THE PHYSICS OF MOVING THINGS (AND NOT A MOMENTUM TOO SOON!)

Grades 7 - 9

Momentum is the fundamental principle behind anything that moves. From the large scale motions of the planets to the twisting and turning of an acrobat, the rules of momentum are continuously obeyed. In this introductory lesson involving motion, both linear and angular momentum are explained in a non-mathematical way.

Through the use of segmented video and stimulating hands-on activities, students will become aware of the important properties of momentum and learn to appreciate its significance in a world of moving things. This lesson is designed to take approximately 45 minutes.

"Bill Nye, the Science Guy: "Momentum"
Newton's Apple: "Angular Motion"

Students will be able to:

• list three examples of momentum found in their local environment;
• describe the importance of mass and velocity on momentum;
• determine what is necessary to produce the greatest amount of momentum within a particular system;
• define angular momentum
• explain how a figure skater can develop a rapid spin on ice;
• give three examples of how angular momentum is used in sports.

One each per pair of students:

• 1 basketball
• 1 tennis or ping pong ball
• 1 12" wooden ruler
• 1 quarter and 1 dime or similar sized washers for demonstrations
• 1 ballistics car (available from most science supply houses)

For demonstration

• 1 rotational inertia wheel
• 1 rotational inertia suitcase (optional) - directions enclosed
• 1 rotating stool or platform
• 1 set of dumbbell weights (approximately 5 lbs. each)
• 1 activity sheet per student

• momentum
• mass
• velocity
• rotation
• radius
• conservation
• axis

Give student teams the basketball and tennis or ping pong ball. Direct them to bounce these objects aw few times to determine which object rebounds the best, elicit responses. Now, direct the teams to place the ping pong ball on top of the basketball and drop them together (prepare for flying objects!). Have students write a brief summary of what just happened and list possible explanations. Use this time to collect the objects.

Ask teams to read their hypotheses. Bring up the term momentum here if students haven't already considered it in their explanations.

First Segment

The emphasis of this portion of the lesson is on linear momentum. This section will provide a god opportunity to introduce the basic physical concepts related to j=objects moving in a straight line and the transfer of moving energy from one object to another. To give students a specific focus for viewing, direct them to watch this portion of the video carefully and to listen for the two properties of momentum. Ask them to write these down in the space provided on their worksheets. [Properties are speed (velocity) and weight (mass)]

Second Segment

Tell students to play close attention to the next portion of the video. In this segment a ballistic car is demonstrated and the reason for behavior of the steel ball is explained. To give students a specific focus for viewing, tell them to carefully watch for the demonstration using a moving model car and a steel ball.

Third Segment

CUE Newton's Apple: "Angular Momentum" (last segment in the episode). Now that the concept of angular (circular) momentum has been introduced, the next portion of this lesson will show how angular momentum is conserved. It will also show how the conservation of angular momentum is used in sports, specifically in the sport of figure skating. In this section of video, Olympic skating champion Scott Hamilton will demonstrate how the conservation of angular momentum is used to execute many of the stunts in figure skating.

To give students a specific focus for viewing, tell them to watch the next short section of video and listen carefully to the explanation of why Scott Hamilton can spin so fast on ice. Have students list the term on their Activity Sheets.

First Segment

START the "Bill Nye, the Science Guy" tape right after the sign for "Bob's Rocks" appears at the beginning of the program. PAUSE after the words "The faster something s going, the more it weighs, the more mass it has, the more momentum it has!" To check for comprehension, ask students to list the most important properties involved in momentum.

Write the symbols for these properties on the blackboard.

(M=Mass) the term weight is used to simply this concept

(V=Velocity) Speed in a particular direction

Momentum is a function of Mass times Velocity.

Momentum = MV

Now the class will apply this knowledge to the problem of the bouncing ping pong ball. In the next segment of the video, the transfer of momentum is explained. To continue their focus, as students to watch for an explanation of why the smaller ball went flying.

PLAY. STOP after the words "The bigger an object and the faster it goes, the more momentum it will transfer!" Check for understanding by asking for an explanation of what happened to the small ball in the previous activity. The next activity will help if the students still have difficulty with this concept.

Second Segment

PLAY from "this little ball has weight and when it's moving it has momentum."

STOP at the end of the segment, after the words,"...the ball went up an came down without changing its momentum."

Third Segment

START after "Science of the Rich and Famous".

PAUSE after "...angular momentum, the spinning law of physics!" To check for understanding, have students list the term that applies to the "spinning law of physics." RESUME PLAY and PAUSE after "...once it starts rotating, it wants to keep rotating in the same direction, with the same speed!"

To check for comprehension, ask students if they noticed something wrong with the last statement. REWIND to the beginning of the section and REPLAY if students missed the concluding statement. (In the video, Scott Hamilton's speed increases dramatically.)

RESUME PLAY at "...but your speed did change."

PAUSE after "...angular momentum stayed the same, it has to, it's the law!"

Ask students whether this change in spin rate actually violates their notion of angular momentum.

RESUME PLAY and PAUSE after "...hey, you're not one of these!"

