Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. To make the slingshot cars, insert a dowel peg into each hole of the wood blocks. The pegs should fit very firmly. If desired, use a small amount of wood glue to secure the pegs in the holes.
2. Cut a 1.8-m length of string for each lab group.

Safety Precautions

Wear safety glasses or goggles during this experiment. Do not aim the rubber bands at anyone or allow the rubber stopper to be projected in the direction of other people. Please follow all laboratory safety guidelines.

Lab Hints

• Enough materials are provided in this kit for 8 groups of students. 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 calculations and questions may be completed the day after the lab.
• Making sure the scissors cut the string and are quickly pulled out of the way may take more than one or two practice trials. Provide extra string in case students need more practice or in the event a trial may need to be “scratched.” Alternatively, if time permits, allow students to perform five trials for each number of rubber bands and average the top three distances.

Teacher Tips

• Newton’s Laws of Motion may sometimes be challenging for students to grasp, and measuring acceleration without special equipment may be difficult. This activity is presented to captivate the natural curiosity of students and help them conceptually understand the laws that govern motion, rather than simply present mathematical problems in which students “plug in the numbers.”
• Students should understand that acceleration is any change in motion, including an increase or decrease in speed or a change in direction.
• The Diving Eggs Inertia Challenge—Newton’s First Law Demonstration Kit, available from Flinn Scientific (Catalog No. AP7419), is a fun activity to further explore Newton’s Laws.

Answers to Prelab Questions

1. For each action force described below, identify the reaction force. The first one has been done for you.
   1. Action force: Canoe paddle pushes water. Reaction force: Water pushes canoe paddle.
   2. Action force: Bat pushes baseball. Reaction force: Baseball pushes bat.
   3. Action force: Person pulls door handle. Reaction force: Door handle pulls person.
   4. Action force: Earth pulls apple downward. Reaction force: Apple pulls Earth upward.
2. Which of Newton’s Laws of Motion best explains the motion of each object described? Explain the reason for each choice.
   1. A skateboarder pushes back against the ground and the rider and skateboard are propelled forward.

      Newton’s Third Law states that for every action force there is an equal and opposite reaction force. The skateboarder pushes back on the ground and the ground pushes against the foot in the opposite direction, propelling the rider and board forward.

   2. A hockey puck is pushed with a hockey stick across the ice in a straight line and at a constant speed.

      Newton’s First Law of Motion states that an object in motion remains in motion unless acted on by an outside force. The force of the hockey stick pushing forward on the puck and the force of friction from the air and the ice cancel each other out.

   3. When a bowling ball hits the side of a bowling pin, the ball is only slightly deflected from its path, while the pin slides across the lane.

      According to Newton’s Second Law, an object of smaller mass will accelerate more than an object of greater mass when acted upon by the same magnitude of force. (According to Newton’s Third Law, the force of the ball on the pin is the same as the force of the pin on the ball, just in opposite directions.)

3. Read through the Procedure. Form a hypothesis to describe the relationship between the number of rubber bands and the distance the car travels by completing the following sentence. Explain your reasoning in terms of Newton’s Laws of Motion.

   “If the number of rubber bands is increased, then the distance the car travels will increase because
   adding more rubber bands will increase the force that causes the stopper and the car to accelerate. According to Newton’s Second Law, the force is directly proportional to the acceleration; therefore a greater force will result in a greater acceleration and the car will travel further.”

4. What safety precautions must be taken during this activity?

   Wear safety glasses or goggles during this experiment. Do not aim the rubber bands at anyone or allow the rubber stopper to be projected in the direction of other people. Follow all laboratory safety guidelines.

Sample Data

Answers to Questions

1. Calculate the average distance the slingshot car traveled with the force of one, two and three rubber bands, respectively.

   See Sample Data table.

For each action force described below, identify the reaction force in this activity.
1. Action force; String pulls rubber band. Reaction force: Rubber band pulls string.
2. Action force: Rubber band pulls pegs. Reaction force: Pegs pull rubber band.
3. Action force: Rubber band pushes stopper. Reaction force: Stopper pushes rubber band.
2. Identify at least two other action–reaction pairs of forces in this activity.

