Science activities for families to share
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.
Objects in Motion
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.
Keyword: Inertia
Newton’s First Law of Motion says that moving objects will continue moving in at www.scilinks.org
the same direction at the same speed. It also says that objects at rest will remain Enter code: SC030404
at rest unless an outside force acts on them.
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.
Teachers: Cut out this page and photocopy it for your students.
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 on the next page. You can even try the tablecloth
trick without getting in trouble!
(see reverse)
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
Kathleen Damonte teaches seventh-grade science at
Julius West Middle School in Rockville, Maryland.
March 2004 57
Soda Bottle Magic Trick
Directions:
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.
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.
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Science and Children