Making Physics Fun: Key Concepts, Classroom Activities, and Everyday Examples, Grades K?8

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Motion and Force

INERTIA AND NEWTON’S FIRST LAW OF MOTION

Concepts

All material objects are stubborn in two senses of the word. On the one hand, an object at rest (not moving at all) tends to remain at rest. Examples of this aspect of the concept of inertia are all around us—a book resting on a table, a house resting on the ground. On the other hand, an object moving in a straight line with constant speed (not increasing or decreasing its speed and/or turning) tends to remain moving in the same straight line with the same constant speed. Examples of this aspect of the concept of inertia are a little harder to identify—a spaceship drifting in interstellar space far from gravitating objects, a hockey puck moving freely across the ice. These two tendencies make up the concept of inertia. Historically, recognizing this duality was a key to unlocking our present understanding of motion. The scientists most responsible for its discovery and clarification, respectively, were Galileo (1564–1642) and Newton (1642–1727).

An object with a larger quantity of matter (say, a jumbo jet) possesses more of these tendencies—that is, possesses more inertia than an object with a lesser quantity of matter (say, a Ping-Pong ball or other table-tennis ball). In other words, both a jumbo jet and a Ping-Pong ball at rest tend to stay a rest, and both a jumbo jet and a Ping-Pong ball moving in a straight line with constant speed tend to remain doing that, but the jumbo jet has much more of these two tendencies than the Ping-Pong ball. The jumbo jet is said to have more mass, mass being the quantitative (numerical) measure of inertia.

Mass is also a measure of the amount of stuff (matter) that makes up an object. In other words, the mass value for an object gives a numerical value for the quantity of matter as well as for the tendency an object has to remain at rest and for the tendency an object has, if moving in a straight line with constant speed, to remain doing that. In the metric system of units (International System of Units), mass is measured in kilograms (kg).

Mass is a distinct concept from and more fundamental than weight. Weight is a measure used to quantify the strength of the gravitational force of attraction an object experiences when in the vicinity of the earth, moon, or any other astrophysical body. In other words, weight is a gravitational force and not a measure of inertia. For example, out in interstellar space, far away from any gravitating astrophysical body, a jumbo jet and a Ping-Pong ball would both be weightless (would have the same weight of zero) but would still possess greatly different amounts of inertia. In fact, each would have the same amount of inertia (each would have the same mass value) they had on earth or anywhere else, for that matter. Weight (gravitational pull) depends on locality; mass (inertia, stuff) does not. In the metric system (International System of Units), weight is measured in newtons (N). Near the surface of the earth, a 1.0-kg object weighs about 9.8 N or about 2.2 lbs.

The concept of inertia sets the default for motion. When there is no force on an object, or the net force on an object is zero (the concepts of force and net force will be examined later), the object will be moving in a straight line with constant speed (the speed could be zero and the object not moving). In other word, objects do not need any cause to keep moving in a straight line with constant speed. That is just the way it is. This is the default of nature. As we will see later, forces are responsible for changing motion (speeding up, slowing down, and/or turning) but are not needed for straight-line, constant-speed motion. If you could snap your fingers and turn off all the forces in the universe, you would see all the fundamental particles that are the building blocks of matter moving in straight lines with constant speeds.

Indeed, the concept of inertia is the backdrop against which all motion is played out. It is absolutely fundamental to our understanding of motion. Unfortunately, we live in a world dominated by electrical and gravitational forces, so it is natural that children possess the misconception that forces are needed for motion. Indeed, many children hold the misconceptions that constant motion requires a constant force, and that if an object is moving there is a force on it in the direction of its motion, with the amount of motion being proportional to the amount of force (Gunstone & Watts, 1985).

Forces are not required to maintain motion. Straight-line, constant-speed motion just happens on its own without the need for any external agents. Forces (external pushes and pulls) cause a change in motion. To be discussed later in more detail, forces cause objects to accelerate (speed up, slow down, and/or turn corners).