   Accept all reasonable answers. Examples include:
   Car pushes down on straws. Straws push up on car.
   Straws push down on table (floor). Table or floor pushes up on straws.
   String pulls peg. Peg pulls string.
   Hand pushes scissors. Scissors push hand.
   Stopper pushes down on car. Car pushes up on stopper.

3. In terms of Newton’s Laws of Motion, describe the sequence of events in this activity that caused the car to accelerate.

   The car remained at rest on the straws as long as no unbalanced forces were acting on it. Once the string was cut, the stretched rubber band returned to its original shape, pushing on the stopper. Even though the stopper pushed back on the rubber band with equal force, the force of the rubber band was enough to overcome friction between the stopper and the car and the stopper was ejected backward from the car. At the same time, the rubber band pulled the pegs of the car in the opposite direction and the car accelerated forward.

4. Why did the car stop moving?

   The force of friction from contact with the straws and air friction opposed the forward motion of the car.

5. The distance the rubber stopper traveled was not measured since it was prevented from being projected too far by the barrier. If no objects were in the way, would the distance the stopper traveled be greater, less or the same as the car? Explain your answer.

   The stopper would have traveled a greater distance than the car. The stopper has less mass than the car and mass is inversely proportional to acceleration.

6. Was your hypothesis from Prelab Question 4 supported by the data? Explain.

   Yes, increasing the number of rubber bands increased the force that caused the car to accelerate. Since the mass of the car did not change, the acceleration of the car increased, according to Newton’s Second Law of Motion.

7. The Voyager 1 spacecraft was launched from Earth on September 5, 1977. It is currently the most distant man-made object in space—more than 13 billion kilometers beyond the outermost planet of our Solar System and traveling at a speed of over 5 million km per year. How might Aristotle explain the behavior of the spacecraft? How would you explain it?

   Aristotle might explain that the natural motion of the spacecraft is toward the sky or outer space and its natural place of rest is somewhere in deep space.
   According to Newton’s First Law, after the spacecraft escaped the gravitational pull of the Earth and if no other unbalanced forces act on it, the spacecraft will continue at the same velocity (speed and direction) forever.

References

Hewitt, P.; Suchocki, J; Hewitt, L. Conceptual Physical Science—Explorations; Addison Wesley: San Francisco, 2003.

Rockets Educator Guide. NASA. http://www.nasa.gov/pdf/280754main_Rockets.Guide.pdf (accessed October 2011).

Introduction

A skateboarder pushes back off the ground and the board is propelled forward. A bowling ball strikes some pins and the pins scatter. A spacecraft is launched and escapes Earth’s gravity. The objects in these scenarios all have something in common—they act according to Newton’s Laws of Motion.

Concepts

• Newton’s laws of motion
• Inertia
• Action-reaction force pairs

Background

Until the end of the Middle Ages, the common understanding of the motion of objects was based on the ideas of the Greek philosopher Aristotle (384–322 B.C.). Aristotle believed that all objects had natural places of rest in the universe, and would follow a natural motion toward those states. For example, it was believed that rocks fell to Earth because their natural place was land and smoke rose to the sky because its natural place was the sky. If a force was applied to an object against its natural motion, such as picking up and throwing a rock, the rock would eventually fall and stop moving because its natural state on Earth was rest.

Isaac Newton (1642–1727), expanding on ideas presented earlier by Galileo Galilei (1564–1642) and others, described three laws of motion. These laws refuted the belief that objects have a natural motion or natural state of rest. According to Newton’s First Law of Motion, an object in motion will remain in motion unless acted upon by an unbalanced force. This means an object will travel in a straight line at a constant speed as long as no outside force is acting on it. If an object is at rest, the object tends to stay at rest unless acted upon by an external force. The tendency of an object to resist change in motion is called inertia. The reason a rock comes to rest on the ground after being thrown is not that its natural state is rest, but rather that other forces are acting upon it. The force of gravity from the Earth causes the rock to fall while friction from contact with the ground opposes the motion of the rock on the Earth, slowing the rock down until it comes to rest. In the absence of all forces—e.g., a rock thrown in the vacuum of space—the rock would remain in motion at a constant velocity.

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). The acceleration of an object is inversely proportional to its mass and directly proportional to the force needed to accelerate the object. 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 with its greater inertia.

Experiment Overview

The purpose of this experiment is to investigate Newton’s Laws of Motion. A device called a slingshot car will be set in motion by the force of stretched rubber bands that launches an object off the back of the car. The distance the car travels will be measured using the force of one, two, and three rubber bands, respectively.

