Investigating Forces
Third Grade Unit: 3 Lesson: 1
EXPLORE/EXPLAIN: Downhill All the Way
1.
Use the toy cars to demonstrate the following concept.
Ask the students to hold the car in the palm of their hand.
What do we call it when the car is not moving? The car is at rest.
Do you think there are any forces affecting the car, now? Accept all answers. Students may say
that they are pushing up on the car, and someone may suggest that a force is pulling down on the
car.

Ask one of the students to demonstrate what happens when he removes his hand from under the
car.

What happened to the car? The car fell to the ground.

Did you push the car down? No. Pull the car down? No.

What might have caused the car to move or fall to the ground? Accept all answers.

We know that an object at rest will stay at rest until a force moves it.

So, what’s this invisible force that pulls the car down towards the ground?

Let’s find out!
2. Show the PowerPoint, “Isaac Newton and Gravity: The Invisible Force,” or read an excerpt from a book
on the same topic.

Tell students to use their Gravity Worksheet to respond each time the PowerPoint asks, “Is Gravity
Working?”.
3. Have students get out a textbook. Instruct them to stretch their arms out to their sides with the textbook
in the palm of one hand and the toy car in the other hand. The book and toy car should be “at rest.”
4. Allow them to hold this position for 15 seconds or until you see that they are straining. Ask students to
describe what they feel. Students may say that the book is heavy, they have trouble holding the book up,
or the book makes their arm tired or sore.
5. Tell them they are experiencing the downward pull of gravity.
6. Which object is being pulled with more (gravitational) force, the book or the car? The book is being
pulled with more force.
7. Why do you think the book is being pulled with more force? Accept all answers. See teacher note.
8. Tell students that although the force of gravity may be invisible, it can be measured and/or compared.
9. Demonstrate how to make a binder clip forcemeter or provide one for each group of students.

Thread the rubber band through one of the wire handles on the binder clip. Loop the rubber band
through itself.

Thread the other end of the rubber band through the opposing wire handle of the other binder clip.
Once again, loop the rubber band through itself. You will have to pull the other binder clip through the
loop.

Securely tape the pen or pencil to the top of a filing cabinet so that it sticks out an inch or two past
the vertical surface of the cabinet. Any hook or nail that attaches securely to a wall, door facing, or
bulletin board will serve the same purpose.

Hang the 1st binder clip’s wire handle (without the rubber band) from the pencil, hook, or nail,
allowing its wire handle with the rubber band to hang down the vertical surface.

The other binder clip attaches to the load.

Tape the sheet or strip of cardstock along the length of the forcemeter. Draw a line on the cardstock
opposite the lowest end of the rubber band.

You are now ready to compare the force of gravity on various loads.

Try attaching the end of a ruler to the forcemeter. Mark the lowest point of the rubber band and label
the mark with “ruler.”

Try attaching a thin book, a bigger book, a box of crayons, a box of playing cards, the toy car, a
baggie of rice, etc. Each time, mark the lowest end of the rubber band and label the mark with the
load.

Students can use the symbols <, >, = to compare the force of gravity on the various loads.

10. Follow the activity with an explanation of weight.

You have felt and compared (or measured) the pull of the Earth’s gravity on various objects. Weight
is a measure of that pull or force.

You measure your weight by stepping on a forcemeter know as a scale. Most of you probably know
your weight in pounds, but in the metric system it is measured in newtons.

Since weight is a measure of the pull of gravity, your weight will change if you go into space or go to
the moon. The force of gravity is less there.



11. This would be a good time to add the word, “gravity,” to the word wall. Make sure that students
understand that gravity is a force that pulls objects toward the ground.
12. Summarize this section by sharing a children’s book about gravity.
© 2009, TESCCC
Notes for Teacher
Suggested Time: Day 8
MATERIALS:
 PowerPoint: Isaac Newton
and Gravity
 Handout: Resources –
Force and Motion.
 Handout: Gravity
Worksheet
 textbook
 spring scales (opt)
 forcemeters
 journals
Misconceptions: An object at rest
has no forces acting on it.
See PowerPoint notes for
accompanying script.
See Handout: Resources - Force and
Motion for books on these topics.
Teachers Only: Mass and weight are
often confused. Mass is the amount of
matter in an object. Weight is a
measure
of the pull of gravity. However, mass
and weight are proportional. So, the
greater an object’s mass the more it
will weigh (in the same location). The
book has more mass, so it feels
heavier (weighs more). If you take the
same book to the moon, it will still
have the same mass, but it will weigh
less because the moon’s gravitational
pull is weaker. Students will address
the difference in mass and weight in 4th
grade math.
If you have spring scales with newton
units, substitute these for the
forcemeters.

Forcemeters – Illustrations
(Key)

binder clips-medium or large
(do not mix sizes)

rubber bands-medium

unsharpened pencil or
capped pen and duct tape.

access to a filing cabinet or
vertical surface to which a
nail, hook, or rod (pen or
pencil) is attached.

sheet or strip of paper or
cardstock for marking
intervals as a scale.

items of teacher’s choice to
serve as the loads.
You can calibrate the forcemeter so
that it is marked off in newtons. Do this
by preparing loads (baggies of rice or
sand) of 100 g, 200 g, 300 g, etc. Mark
the interval for each load and label it 1
N, 2 N, etc. as far as your device will
allow.
Keep the forcemeters or spring scales
for use in the next lesson.