Build a Gauss Rifle!
AbstractWhat can you do with magnets and ball bearings that makes a lot of noise? Why, build a magnetic linear accelerator, called a Gauss rifle, of course! Now, this magnetic accelerator is not a weapon, but a way for you to learn a lot more about physics concepts, like momentum. In this physics science project, you will investigate how far a ball bearing launched by a Gauss rifle will fly, depending on how many magnetic acceleration stages are in the setup and the ball bearing's initial velocity. This science project makes for a really cool visual demonstration.
Terik Daly and Michelle Maranowski, PhD, Science Buddies
This science project is based upon the following Science Buddies Clever Scientist Award winning project: Agajanian, L. (2010). Gauss Rifle Magnetic Linear Accelerator.
- Elmer's® is a registered trademark of Elmer's Products, Inc.
Recommended Project Supplies
In this science project, you will build a Gauss rifle, also called a magnetic linear accelerator, and determine the effect the number of magnet stages has on the flight distance and velocity of ball bearings.
Have you ever played with magnets, sticking them together and pulling them apart? Maybe you have seen how magnets attract paper clips and pins? Have you ever felt that tug when you tried to remove something from the magnet? If you have, then you have experienced the effects of magnetic fields. A magnetic field creates a force (a push or a pull) on other magnets or magnetic materials in the field. This force has a magnitude (meaning it can be strong or weak) and a direction. It can be attractive (meaning it attracts or pulls on magnetic materials or other magnets) or repulsive (meaning it repels or pushes away other magnets). To learn more about magnets, check out the Science Buddies Electricity, Magnetism, & Electromagnetism Tutorial.
You might be thinking, "Great, magnets have magnetic fields, so what? What kinds of things can I do with magnets, other than just sticking them to stuff?" Well, it turns out that magnetic fields are used in all kinds of things, like circuits, motors, compasses, and MRI equipment. In this project, you will see a magnetic field at work and use it to make a neat toy called a Gauss rifle. This setup is named after Carl Gauss, who discovered the equations that describe magnetic accelerators like this one. Don't worry; the accelerator is not used to shoot things but to demonstrate physics principles like magnetism, and others that are discussed below.
A Gauss rifle is made up of at least one magnet stage, but it could have several successive magnet stages. A magnet stage is a magnet with several ball bearings touching it on one side. The first magnet stage in this project will have another ball bearing on its other side, which we will call the "starter" ball. To get the Gauss rifle to launch a ball bearing, the starter ball rolls toward the first magnet stage and then hits the first magnet. This starts a chain reaction that ends with the last ball bearing being ejected from the launcher. Watch the video below to see a Gauss rifle in action.
So how does a Gauss rifle work? When you give the starter ball a slight nudge, it moves forward. As it gets closer to the neodymium magnet, the magnetic force pulls the ball toward the magnet. The ball bearing accelerates (meaning it gains speed) toward the magnet due to the magnetic force acting on it. When the starter ball bearing hits the magnet, it transfers its energy and momentum (a quantity that depends on both mass and velocity) to the magnet. Without moving, the magnet then transfers the momentum from the starter ball to the first ball bearing on opposite side of the magnet. This concept is called conservation of momentum. The ball bearing then hits the ball bearing next to it and momentum keeps getting transferred until the last ball bearing launches off. In a launcher with more than one magnet stage, the magnetic field from the second magnet attracts this last ball bearing, the ball bearing accelerates toward the second magnet, and the process starts again. The only difference is that the ball bearing from the previous magnet stage gives the second magnet more energy than the ball bearing that started the chain reaction.
So how fast is the final ball bearing going when it leaves the launcher? What is its velocity? How far will it go? In this project, you will answer these questions and look at how the velocity of the final ball bearing depends on the number of magnet stages. The distance that the ball travels will depend on how fast the ball was going when it was launched, as well as on earth's gravitational force—the force that pulls the ball towards the ground—which will eventually pull the ball down. The Gauss rifle is not only a great toy for demonstration, but also a cool way to gain a whole lot of physics knowledge.
