how a rocket “works.”

   Newton’s third law states that for every action force there is an equal and opposite reaction force. The “action” of rocket gases being forced out the open nozzle of a rocket engine causes a “reaction” on the rocket engine itself. The solid rocket pushes on the gas from the burning fuel, and the gas pushes on the rocket body causing it to travel in the opposite direction.

2. What are some variables or conditions that may affect the distance a balloon rocket travels? Suggest some possible modifications in the rocket or launch design that may improve the rocket performance. (Hint: The fishing line must be as taut as possible, without breaking, for the best performance.)

   If the balloon is not filled with enough air, then there will not be enough thrust to push it across the room. The balloon may get “hung up” on the fishing line if there is too much friction on the straw. The balloon may get “hung up” on the fishing line if it is not properly lined up to shoot straight across the classroom.
   
   To improve the performance of the balloon rocket: Make sure the fishing line is taut so the balloon does not get hung up on the sag of the fishing line. Use as thin of fishing line as possible to limit the amount of friction produced. Be sure the straw lines up on the balloon so that the balloon will travel in a straight line down the fishing line. The nose of the balloon should point toward the target wall and the nozzle opening of the balloon should point toward the fishing line attachment point on the launch wall.

Sample Data

Activity A. Newton’s First Law: Air Pucks

Activity B. Newton’s Second Law: Hanging Mass

Distance: ___0.7___m
Mass of car: ___116.11___g Activity C. Newton’s Third Law: Balloon Rockets

Answers to Questions

Activity A. Newton’s First Law: Air Pucks

1. In your own words, define the following terms.
   1. Force is a push or a pull, or any influence that acts on an object to cause acceleration.
   2. Inertia is an object’s resistance to change in motion. Inertia is related to the mass of an object.
   3. Acceleration is a change in velocity—whether speeding up, slowing down, or changing directions.
2. How can you tell whether or not all the forces acting on the non-accelerated puck in step 7 are balanced?

   Note: Student data will vary depending on whether or not the puck remained stationary.
   Sample acceptable answers are below.
   
   The puck did not move after the balloon was released, but hovered in one place. The force of gravity pulling down on the puck and the force of the air pushing up must be balanced since the puck did not accelerate.
   
   The puck moved around after the balloon was released. The forces acting on it must be unbalanced. The surface may not be level or the air currents are not pushing equally along the bottom of the puck.

3. Imagine an air puck with a limitless air supply (i.e., a level air table of infinite length).
   1. Once the puck was pushed, would it continue to travel forever?

      No, the air puck would not continue to travel forever.

   2. Why or why not?

      Air friction would slow the puck down and it eventually would stop, hovering in one place.

4. List three more examples of Newton’s first law in action in everyday life.

   Student answers will vary. Examples include: Shaking bottles upside-down to dislodge a viscous material stuck at the bottom, headrests and seat belts in cars to prevent serious injuries during collisions, a satellite’s motion in space, the feeling of one’s stomach lifting during a sudden drop on a roller coaster, etc.

Activity B. Newton’s Second Law: Hanging Weight

1. Using Equation 2 and the average time recorded for each hanging mass, calculate the acceleration of the cart for each hanging mass used and record in the data table.

   a = 2d/t² = 2(0.7 m)/(1.96 s)² = 1.4 m/3.84 s² = 0.36m/s²

2. Plot a graph of the hanging mass vs. acceleration.
   {12144_Answers_Figure_10}
3. A heavy box and a light box are accelerated to the same speed and then released. Ignoring friction, which mass will require more force to bring it to a stop?

   The heavier box will require more force to bring it to a stop because its greater mass requires more force to give it the same negative acceleration as the lighter box.

4. (Advanced) Using the free body diagram from the Prelab Questions, solve for the acceleration of the cart. Assume the pulley is weightless, and the system is without friction. Prove that the acceleration of the system is given by Equation 4.
   {12144_Answers_Equation_4}

   Hint: Start by totaling the forces on the hanging weight, then totaling the forces on the cart.
   
   The forces on the hanging mass: m_hg – T = m_ha
   The forces on the cart: T = m_ca
   Substituting the force of tension, T, into the first equation gives: m_hg – m_c x a = m_ha.
   Solving for acceleration: m_hg = (m_h + m_c)a
   a = mhg / (m_h + m_c)

5. 1. (Advanced) Based on Equation 3, what constant must be compared to the acceleration to show a directly proportional relationship? Calculate the appropriate relationship, and plot this data.

