Pulling
a Car
A laboratory experiment from the
Little Shop of Physics at
Colorado State University
CMMAP
Reach for the sky.
Grade Level
• Activities designed to address
Kindergarten and 2nd Grade
Standards
Science Focus
• Inertia
• Pulling
• Friction
Time Required
• 20 minutes if other adults are
helping you
• 10 minutes for Extension 1
Unbelievable as it seems, this teacher is changing
the car’s inertia. The car was at rest, but the
teacher applies a constant force on the rope, and
the car starts to move!
Overview
In this exciting, surprising, and dramatic
demonstration, a teacher pulls a car several feet,
changing its inertia, and amazing all the students!
Theory
Inertia means that objects continue to do what they are
doing. If an object is moving, it will continue to move.
If an object is sitting still, it will continue to do so. If
you want to make something move, you need to apply a
force. A force is also needed when an object in motion
slows down or stops.
In the last activity, students applied a steady force to
one hover disk, and then three. They discovered that
the object with more mass (and thus more inertia)
didn’t change motion as quickly.
Necessary materials:
• A car
• An area with a level surface, no slope,
and large enough for a car to be pulled
for at least 10 to 20 feet
• A long rope attached to the front of the
car
• 1 super hero costume (Optional)
Extension 1:
• Bingo marker device
• Timing device - seconds, beats, claps,
etc.
You can understand this using Newton’s Second Law, a = F m.
The mass of the car we used in this lesson was approximately 40 times that of the teacher in the photo
above. But she was able to move the car.
The unit of force in the English system is pounds—the same unit as your bathroom scale measures.
Weight is a force, a measure of how hard the earth pulls on you.
The car in the photo weighs about 4,000 pounds. The friction force impeding its motion is about 40
pounds. If someone pulls forward on the car with a force that’s bigger than 40 pounds (easy enough!) the
forward force exceeds the friction force, and there’s a small net forward force.
It’s a (relatively) small force and a very large mass, so the rate of change will be small. But the speed
does change, albeit slowly!
Doing the Experiment
Preparation before you begin:
You will want at least two other adults or more to assist you with this activity. One person will need to
drive the car to a level area that is large enough to pull a car several feet. Once in the location, be sure to
put on the emergency brake and turn off the car. Attach the rope in two places at the front of the car, and
to a solid part of the frame (not the bumper). The car we used had two tie on areas, one on each side of
the front of the car. Tie a secure knot on both ends of the rope and pull the rope forward so it forms an
upside down V or U shape. The person planning to pull the car should stand on the inside of the rope
(between the car and the rope) at the narrow point of the V or the curved shape of the U.
Now begin the lesson:
Tell your students that they are going to see if someone can pull a car. They will probably be quite vocal
about this! Remind them that they just did an activity that taught them about the mass of an object and its
inertia. If something is sitting still, you have to apply a force to make it change its motion. If it is moving,
you have to apply a force to make it stop. Ask what happened when they used a fan to make a force on
one hover disk. It moved across the floor. Ask what happened when they tried to use the same force on
the three hover disks. They did move, but very slowly at first.
Have one or two adults in charge of your students and bring them outside. Have the students stand a
safe distance away from the car and to the side of the car so they can see when it is pulled by them.
Once you are ready, have the person pulling the car and the driver assume positions. The driver will either
turn on the car or turn the key far enough in the ignition, that the steering wheel will turn during the
activity. Put the car in neutral. Once the person pulling the car is ready, the driver will release the
emergency brake and the car will start slowly, slowly moving forward. If it is pulled over a distance with
a steady force, it will keep speeding up like the propeller car. After the car is pulled for a few feet, the
driver should apply the brake, so the forward motion stops.
Pull your students together and discuss what they observed about the motion of the car. They most likely
will have many questions. The car is too heavy. How could the adult pull the car? The force the adult used
would be similar to what force it would take to lift a small second grade student. How could the car start
going faster? Just one person was pulling it. The person pulling the car kept using a steady force and as
the car changed its inertia from a resting object to a moving one, the steady force kept changing the car’s
speed, going faster and faster.
Extension 1:
You may want to have another adult help you with this demonstration, and they could mark the level area
with the bingo marker. Be sure to make a mark every second near the front tire of the car. Observe the
marks with your class and discuss.
Extension 2:
Do a thought experiment with your students. Ask them how the car would move if one person was pulling
the car from the front, and another person, using the same steady force, was pulling the car from the back
at the same time.
Extension 3:
Brainstorm what things have just a little inertia, so it would be easy to change their motion.
(Dandelion seeds, mosquitoes, beads, etc.)
Now brainstorm some things that have a big inertia, so it would be more difficult to change their motion.
(Boulders, a school building, elephants, etc.)
Summing Up:
Doing a demonstration as dramatic as this will help students transfer their learning about force, friction,
and inertia to many different situations. They will hopefully remember this activity for a very long time.
For More Information
CMMAP, the Center for Multi-Scale Modeling of Atmospheric Processes: http://cmmap.colostate.edu
Little Shop of Physics: http://littleshop.physics.colostate.edu