Materials

Prelab Questions

1. For each action force described below, identify the reaction force. The first one has been done for you.
   1. Action force: Canoe paddle pushes water. Reaction force: Water pushes canoe paddle.
   2. Action force: Bat pushes baseball. Reaction force:
   3. Action force: Person pulls door handle. Reaction force:
   4. Action force: Earth pulls apple downward. Reaction force:
2. Which of Newton’s Laws of Motion best explains the motion of each object described? Explain the reason for each choice.
   1. A skateboarder pushes back against the ground and the rider and skateboard are propelled forward.
   2. A hockey puck moves across the ice in a straight line and at a constant speed.
   3. When a bowling ball hits the side of a bowling pin, the ball is only slightly deflected from its path, while the pin slides across the lane.
3. Read through the Procedure. Form a hypothesis to describe the relationship between the number of rubber bands and the distance the car travels by completing the following sentence. Explain your reasoning in terms of Newton’s Laws of Motion.

   “If the number of rubber bands is increased, then the distance the car travels will (decrease, remain the same, increase) because __________________________________________.”

4. What safety precautions must be taken during this activity?

Safety Precautions

Wear safety glasses or goggles during this experiment. Do not aim the rubber bands at anyone or allow the rubber stopper to be projected in the direction of other people. Please follow all laboratory safety guidelines.

Procedure

Preparation

1. Cut the string into twelve 15-cm pieces.
2. Form one piece of string into a loop by tying the string’s ends together with a knot as shown in Figure 1. Tie the knot close to the ends of the string to make the loop as large as possible.
   {12489_Preparation_Figure_1}
3. Repeat step 2 with the other 11 pieces of string.
4. Place a meter stick on the lab table or floor according to the teacher’s instructions.
5. Starting at the 5-cm mark, place 20 straws perpendicular to the meter stick every 5 cm. Use the meter stick as a guide to align the straws as parallel to each other as possible (see Figure 2).
   {12489_Preparation_Figure_2}
6. Once the straw “track” is in place, move the meter stick about 5 cm away from the track so the straws will be able to roll freely.
7. Place a barrier, such as a large book, about 20 cm away from the zero end of the meter stick. The barrier will prevent the rubber stopper from being projected too far away from the workstation. Note: A wall may also be used as a barrier.

Procedure

1. Place one loop of string over the single peg at the front end of the slingshot car.
2. Place one rubber band through the loop of string.
3. With the string in the middle of the rubber band, pull the ends of the rubber band toward the back of the car and slip one end over each of the two pegs (see Figure 3).
   {12489_Procedure_Figure_3}
4. Place the stopper narrow end down in the “V” made by the stretched rubber band. Push the stopper as far into the V as possible (see Figure 4).
   {12489_Procedure_Figure_4}
5. Adjust the level of the string and rubber band on the pegs so they are horizontal and the V of the rubber band is near the vertical center of the stopper.
6. Place the car with the stopper on top of the straws, with the back (two-peg) end at the zero end of the meter stick. Note: Three straws should be supporting the car. Adjust the position of the third straw if necessary so it supports the front end of the car.
7. Use a ruler or straight edge to make sure the back end of the car is lined up with the zero end of the meter stick. Adjust the position of the meter stick if necessary.
8. Using scissors, cut the string. This may take one or two practices to cut the string and pull the scissors away quickly so they do not interfere with the motion of the car. Tip: Hold the scissors vertically near the single peg and use just the tip of the scissors to cut the string, and then raise your hand up quickly (see Figure 5).
   {12489_Procedure_Figure_5}
9. Once the car stops rolling, measure how far the car traveled by noting the position of the back of the car with respect to the meter stick. Record the distance in cm in the first row of the data table for trial 1 on the Newton’s Laws worksheet.
10. Reposition the straws and meter stick as in steps 5 and 6 of the Preparation section.
11. Repeat steps 1–10 of the Procedure two more times, recording the distance the car traveled for trials 2 and 3, respectively. Use a new loop of string each time.
12. Repeat steps 1–11 with two rubber bands and again with three rubber bands. Make sure the rubber bands are close together on the pegs.

Student Worksheet PDF

12489_Student1.pdf