It is important to have your students "feel" the law of conservation of angular momentum. Have one student sit on the rotation stool and give him/her the dumbbells to hold. Starting with the dumbbells held away from body, initiate a slow spin and then have the student bring the dumbbells close to the body. The results are immediate. Have the students take turns and have them alternate the position of the dumbbells to speed up and slow down. Ask the students to consider themselves as part of a large wheel. What measurement would be changed if the dumbbells represented the tire portion of the wheel? (Answer: the radius)

Now go back to the conservation of momentum expression (mv=mv). Ask students to add the ingredient that was changing (r for radius) to each side of the conservation of momentum expression. The expression now becomes:

mvr (before) = mvr (after)

With the dumbbell's mass remaining constant, the only change in the expression is the velocity and radius. When the initial rotation (momentum)_ is given, the radius is large and the spin is low. This can be expressed as the following:

mvR

After the students bring the dumbbells close to the body, of the spin axis, the radius gets smaller and the visible effect is a dramatic increase in velocity. This part of the expression can be written as:

mVr

In keeping with the conservation of angular momentum, the expression can be stated as:

mvR = mVr

Thus, the reason for the increased spin when the dumbbells are brought close to the spinning body is explained by the Law of Conservation of Angular Momentum.

In the final section of the video, Scott Hamilton demonstrates how his spins on ice are a function of the changing radius of his body. Have the students pay close attention to each of the spins performed and see if they can pick out which part of the body is being used to change the radius.

RESUME PLAY. STOP after "...once again, angular momentum wins the day!" Check for comprehension by having students list at least three other applications of angular momentum in sports.

To summarize, ask students to describe, in their own words, the meaning of momentum. Have them give examples of things around them with large momentums. (E.g.- the packed school bus that they took to school has a lot of momentum!) When momentum is started it wants to keep going. What examples can students cite for this principle? (Ask students if they've ever seen a speed skater fall?)_ Lastly, have the students write a summary of Scott Hamilton initiates any one of the spins shown in the video.

First Segment

Explain the activity and direct students to do the activity in groups or pairs.

Place a ruler horizontally on a flat surface. Put the dime face down in contact with one end of the ruler. Now, slide the quarter sharply up against the other end of the ruler. Momentum is transferred from the moving quarter to the stationary rulerdime system. Notice that the dime will move much further than the ruler. It should be mentioned here that the amount of momentum given to the quarter should be approximately equal tot he total momentum transferred tot he dime, minus some loss due to friction. It might be helpful to use letter size to demonstrate the magnitude of the behavior of the dime. The following is an example:

mv (before) = mv (after) for two objects with equal mass

M (quarter) v (quarter = m (dime) V (dime)

Have students reverse the process to see how much the quarter will move when the dime is projected against the ruler.

Ask students to predict the behavior of the objects before performing the experiment. When energy loss due to friction is considered, the momentum of the quarter is transferred completely to the dime. When this happens, momentum is said to be conserved.

Second Segment

1. Allow students to explore this concept further by actually using a ballistic car (easy to follow instructions come with the car). In the previous section of this lesson, emphasis was placed on the general principles behind momentum. In the next section, the emphasis will be placed on momentum that takes a more or less circular path. This concept is known as angular momentum.

2. To introduce this topic a fairly simple wheel, with some modifications, will be used. The bicycle wheel is modified by placing handles on each side of the axle. One of these handles should have a hole drilled through it so that a small rope can be tied to it. In addition, the inner tube of the bicycle wheel has been filled with sand to add more mass to the portion of the wheel furthest from the axis.

Holding the wheel with one hand on the rope ad the other on the rim of the wheel, challenge the students to balance the wheel in an upright (vertical) position using only the rope for support. Give them a few moments to try to figure this problem out. The concept in this balancing act involves momentum similar to that of the ball in the ballistics car. Things that are in motion tend to continue that motion. Some students may realize that the wheel must be moving for the "trick" to work.

Before the students' frustration level becomes critical, have one student hold the handle and rope while you spin the wheel. Have the student slowly and carefully release the handle, while continuing to hold the rope. As long as the wheel is spinning, the wheel remains vertical. Allow the students to pas the wheel around so they can each feel the momentum.

Ask students to write a brief paragraph explaining why the "trick" would work only while the wheel was in motion. Given a certain amount of momentum - speed in a particular direction and mass - the wheel continued to travel the same way. Ask them to list any important applications of this motion.

3. Enlist the aid of your school's technology teacher to fashion a similar wheel inside a suitcase. Using a crank, or an electric drill, give the wheel inside the suitcase a rapid spin. Have an unsuspecting student carry the suitcase to a point in the room that requires a turn in direction. The result s hilarious! The cause is angular momentum.

Third Segment

• Have your students work through the following questions:
  
  Which has more momentum? A bus at rest, or the student running to catch it?
  
  Which has greater momentum when they move at the same speed? A truck or a student running beside it?
  
  
• Have students analyze this statement put out by a national safety committee. "When you are driving in your car and not wearing a seatbelt, your face is tailgating your windshield."
• To bring a huge oil tanker to a stop, its engines are usually cut off at least twelve miles from port. Why is it so hard to stop or, for that matter, turn an oil tanker?
• Why is it easier to sit on a bike that is moving rather than one that is standing still?
• Why do tall animals usually walk with a slower stride than animals with shorter legs?
• Which moves faster on a carnival merry-go-round - the inside horse or the outside horse?
• When a spinning skater spreads his/her arms outward, does the speed at which he/she spins decrease? Does his/her angular momentum decrease?
• Explain how a gyro is able to keep its balance.

• Take a field trip to an amusement park. Student pairs (or groups) are to locate and describe examples of momentum and angular momentum used for rides and games. For each right answer the teams get 1 point, 2 points if their answer is unique and no other team has it; -1 point if the answer is wrong.

MATH:
Compute the changes in radius vs/ velocity for the turntable experiment.

SOCIAL STUDIES:
Research an application of these physics principles and create a display showing how physics is used in invention.

Master Teacher: Gib Brown
Mountain Lake Public Broadcasting/ Plattsburgh, NY