The concept of inertia is clearly articulated in Newton’s First Law of Motion, often called the Law of Inertia: When there is no force on an object or when the net force on an object is zero, the object will be either at rest or moving in a straight line with constant speed. The law also works in reverse: If an object is at rest or is moving with constant speed in a straight line, you can conclude either that no forces are being exerted on the object or that the net force on the object is zero.

Before closing on this topic, some interesting extensions of Newton’s First Law of Motion must be mentioned, not because they are developmentally appropriate for the student, but only because they should be of philosophical interest to the teacher. The fact that being at rest and being in motion in a straight line with constant speed are equivalent, in the sense that no forces are need for these types of motion, led Einstein to conclude that there is no way to know whether something is actually moving or not. You can feel only acceleration, motion that is changing. In other words, there is nothing you can do and no experiment you could perform to know whether you are absolutely at rest or moving in a straight line with constant speed. Consequently, Einstein argued that the absolute motion of an object through a fixed space is just a false construction of the human mind. There is no such thing as absolute motion and absolute space. This thinking led him to conclude that all motion must be relative. You can reference the motion of an object relative to some other object (e.g., the earth moves relative to the sun or a car moves relative to the road), but the absolute motion of an object in some fixed and absolute space does not exist. These ideas, along with some additional reasoning about the speed of electromagnetic radiation, eventually led to the knowledge that both space and time are also relative concepts and to the equivalence of mass and energy, cornerstones of Einstein’s Special Theory of Relativity.

Activities

Bowling Ball Versus Nerf Ball: You will need to obtain a bowling ball (more mass and inertia) and a Nerf ball or beach ball (with much less mass and inertia). Place each ball at rest on the ground in front of you. Hit each with a yardstick like you would hit a golf ball. Observe that the bowling ball does not move or moves very little (more tendency to stay at rest) and that the Nerf ball moves quickly away (less tendency to stay at rest). Next, get each ball rolling across the classroom floor and try to stop each with the yardstick. Observe that it is very easy to stop the Nerf ball (less tendency to keep moving) and very difficult to stopped the bowling ball (more tendency to keep moving).

Pendulums: You will need to obtain a variety of balls of various masses (from Styrofoam to steel) that can be hung from individual strings. You will also need to obtain a rubber-tipped dart gun (from your local toy store) and a drinking straw. With the pendulums hanging side by side, shoot each one in turn with the dart gun. Note the motions. The more massive ball will move the least and the least massive one the most. If you want, you can then weigh the balls on a scale to determine their masses to see if you ordered them properly. Instead of using the dart gun, you can push on the balls by blowing air at them through a straw. You can also set the pendulums swinging and try to stop them with a dart gun or by blowing air. The more massive pendulum should be harder to bring to rest. You might investigate other ways of providing a controlled push on the pendulums beside the dart gun and blowing air.

Ball on a Truck: You will need to locate a toy truck (or any flatbed cart), a metal ball (or any ball with a reasonable amount of mass), and a small piece of putty. Rest the ball in the center of the toy truck on a small piece of putty. The putty is used to fix the ball to the bed of the truck in order to keep it from initially rolling around inside the truck. It is important that the putty barely keeps the ball fixed to the truck. Move the truck forward quickly and observe that the ball wants to stay where it was (relative to the ground) and hits the back of the truck. An object at rest tends to stay a rest. Repeat, but now ease the ball and truck into motion, with the ball moving along with the truck and still stuck to the truck. Stop the truck suddenly by running it into some object (say, a wall). Notice that the ball has a tendency to keep moving in a straight line and hits the forward part of the truck. Repeat, but this time, as the truck and ball are moving together, turn the truck in a sharp circle. Notice that the ball still tends to move in a straight line and hits the side of the truck. You might try this activity using a smooth block (no putty needed) in the truck, instead of with a ball. You might also try scaling up these activities by using a push cart, the kind used for moving supplies around the school, and a larger ball (e.g., a basketball).