      The mass of the hanging mass divided by the total mass of the system is proportional to the acceleration.

      {12144_Answers_Figure_11}
   2. Calculate the slope of the “best-fit” line. Select two points—(x₁, y₁) and (x₂, y₂)—that are closest to the actual line. The slope (m) is calculated using Equation 5. Show all of your work! What are the units of the slope? What is represented by the slope?
      {12144_Answers_Equation_5}

      The slope of the line is 10.56. The slope of the line should be the acceleration due to gravity.

6. (Advanced) A heavy box and a light box are accelerated over the same distance using the same force. Ignoring friction, which mass will require more force to bring it to a stop?

   The two boxes will require the same amount of force to bring to a stop. Even though one box is heavier, it will have accelerated less than the smaller box for the same amount of force.

Activity C. Newton’s Third Law: Balloon Rockets

1. Why does the balloon move when it is blown up and the pressure inside the balloon is released? The air is expelled out the opening and the air exerts an equal and opposite force on the balloon. The balloon pushes the air out, and the air pushes back on the balloon in the opposite direction.
2. Why is the air pushed out of the balloon? The stretched rubber exerts a force on the air inside the balloon as the stretched rubber relaxes back to its original size. The force from the relaxing balloon forces the air out the opening. The original force comes from the initial inflation process when air was forced into the balloon and the balloon stretched.
3. List some suggestions that might improve the performance of the balloon rocket. If students used straw pieces, they may suggest that one long straw may work better than two or more straw pieces. The use of thinner or smoother string may have reduced the frictional forces and prevented the balloon from slowing down. A larger, more streamlined balloon would have more thrust and may be more aerodynamic, thus limiting its drag. Inserting a rigid tube, such as a piece of a drinking straw, into the balloon nozzle may create a more uniform and constant thrust.

References

Halliday, D., Resnick, R., & Walker, J., Fundamentals of Physics, 8th ed.; Wiley: Cleveland, OH, 2008.

Hyperphysics: Georgia State University, http://hyperphysics.phy-astr.gsu.edu/hbase/HFrame.html (accessed May 2010).

Introduction

Whether you’re lifting a heavy box, playing hockey, or flying to the moon, Newton’s laws of motion explain a great deal of motion and physical interactions in the world. Explore each of the three laws as you rotate through a series of activity stations.

Concepts

• Newton’s laws of motion
• Inertia
• Acceleration
• Force

Background

Activity A. Newton’s First Law: Air Pucks

Until the end of the Middle Ages, the common European understanding of objects and motion was based on the ideas of the Greek philosopher Aristotle (384–322 BC). Aristotle believed that all objects had their natural place of rest in the universe, and would progress toward those states. Rocks fell to Earth because their natural place was land; smoke rose to the sky because its natural place was the sky. Once an object reached its natural state, it would remain at rest. When slid along the ground, rocks would eventually stop moving because their natural state on Earth was rest.

Isaac Newton (1642–1727), basing his work on ideas already in place by Galileo Galilei (1564–1642) and René Descartes (1596–1650), developed his First Law of Motion, also called the law of inertia. Rather than believing an object’s natural state was at rest, Newton proposed that an object in motion with a constant velocity tends to stay in motion maintaining that velocity unless acted upon by an external force. If an object is at rest, the object tends to stay at rest unless acted upon by an external force. Inertia may be defined as the tendency of an object to resist change in motion. Inertia is directly related to mass—the greater the mass of an object, the greater its inertia. The reason a rock comes to rest when slid across the ground is not that its natural state is rest, but rather another force is acting upon it—in this case, friction from the ground and the air. In the absence of all forces (e.g., a rock thrown in the vacuum of space) the rock would remain in constant motion.

Activity B. Newton’s Second Law: Hanging Weight

Newton’s Second Law of Motion states that force applied by an object is equal to the mass of the object multiplied by the object’s acceleration (see Equation 1). For a given force, the mass of an object is inversely proportional to its acceleration, while for an object of specific mass, the force needed to accelerate the object is directly proportional to its acceleration. In other words, if the same force were applied to two objects of different masses, the object with less mass would experience a greater acceleration than the more massive object.