Terms and Concepts
- Magnetic field
- Conservation of momentum
- Gravitational force
- Square root
- What is a magnet and what are some magnetic materials?
- What is the difference between velocity and acceleration?
- What is conservation of momentum and what are Newton's laws of motion?
These sources go into more detail about the physics of horizontally launched projectiles, the kind of physics that will help you understand how far a ball bearing launched by the Gauss rifle will travel.
- Build a Gauss Rifle Kit (1). Includes:
- Wooden dowels, 16 inches long and ⅜ inch diameter (2)
- Neodymium magnets, ½ inch thick and ½ inch diameter (4)
- Nickel-plated steel balls, ½ inch in diameter (10)
- Elmer's® Carpenter's Wood Glue
- Clear tape, ½ inch in diameter
- Plastic box, approximately the size of a shoe box
- Sand, 2 cups
- Tape measure (metric)
- Table in front of which there is ample room and will not be a lot of foot traffic
- Calculator with square root function
- Lab notebook
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Neodymium magnets are very strong. Adult supervision is recommended when using them. Do not let the magnets slam together. They may pinch your fingers or crack. Keep them away from small children, pets, credit cards, and pacemakers.
Building the Slide for the Rifle
- You are going to use the two wooden dowels to make a slide on which the magnets and balls will sit on and move down.
- Place the dowels evenly next to each other and make sure the ends are flush (lined up). Use clear tape to tape the dowels together at both ends. The tape will temporarily hold them together.
- Place the taped dowels on the table and then carefully glue them together with wood glue. Try to prevent the glue from leaking through to the other side.
- Let the glue dry (this may take a few hours, depending on the humidity) and then take the pieces of tape off.
Setting Up the Experiment
- Place the wood slide on the table with the glued side down. Put one neodymium magnet on the slide, toward one end. Place two ball bearings on one side of the magnets such that the last ball is at the end of slide. See Figure 1. This step is represented by the magnet and ball bearings on right edge of the table in the figure.
A Gauss rifle has two parts, one is a magnet that attracts two metal balls on one end and the other is a single metal ball that rolls towards the magnet on the side opposite of the other two metal balls. When the single metal ball strikes the magnet, the force is transferred into the two metals balls on the opposite end of the magnet. This force will cause the second metal ball to shoot out in the same direction that the single metal ball was originally rolling.
Figure 1. A one-magnet stage Gauss rifle.
Now wrap a piece of tape around the magnet so that the ends of the tape wrap around the wood slide. Remove the ball bearings.
- Troubleshooting Tip: If you have problems with the magnet pulling out from the tape or with the tape breaking, then add a second layer of tape for reinforcement.
- Place the wood slide on the table so that the end of the slide is flush with the end of the table. See Figure 1.
- Place two ball bearings on one side of the magnet, at the end of the slide (where they were located in step 1).
- Pour the sand in the plastic box and smooth it out so that the sand is approximately level. Place the box on the floor a couple of feet away from the edge of the table.
- Place one ball bearing on the other side of the magnet, about 5 centimeters (cm) from the magnet. This is the starter ball. See Figure 2.
Figure 2. A one-stage Gauss rifle. Note the two ball bearings touching the magnet stage at the end of the slide (as described in step 4) and the single ball bearing (the starter ball) 5 cm behind the magnet stage (as described in step 6).
- Measure the height of the table, in meters (m), on which the Gauss rifle is sitting. Record this value in your lab notebook.
Launching the One Magnet Stage Gauss Rifle
Practice launching the Gauss rifle so that you can place the plastic box at the correct distance for accurate measurements, as follows. Your experiment will look something like Figure 1.
- Lift the slide just a little to get the starter ball rolling toward the magnet. Keep an eye on the last ball and watch where it lands so that you know where to place the plastic box. Caution: Be sure there is nothing breakable and nobody in front of the setup before you begin testing.