Marker Pen Person on a Cart: You will need a marker pen (like a Magic Marker) and a flatbed cart. Balance the marker pen vertically in the center of the cart so it simulates a person in a car. In other words, the maker pen is standing straight up, balanced on its bottom. With your hand, thrust the cart forward and watch the maker pen fall backward. Relative to the ground, the marker pen has a tendency to remain at rest and falls backward. Now repeat, but this time ease the cart into motion—making sure that the pen does not fall over—and run it into some object (like a wall). Observe now that the maker pen falls forward. A marker pen in motion tends to stay in motion. Like the previous activity, this activity is very useful for illustrating the role of seat belts, air bags, padded dashboards, headrests, and child safety seats in automobiles and other vehicles.

Ramp and Slider Investigation: You will need to collect a set of small balls of approximately the same diameter but with different masses. You will also need to construct a ramp (say, from a Hot Wheels track or with a grooved ruler) that runs down to the floor. Position a catcher-slider (anything, like a paper cup, that can catch a ball and slide across the floor will do) on the floor a few inches from the bottom of the ramp. From the same height on the track, release each ball in turn down the ramp and into the catcher-slider. Observe or measure how far the slider moves in each case. The more massive balls have more inertia and tend to keep on moving more than the less massive balls. Other versions of this activity would change the mass of the catcher-slider and/or change the surface over which the catcher-slider slides. Instead of using the slider, you can set up a row of cards (bent to stand up) in front of the ramp and see how many cards each ball can knock over.

Stack and Pull: You will need to cut a two-by-four into seven or more eight-inch-long blocks or locate some wood blocks of similar size. You will also need to sandpaper these blocks smooth. Attach a screw eyelet to one of the blocks in the middle of one of its narrow ends. Stack the blocks directly on top of each other, with the eyelet block at the bottom. Using a string attached to the lower block’s eyelet, pull that block quickly out from under the others. Be careful; do not pull the block so hard that it flies into your hand or body or something else in the room. As in the previous activity, the stack of blocks will tend to remain at rest and will not fall over and will remain stacked. You might try different arrangements, like standing some of the blocks vertically or inserting the eyelet block at different places in the stack. Blocks at rest tend to stay at rest.

Stack and Shoot: You will need to locate some circular-shaped playing pieces from a backgammon game. The pieces must be smooth and without ridges (ridged checkers will not work). Stack the pieces (10 or so) on top of each other on a smooth table or floor. Take another piece and, with a flick of a finger, send it across the table toward the bottom piece in the stack. A direct hit will flip out the bottom piece in the stack while the others will fall down in place and remained stacked. The stack of pieces tends to stay at rest. Another version of this activity places a small strip of paper under the stack. The paper can be pulled out quickly without toppling the stack.

Dollar Bill Trick: You will need to locate two plastic water bottles, both filled with water and sealed tightly. You can also use empty soda bottles filled with sand and sealed with a smooth cap. Take a dollar bill and place it between the two bottles balanced on each other, cap-to-cap. In other words, the dollar bill is sandwiched between the two bottles, which are precariously balancing cap-to-cap. One edge, not the middle of the dollar bill, should be between the bottles. Try to pull out the dollar bill quickly without upsetting the balance. One technique that works well is to hold the dollar bill straight out from the bottles with one hand and then quickly karate chop downward with the index finger of the other hand in the middle of the dollar. A bottle at rest tends to stay at rest.

Tablecloth Pull: You will need to locate a smooth piece of cloth (a piece of silk works well) to act as your tablecloth. There should be no seams around the edges. You will also need to locate a set of dinnerware (smooth plate, cup, etc.) and a very smooth table with a sharp edge. Place the tablecloth partway on the table, no more than a foot and a half from the table’s edge. Allow the rest of the tablecloth to hang to the floor. Again, make sure the table has a sharp edge and the tablecloth has no edge seam. Set the table. Grab the cloth with both hands in the middle of the cloth hanging to the floor. Pull it quickly downward. The table setting should remain at rest.