Multiple forces acting in complex systems can be summed to find the resulting acceleration. Acceleration along a given axis, however, can only be the result of all the forces in the same axis—forces in the y-direction will not affect the acceleration in the x-direction and vice-versa. Imagine a game of tug-of-war, with two teams pulling on a rope with great force. The whole system will often experience very little acceleration because the two forces are acting in opposition along the same axis, and thus cancel each other out. In this lab, the acceleration of a hanging weight system will be measured by recording the time it takes the weight to traverse a measured distance. The acceleration can be calculated using Equation 2. By modifying the weight of the hanging mass and analyzing the forces on the system, you will be able verify the relationship between force, mass, and acceleration.

Activity C. Newton’s Third Law: Balloon Rockets

Newton’s Third Law of Motion states that for every action force there is an equal and opposite reaction force. Rockets clearly show Newton’s third law in action. When a rocket burns fuel, hot gases are forced out the bottom of the rocket at high speed. The fast-moving gas particles are pushed by the rocket chamber in one direction and the gas particles, in turn, push on the rocket in the opposite direction. A common misconception about rocket thrust is that when the fast-moving gas particles exit a rocket engine, the gas particles push against the air outside the rocket and this causes the rocket to shoot upward. However, if this were the case, then rockets would never work in outer space because there are no air molecules in space for the fast-moving gases to push against. Instead, the fast-moving particles are forced out the rocket engine by the body of the engine.

When the fuel burns, a great amount of heat is created and the pressure inside the rocket combustion chamber increases. At the same time, the walls of the combustion chamber push back on the fast-moving gas particles. Rockets are composed of strong, solid materials with a small opening at the bottom. This opening is the only region on the engine where the pressure can be released. Since gas particles move from high to low pressure, the gas shoots out the bottom of the rocket. The rocket accelerates in the opposite direction of the ejected gases (see Figure 1). An enormous amount of fast-moving gas particles need to be generated in order to lift a rocket into orbit. A small thrust channel increases the speed of the hot gases as they exit from the larger combustion chamber. Gases always accelerate toward lower pressure, so the high-pressure gas moves faster and faster as it rushes out of the nozzle. The constricted flow path increases the speed of the gas particles. This increase in particle speed in a chamber as the diameter decreases is an example of Bernoulli’s principle (see Figure 2). The small-diameter chamber increases the speed of the exiting particles and therefore increases the net force that blasts off the rocket.

Experiment Overview

The purpose of this “activity-stations” lab is to investigate and explore Newton’s three laws of motion. Three distinct activity stations are set up around the lab. Each activity focuses on one of the laws and is a self-contained unit, complete with background information and discussion questions. The activities may be completed in any order.

Activity A. Newton’s First Law: Air Pucks
Activity B. Newton’s Second Law: Hanging Weight
Activity C. Newton’s Third Law: Balloon Rockets

Materials

Prelab Questions

Activity A

1. A truck driver places a crate on his perfectly slick flatbed, but forgets to secure it to the truck. As he drives off, the crate slides and falls off the back. Explain why this occurred in terms of Newton’s first law.
2. Why are balloons used with the pucks for this activity? What advantage do the balloons provide?

Activity B

1. Sketch and label the following free body diagram, filling in all force vector arrows.
   {12144_PreLab_Figure_3}
2. Only the mass of the hanging weight will be varied during this activity. Does the mass of the cart make a difference? Why or why not?

Activity C

1. Define Newton’s third law of motion in terms of how a rocket “works.”
2. What are some variables or conditions that may affect the distance a balloon rocket will travel? Suggest some possible modifications in the rocket or launch design that may improve the rocket performance. (Hint: The fishing line must be as taut as possible, without breaking, for the best performance.)
3. List two other examples of Newton’s third law in action, and describe the forces at work in each case.

Safety Precautions

Be aware of your surroundings as you walk through the classroom. Latex balloons may be an allergen. Wear safety glasses. Aggressive or excessive pushing of the pucks is not permitted. Projectiles may be inadvertently launched during this activity. Use caution when launching the balloons. Be sure no one is in the path of the balloon rocket on the string before launching the balloon. The fishing line may be difficult to see. Do not over-inflate the balloons and cause them to pop. Wash hands thoroughly with soap and water before leaving the laboratory. Follow all normal laboratory safety guidelines.

