During this activity, participants will experience the contrast between gravity and other forces. The activity consists of four 'stages,' and in each of them, gravity will "compete" with one of the following forces:
- Stage1 - Buoyant force
- Stage 2- Friction force
- Stage 3- Magnetic Force
- Stage 4- Reaction Force
At the end of the guided activity, students can be asked to experience forces on their own on a Tinkering table.
Preparation
Preparation of "metallic cubes": half of the wooden cubes are prepared with 12 nails (2 nails for each face of the cube), covered with copper wire, and then with an aluminum layer. These are the the 'metallic' cubes referred to in the text. (See Figure 5)
Figure 5: preparation of the metallic cube
Buoyant force stage: put the boxes on the table, far enough from the edges and from each other. Then put water into each box up to 4 cm from the top of the box.
Figure 6: material prepared for stage 1 - Buoyant force
Friction stage: place the inclined plane on the table and secure it with something stable (rocks work well; alternatively, try to keep the plane steady with your hands). Initially, the plane should have a low inclination (Figure 7).
Figure 7: Set-up of the inclined plane used for Stage 2 - Friction force.
Magnetic force stage: remove the lids from the shoeboxes and place the boxes on the table, using the smaller face as the base. Then, fix the magnets to the inner side of the upper face using glue (Figure 8).
Figure 8 : Set-up of the box with magnets used for the Stage 3 - Magnetic force.
Normal force stage: place two shoeboxes (with lids) on the table, using the larger faces as the base. Keep some sheets of paper within reach as well.
Figure 9 : Material needed for the Stage 4- Normal foce
Figure 10 : Set-up for Stage 4 - Normal force.
Beginning of activity
Every participant receives from the teacher one wooden and one "metal cube". The teacher explains the activity, telling the participants that there is a "tug-of-war" between gravity and other forces, and that they will discover in which cases gravity wins and in which cases other forces win.
Stage 1- Buoyant force
The teacher briefly introduces the buoyant force and asks the participants to put the wooden and metal cubes in water (Figure 11) to demonstrate how the force acts on different materials.
Figure 11 : Buoyant force in action
The teacher can ask the students to observe the phenomenon and reflect on what they see:
- What happens? (the wooden cube floats, while the metal one sinks)
- Why? (the buoyant force overcomes gravity as wood has a lower density than water. But the opposite occurs for the metal cube).
Stage 2 - Friction
The teacher briefly introduces friction, and then asks the participants to try dropping both their cubes along the inclined plane, using only the unpainted part (Figure 12). The metal and wooden cubes fall at different speeds because friction depends on the materials.
Figure 12 : Friction in action: two cubes falling down the unpainted part of the inclined plane.
Reflect on what you observe:
- What happens? (the cubes do not fall)
- Why? (friction is stronger than gravity, as it is made stronger by the tilting of the plane)
Now let’s make friction weaker by increasing the inclination of the plane, until the cubes start falling. Now gravity wins. However, cubes fall slowly: the teacher asks the participants to check if this fall is slower than the free fall (it is). The teacher can also ask the students to observe what cube falls more quickly (the metal one) and highlight that friction depends on the materials involved.
The teacher can also ask participants to place their wooden (or metal, as long as they are of the same material) cubes on the two different parts of the plane (Figure 13): one cube on the painted part and one on the unpainted part, at the same height, and let them fall. Even though the cubes have the same weight, the one falling along the painted part of the plane is slower because the paint increases the friction on the plane.
Figure 13 : Friction in action: one cube falls down the unpainted part of the inclined plane, while the other one on the painted part.
Stage 3 - Magnetic force
The teacher briefly introduces the magnetic force, then asks the participants to place the cubes against the inner side of the upper face of the box (where the magnets are glued) and to let them fall (Figure 14). We can observe that the magnetic force only acts on metals.
Figure 14 : Magnetic force in action.
Stage 4 - Normal force
The teacher briefly introduces the normal force, then places the lid of one of the boxes as a bridge between them and asks the participants to put the cubes on top (Figure 8).
Figure 15 : Normal force in action using the show box lead.
The cubes do not fall. Afterward, the lid is replaced with a sheet of paper. The cubes fall (Figure 9).
Figure 16 : Normal force in action using a sheet of paper.
At this point, participants can be asked how to make the paper more resistant. They may discover that folding the paper can make the sheet strong enough to prevent the cubes from falling (Figure 10).
Figure 17 : Normal force in action using a folded sheet of paper.