- Place the box so that the ball will land approximately in the middle of the plastic box.
- Now make a data table, like the one below, in your lab notebook so you can record the data that you get from your experiments.
|Number of Magnet Stages||Trial||Distance the Ball Traveled (m)|
Replace the correct number of balls on either side of the magnet.
- Troubleshooting Tip: You may find it helpful to stabilize the magnet with one hand while repositioning the ball bearings. This keeps the tape holding the magnet to the wooden dowels from stretching or breaking as you reset the launcher. It is often easier to reset the launcher by sliding the ball bearings around the magnets instead of trying to pull them off the magnets.
Now lift the slide just a little to get the starter ball rolling toward the magnet. After the launched ball lands, take the tape measure and measure the horizontal distance from the edge of the table to the spot where the ball first landed in the box. Record this distance in meters (m) in the data table in your lab notebook.
- Troubleshooting Tip: Sometimes the ball bearing will roll or bounce after it first hits the sand. You want to measure the distance from the edge of the table to where the ball first hit the sand, not the distance from the edge of the table to where the ball finally comes to rest. See Figure 3.
Figure 3. In this case, the launched ball bearing landed in the crater at left, then bounced and slid to where it finally stopped at the right side of the box. The tape measure is positioned to start measuring from the circular crater at left—the place where the ball bearing first hit the sand.
- Retrieve the ball from the box, smooth the sand, and replace the ball in its original position on the slide.
- Repeat steps 4-5 four more times for a total of five trials. It is a good idea to repeat your experiments to make sure that your data is reproducible and accurate.
Launching a Multiple Stage Gauss Rifle
- You now have data for a one-magnet stage Gauss rifle. But what happens when you have more than one magnet stage?
Build a two-magnet stage Gauss rifle. Remove the ball bearings from the launcher. Place the second magnet stage 10 cm to the left of the first magnet stage (as measured from the front of the first magnet stage to the front of the second magnet stage) and tape it to the wood slide. Cut off any excess tape, if needed. See Figure 4.
- Troubleshooting Tip: Depending on the strength of your magnets, 10 cm between stages may not be enough space. If you have problems with the magnet stages pulling together (instead of staying separated) due to their magnetic attraction, simply increase the spacing between magnet stages. If you do change the spacing, be sure to use the same spacing between all of the magnet stages in your Gauss rifle.
Figure 4. The magnet stages are spaced 10 cm apart, as measured from the front of the first magnet stage (at right) and the front of the second magnet stage (at left).
- Now place two ball bearings on one side of each magnet stage. Place the starter ball 5 cm to the left of the second magnet. Figure 5 shows how to arrange the magnets and ball bearings.
Figure 5. A four-stage Gauss rifle with the "starter" ball bearing 5 cm to the left of the fourth stage. A two-stage launcher has two magnet stages, with the "starter" ball bearing 5 cm from the second stage. Similarly, a three-stage launcher has three magnet stages, with the "starter" ball bearing 5 cm from the third stage. In each case, the magnet stages are 10 cm apart from one another.
Practice launching the Gauss rifle so that you can place the plastic box at the correct distance for accurate measurements.
- Lift the slide just a little to get the starter ball rolling toward the magnet. Keep an eye on the launched ball and watch where it lands so you know where to place the plastic box.
- Place the box so that the launched ball will land approximately in the middle of the box.
- Retrieve the ball from the box, smooth the sand, reset the launcher, and replace the launched ball in its original position on the slide.
- Lift the slide a tiny amount to get the starter ball rolling toward the magnet. After the launched ball lands, take the tape measure and measure the horizontal distance from the edge of the table to the spot where the ball first landed in the box. Record this distance in meters (m) in the data table in your lab notebook.
- Repeat steps 5-6 four more times for a total of five trials. It is important to repeat your experiments to make sure that your data is reproducible and accurate.