Jar and Screw Trick: For this barroom trick, you will need to locate a small-mouthed jar and a flathead screw. You also need to construct a plastic ring about six inches in diameter out of a half-inch strip of flexible plastic. Balance the plastic ring on top of the small-mouthed jar with the plane of the ring vertical. Then balance the screw on top of the ring directly above the mouth of the jar. Swipe out the ring with your finger and watch the screw fall into the jar. You must swipe it from the inside of the ring to the outside to make this work. A screw at rest tends to stay at rest.

Inertia Apparatus: You will need to buy this inexpensive inertia apparatus from a science supply house (Inertia Demonstrator, Sargent-Welch). A small piece of cardboard is placed on the apparatus and a ball is placed on top of the cardboard. A flipper mechanism flips the cardboard out from under the ball and the ball remains on the apparatus after the cardboard has been ejected. A ball at rest tends to stay at rest.

Coin and Finger Trick: This is another version of the previous activity, but does not require a commercial apparatus. You will need to cut out a one-and-a-half-inch square piece of thin, smooth cardboard. The thin cardboard from the back of a pad of paper works well. Place the piece of cardboard on the pad of one of your middle fingers. Balance a quarter on top of the cardboard and above your finger. With the middle finger of your other hand, flick out the piece of cardboard without hitting the quarter. The quarter should remain balanced on your finger. A quarter at rest tends to stay at rest. This activity may take a little practice to perfect.

Balloon Puck Fun: You need to buy some inexpensive balloon pucks from a science supply house (Puck Set, Carolina Science and Math). These plastic pucks can be lifted off a smooth surface by the balloon that directs air underneath the puck’s bottom surface. This provides a cushion of air that greatly reduces the friction between the puck and a smooth table or floor. As long as the balloon is exhausting air, the puck acts like a hovercraft. When launched across a smooth and level table or floor, the puck tends to glide in a straight line with relatively small deceleration. A puck in motion tends to stay in motion.

Toy Hovercraft: You can now buy a relatively inexpensive toy hovercraft from a science supply house (Kick Dis-Air Puck, Educational Innovations). When launched across a smooth level floor, the hovercraft will continue in a straight line at relatively constant speed.

Rotating Table and Pickle Play: You will need to buy a rotating table from a science supply house for these activities (Rotating Platform, Arbor Scientific). A lazy Susan might also work. You will also need a ball (steel ball works best) and a jar of pickles (with pickle juice). Place the ball on the table about halfway out from the center. Rotate the table with your hands back and forth and watch the ball attempt to stay in place. A ball at rest tends to stay at rest. Now attach the ball to the table with a small piece of putty a few inches from the edge of the table. Use just enough putty to keep the ball in place. Ease the table and ball into motion so that the ball is rotating along with the table. Stop the table abruptly and watch what happens to the ball. A ball in motion tends to say in motion and should move off the table in a straight line, tangent to the circle. Place a sealed jar of pickles (with pickle juice) at the center of the rotating table. You will need to remove a few of the pickles from the jar so the pickles in the jar can rotate around freely with the juice. Rotate the table back and forth with your hands. Observe what happens or doesn’t happen to the pickles inside the jar. Pickles at rest tend to stay at rest. Now rotate the table steadily in one direction until the pickles and juice start to rotate along with the jar and table. Once the pickles and pickle juice are in motion, reach out and grab the pickle jar with both hands and pick it off the table. Observe the subsequent motion of the pickles. Pickles in motion tend to stay in motion.

Raw Versus Hard-Boiled Eggs: You will need a raw egg (in its shell) and a hard-boiled egg (still in its shell). On a smooth table, spin the hardboiled egg on its side. Stop the egg momentarily by pushing down on it with a couple of fingers and then releasing. The hard-boiled egg will stop spinning. Now repeat with the raw egg. Once released, the raw egg inside the shell will continue to spin some more. The raw egg in the shell in motion tends to stay in motion. See Rotating Table and Pickle Play, above. This is a good way to tell a raw egg from a hard-boiled one when you are not sure which is which.

