# Science and Children : Home Connections

## Objects in Motion

2/27/2004 - Kathleen Damonte

You have probably seen a magician pull a tablecloth off a table without upsetting any of the table settings. Hopefully you have never tried this trick yourself with your family’s good dishes and glasses! The magician performing this trick wasn’t really using magic; he or she just had a good understanding of a scientific principle called inertia. This activity will help you understand inertia and how that magic trick works.

One thing scientists study is how objects move. A famous scientist named Sir Isaac Newton (1642–1727) spent a lot of time observing objects in motion and came up with three laws that describe how things move. This explanation only deals with the first of his three laws of motion.

Newton’s First Law of Motion says that moving objects will continue moving in the same direction at the same speed. It also says that objects at rest will remain at rest unless an outside force acts on them.

 Explore thetopic of Inertia

The idea of an object resisting change in its motion is called inertia. All objects have inertia. The larger the mass of an object, the more inertia it has. It is much harder to get a large boulder moving than a tiny pebble. The boulder has a greater amount of mass and therefore more inertia than the pebble.

The Magic Trick Explained

The plates, glasses, and silverware on the table in the magic trick all have inertia. Pulling the tablecloth is an outside force acting on those objects. If the outside force acts quickly enough, the inertia of the objects keeps them from moving. In order for the trick to be successful, the tablecloth has to be yanked out very quickly while the inertia of the objects on top of the table keeps them in place. In addition, the “slippery” tablecloth lessens the force. Imagine if the magician used sand paper instead of a tablecloth; he or she then wouldn’t be able to pull it out fast enough to avoid causing the items on the table to move.

Seat Belts and Inertia

When the driver of a car puts on the brakes, the car slows down. The people in the car, however, will continue at the same speed the car was going because of inertia, the tendency to stay in motion. The seat belt acts as a force to keep the people in place. You feel the seat belt pull against you when you stop because your body wants to continue moving, but the seatbelt prevents you from doing so. Seat belts help keep people from getting hurt when a car comes to a sudden stop.

Experiment with the inertia of objects in the two activities below. You can even try the tablecloth trick without getting in trouble!

To learn more about Newton’s Laws of Motion and try other activities visit:
Newton’s First Law and Inertia
www.at-bristol.org.uk/Newton/experiment. htm#1st_law
Newton’s Laws of Motion
brainpop.com/science/forces/newton/index.weml

Soda Bottle Magic Trick

Materials:

• Empty plastic soda bottle
• Water
• One strip of thin cloth

Time needed:
20 minutes

Safety Warning:
Do not try this activity with a glass bottle. The bottle may fall and break.

Directions:
1. Fill the soda bottle three-quarters of the way with water. Tightly screw on the top.
2. Place a strip of thin cloth near the edge of a flat surface, such as a table, and put the soda bottle on top of the cloth.
3. Yank the cloth out from underneath the bottle very quickly. Keep your hand at the same level as the top of the table when you pull out the cloth. You might need to try this several times until you can do it quickly enough for the bottle not to move.

Explanation: The inertia and mass of the soda bottle filled with water kept the bottle in place as the slippery cloth was quickly pulled out from underneath it.

Questions:
1. What was the outside force acting in this activity?
2. Would this activity work as well with an empty soda bottle? Try it.
3. What happens if you pull the cloth slowly?

Coin Challenge

Materials:

• 5 nickels
• One plastic knife
*for better results, try this using a butter knife with adult supervision.

Time needed:
10 minutes

Directions:
1. Stack five nickels in a pile on top of each other.
2. Knock the bottom coin out from underneath the other coins by hitting it with the thin edge of the knife. You will need to do this quickly.
3. You may not be successful in knocking just the bottom coin out on the first try. You may need to practice several times before you get it.

Explanation: The bottom coin will move because the force of the knife acts on it. Inertia will keep the other coins in the stack above it from moving with the bottom coin. The other coins will drop straight down to form a stack of four.

Questions:
1. What was the outside force acting in this activity?
2. What type of coin will work best for this activity (dime, penny, nickel, or quarter)? Why do you think so? Test your theory.

Kathleen Damonte teaches seventh-grade science at Julius West Middle School in Rockville, Maryland.