Discovery Lab
PUSHES AND PULLS
2nd Grade
Station #4: Gravity and Friction
Supplies:
Wooden Ball
Glass Ball
*Additional balls can be ordered from Arbor Scientific (arborsci.com)
2 Aluminum Pans
Fine gravel
Goggles
Ruler
1-meter board- plain wood
1 meter board- covered with sand paper
1-meter board- covered with carpet
* All three boards are labeled and can be found in the Discovery Lab closet
Large wooden block with hook attached
3 kg spring scale
Part 1: Gravity
Background Information:
When you jump into the air, why do you fall back down again? You fall because of an
invisible force named gravity. Gravity is a force that pulls objects toward one another.
The strength of this pull depends on the amount of matter an object has—it’s mass. The
larger an object’s mass, the greater the pull. The earth is very massive and its gravity is
very strong. It pulls everything on it towards its center, from the smallest animal to the
tallest skyscraper.
Gravity pulls you downward with a certain amount of force. This amount is called your
weight. You can measure it by standing on a scale.
Because the moon has less mass than the earth, its gravity is much weaker. Astronauts
have only one-sixth of their normal weight when they are on the moon.
Gravity makes the planets orbit the sun and even holds the stars together in immense
groups called galaxies.
Everything has gravity. Even an apple pulls with a small force but the earth’s pull is so
much stronger that the apple falls to the ground.
Concepts to be learned:
Gravity is a force that pulls objects toward one another.
Because the acceleration of gravity is constant, two balls of different weights fall at the
same speed but the heavier ball falls with greater force.
Does a heavy object fall faster than a light object? We will drop two balls—a light one
and a heavy one—and see which one lands first.
1. Have the students take turns “weighing” the balls in their hands. The wooden
ball should feel lighter than the glass ball.
2. Place an empty aluminum pan on the table. Ask the students to put on their
goggles. Have one student stand where the station leader sits facing the other
students. Have the student hold the two balls about 12 inched above the pan.
Make sure his/her hands are at the same level when he drops the balls. Tell the
students to look at the pan and listen while the balls are being dropped so they
can see if they land at the same time. Release the two balls at exactly the same
time. Listen…. the two balls should hit the cookie pan at the same time. You
may need to repeat this demonstration a couple times to make sure everyone
agrees that both balls hit the pan at the same time. Allow a different student to
drop the ball each time.
3. Fill another aluminum pan with fine gravel. You need a thick layer so fill to the
top. Make sure the gavel is smooth and level.
4. Have another student drop the two balls making sure their hands are level and
they drop the balls at the same time.
5.
Have a third student carefully lift the balls to see which ball made the deeper
impression in the gravel.
6. The glass ball makes the deepest impression because the gravity pulls on it with
the strongest force.
7. Have a student use a ruler to measure the diameter of the impressions (It is
easier to measure the distance across the dents than their depth.) Again, the
heavier ball, which hit the gravel with the greater force, should leave an
impression with a larger diameter than the lighter ball.
If time permits, you can repeat this experiment one or two more times. Be sure to smooth
out the gravel between “drops.”
Part 2: Friction
Concepts to be learned:
Friction is a force that causes two things to resist motion when they are in contact with
each other.
Over time, friction slows down motion and stops it. Friction is measured in newtons, just
like other forces.
Lay the three boards side by side on the floor. Place the block that you will be pulling on
the plain wooden board first.
1. Hook the spring scale to the hook on the block. Have a child pull the block across
the board and read the amount of force (newtons) on the scale. Record the results.
Ask them if this was a push or a pull.
2. Repeat the above experiment on the carpeted board pulling the block over the
same distance. Record results.
3. Repeat with the sandpaper board and record results.
4. Discuss that it took more force to move the same block over the same distance on
the carpeted board and the one with sandpaper than it did with the plain board.
Ask them why they think this happened. Share the following information:
Friction is a pull on an object. Friction is a force that resists the motion of objects when
they are touching. It is between two things that are touching and sliding past each other.
Friction always acts against sliding motion. Friction is measured in newtons, just like
other forces. The friction between the block and the board was not as strong as the
friction between the block and carpet or the block and sandpaper. Which material creates
the most friction? If the force of friction is large enough, an object won’t slide at all. Can
you think of an example where friction created would be helpful? Sport shoes have
rubber soles that increase friction. This helps keep you from slipping while you run and
play. Your bike slows down when you apply the brakes. The brakes are rubber pads that
create friction when they press against a wheel. The friction slows the wheel and finally
stops it.