C-Shaped Track: You will need to construct a C-shaped wall or barrier on a smooth and flat table. The wall can be made out of a piece of cardboard bent into the shape of a C and fixed to the table with tape. A ball is then made to move against the C-shaped wall, following its curve. The activity is to predict, once the ball leaves the C-shaped path and when the force of the wall is no longer acting on the ball, what path it will follow on the table. Students can predict the path by drawing the anticipated path on a sheet of paper fixed to the table. When the net force on a ball is zero, the ball already in motion will continue in motion in a straight line at constant speed.

Everyday Examples

Parachute Brakes: Dragsters, some jet planes, and the Space Shuttle have parachutes to help them stop. Dragsters and aircraft have lots of inertia (large mass), and once in motion really tend to stay in motion.

Car Inertia: If you are in a car that suddenly accelerates from a stop, your head and body seem to move backward for a moment. An object at rest tends to stay at rest. When you are in a car that is moving along and suddenly decelerates, your head and body seem to move forward. An object in motion tends to stay in motion. When you are in a car that is turning a corner, your head and body seem to move toward the outer side of the car. An object moving in a straight line with constant speed tends to remain doing that.

Whiplash: When a parked car is suddenly hit from behind by another car, a passenger in the parked car can suffer whiplash, where the upper spine is bent backward. A head at rest tends to stay at rest. Headrests in cars are safety features used to avoid whiplash.

Seat Belts, Airbags, and Padded Dashboards: Seat belts, shoulder straps, air bags, and padded dashboards save lives. People in motion tend to stay in motion.

Airplane Flight: When you are inside an airplane that is cruising in a straight line with constant speed, you do not even know you are moving. In fact, there is nothing you can do inside the airplane to know whether or not you are moving. Everything inside the plane is in motion and no forces are needed to maintain the motion of these objects. But when an airplane accelerates during takeoff or decelerates during landing, please fasten your seat belt.

Airport Runways: Runways for large jumbo jets are very long. Jumbo jets are very massive with lots of inertia; thus it is difficult to get them up to takeoff speed and difficult to bring them to rest when landing.

Oil Tankers and Tugboats: An oil tanker full of oil is very massive, so it takes both a long time and distance for it to stop after the engines have been turned off. Once in the harbor, large ships require tugboats to help them stop, turn, and dock.

Magazine Pull: To get the bottom magazine out from under a large stack of magazines, you can apply a quick jerk to the bottom magazine without disturbing the magazines on the top. Magazines at rest tend to stay at rest.

Tightening a Hammerhead: One way to tighten a loose head of a hammer is by striking the base of the handle on a solid surface. A hammerhead in motion tends to stay in motion.

Slip-n-Slide: Sliding on a Slip-n-Slide gives a long ride. With the water film reducing the frictional force, your body in motion tends to stay in motion.

Curling: In the sport of curling, a stone is released on a flat surface of ice and tends to move in a straight line with constant speed.

Sand Off a Towel: You can get sand off a towel by snapping the towel. This is done by first getting the sand and towel in motion, and then stopping the towel suddenly. The sand stays in motion and flies off the towel.

Water Off a Dog’s Back: A dog shaking water off its back is much like getting sand off a towel. The dog gets the water and its back in motion, and then stops its back and there goes the water.

Water Off a Brush: Shaking a wet paintbrush to remove water is like the dog getting water off its back. Get the water and brush in motion, then stop the brush and there goes the water.

Ketchup Bottle: You can get ketchup out of a ketchup bottle by first moving the bottle and ketchup toward the food and then stopping the bottle. The ketchup keeps moving.

Ice Skating, In-Line Skating, and Skateboarding: Ice skating, in-line skating, and skateboarding are fun because once you get moving, you tend to keep on going.

Icy and Wet Roads: Driving on an icy or wet road can be dangerous because once you get moving, with less friction to stop you, you tend to keep moving in a straight line.

Out of Bounds: As you race to keep