- Repeat steps 2-7 for a three-magnet stage Gauss rifle. Remember to record all data in your data table in your lab notebook.
- Repeat steps 2-7 for a four-magnet stage Gauss rifle. Remember to record all data in your data table in your lab notebook.
Analyzing the Data
- Now review the data you collected in the previous two sections.
- Create a plot showing the relationship between distance traveled and the number of magnet stages.
Now use Equation 1 to calculate the
velocity at which the ball was launched from the wood slide (the Gauss rifle).
Record your results in a table, like the one shown below.
- Troubleshooting Tip: Make sure that you use consistent units in your calculations. If you measured the table height and distance traveled in units other than meters, you will need to convert those measurements to meters.
Velocity (m/s) = Horizontal distance between the table and the ball (m) X Square root of
(gravitational acceleration (m/s2) divided by [2 X height of the table (m)])
|Number of Magnet Stages||Trial||Distance the Ball Traveled (m)||Velocity of the Ejected Ball (m/s)|
- Average the data from the table above. Average the velocity for each magnet stage (1, 2, 3, or 4) over the five trials. Record your data in a table like the one shown below.
|Number of Magnet Stages||Average Velocity of the Ejected Ball (m/s)|
- Now make a plot of the average velocity dependent upon the number of magnet stages. Label the x-axis Magnet Stages and the y-axis Average Velocity.
- What do the plots that you made tell you? How is velocity affected by the increase in magnet stages?
For troubleshooting tips, please read our FAQ: Build a Gauss Rifle!.
Ask an Expert
- Buy additional components and incrementally increase the number of magnet stages until you get to eight magnet stages. Does the velocity increase or decrease with the increase in magnet stages?
- Try increasing the distance between magnet stages. How does this affect velocity?
- What happens if you use two magnets in each stage instead of one?
- Equation 1 does not account for the effect of air resistance. How do you think air resistance affects the distance and trajectory that the ball bearing will travel? Check out the references listed in the Bibliography for more information on how to derive equation 1.
Frequently Asked Questions (FAQ)
Single-Magnet Stage Rifle. If the tape broke while you were working with a single-magnet stage Gauss rifle, then it was probably from the stress of adjusting or resetting the ball bearings. Try reinforcing the tape by adding a second (or even a third) layer of tape to secure the magnets to the dowels. You can also use a longer piece of tape to wrap completely around the dowels and magnets. In addition, you can reduce the stress on the tape by sliding the ball bearings off the magnets, instead of pulling them straight off. Another way to decrease the stress on the tape is to hold the magnet with one hand while you remove and replace the ball bearings with the other hand.
Multi-Magnet Stage Rifle. If the tape broke while you were working with a multi-stage Gauss rifle, think about when the tape broke. Did it break while you were setting or resetting the Gauss rifle? Or did the tape break suddenly when you were not handling the rifle?
- It broke while I was setting or resetting the Gauss rifle. If the tape broke while you were setting or resetting the rifle, then it was probably from the stress of adjusting or moving the ball bearings. Try reinforcing the tape by adding a second (or even a third) layer of tape to secure the magnets to the dowels. You can also use a longer piece of tape to wrap all the way around the dowels and magnets. In addition, you can decrease the stress on the tape by sliding ball bearings off the magnets, instead of pulling them straight off. Another way to decrease the stress on the tape is to hold the magnet with one hand while you remove and replace the ball bearings with the other hand.
- It broke suddenly while the Gauss rifle was resting on the table. If the tape broke suddenly while the rifle was resting on the table, then the problem might be that the magnets are too close together, especially if they snapped together after the tape broke. Check to make sure the magnets are the correct distance apart (10 cm; see Figure 4 in the Experimental Procedure). If they are 10 cm away from each other, but you still have this problem, move the magnets slightly farther apart, retape them (using a couple of layers of tape for added strength), and try the experiment again.
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