Procedure

Activity A. Newton’s First Law: Air Pucks

1. Inflate one balloon and twist (but do not tie) the neck shut to prevent air from escaping. Alternatively, use a clothespin to seal the neck of the balloon.
2. Without allowing the neck to untwist, carefully stretch the mouth of the balloon over the stem of the air puck assembly. Note: The balloon may tear if overstretched.
3. To levitate the puck, untwist the neck of the balloon.
4. Gently push the puck to accelerate it over any smooth surface. Record observations of the movement of the puck on the worksheet. Note: If using a surface such as a lab table, do not allow the puck to fall off.
5. Repeat steps 1–4 if necessary to make detailed observations.
6. Repeat steps 1–3.
7. Allow the puck to hover motionless until the balloon deflates. Record observations on the worksheet.
8. Repeat steps 1–3 with two balloons and two air pucks.
9. Gently push one puck toward a levitating stationary puck so the two pucks collide.
10. Record observations of the collision on the worksheet. Describe the observations in terms of Newton’s first law of motion.
11. Increase the mass of the stationary puck using pennies or washers, and repeat steps 8–10.

Activity B. Newton’s Second Law: Hanging Weight

1. Mass the cart and record this value on the Newton’s Second Law worksheet.
2. Measure the distance between the two tape marks on the table surface (placed by the instructor), and record the distance in meters on the worksheet.
3. Obtain the 130-cm length of string, with loop knots on both ends.
4. Obtain the plastic bag and 5-g mass. Place the mass in the bag and weigh the combination. Record this value on the worksheet.
5. Attach the bag to one end of the string using a looping knot (see Figure 4). 
   {12144_Procedure_Figure_4}
6. Attach the other end of the string to the hook on the cart.
7. Set the cart at the beginning tape mark, and line the string up over the top of the pulley (see Figure 4). Note: Have one partner hold the cart in place until both partners are ready to take data.
8. When ready, have one partner release the car while the other partner uses a stopwatch to measure the time it takes to reach the end tape mark. Note: Ensure that someone is ready to catch the cart before it is pulled off the table.
9. Record the time in seconds on the worksheet.
10. Repeat steps 4–9 once more and average the results.
11. Increase the hanging mass by increments of 5 g, and repeat steps 4–10 twice for each new mass combination. Record all data in the data table. Note: Replace the 5-g mass with a 10-g mass for the second set of trials and then use the 5-g and 10-g masses together for a total of 15 g for the third set.

Activity C. Newton’s Third Law: Balloon Rockets

1. Obtain a straw. Use scissors to cut it to three inches long.
2. Detach one end of the fishing line attached to the wall, and slide the straw piece onto the string.
3. Reattach the string, and slide the straw piece toward the fixed end of the fishing line.
4. Obtain a long, thin balloon. Carefully blow up the balloon to stretch it out, and then allow it to deflate.
5. Place two 5-cm pieces of masking tape on the straw piece. Place the midpoint of the tape on the straw so the tape ends extend equally from each side of the straw (see Figure 5).
   {12144_Procedure_Figure_5}
6. Inflate the balloon to about ¾-full. Pinch the end of the balloon closed with fingers and twist it several times to seal the balloon. Use a clothespin to clamp the opening closed.
7. Tape the inflated balloon to the straw piece, making sure the balloon is balanced, the straw piece is lined up along the string and is not crooked, and the balloon is aimed down the string towards to opposite wall. Continue to pinch the opening closed with the clothespin so no air can escape.
8. Hold the balloon near the wall and line up the balloon to “shoot” straight down the fishing line.
9. When the balloon is in position, release the balloon opening. Do not give the balloon any extra push.
10. Launch results: How far did the balloon travel? Did it make it all the way to the opposite wall? If not, what modifications need to be made to achieve the goal of rocketing to the opposite wall? In a data table, record the results of the launch, the distance the rocket traveled, and any sources of problems and corrective action that is required.
11. If the rocket did not travel all the way across the room, make necessary modifications and repeat steps 3–10. It is best to remove the original balloon and tape from the straw piece, and then use new tape.
12. Repeat step 11 until the balloon rocket reaches its goal of traveling across the classroom on the fishing line or as time permits.

Student Worksheet PDF

12144_Student1.pdf