District of Columbia Grade 3 Science

Use a force model to plan a fair motion investigation

Compare balanced and unbalanced horizontal pushes, separate a model prediction from observation, and follow a selected DC Grade 3 NGSS reference.

The DC science standards directory identifies NGSS adoption in December 2013 and links the standards publisher. Use the Publisher NGSS document for the selected performance expectations and their examples and boundaries. This activity selects a Grade 3 expectation. It is one part of a science course, not a complete curriculum.

Predict what happens to a block that starts at rest

The table describes a simplified model of a block initially at rest. It treats the listed opposing pushes as all the horizontal forces on the block. Compare equal pushes with a stronger push from the left. Predict whether the motion will change, then explain what observations a classroom investigation would need.

Explain the evidence or model

In Model A, the equal pushes act in opposite directions. They balance, so this initially resting block remains at rest in the simplified model. In Model B, the push from the left is stronger than the opposing push. The overall horizontal push is toward the right, so the block’s motion changes toward the right. The important distinction is not merely that there are two pushes: their relative strengths and directions matter. These statements are predictions from the model, not reports that a real block was tested.

What to check in the explanation

“Two pushes always cancel” overlooks unequal strengths. “Balanced forces mean no forces exist” overlooks the two opposing pushes. In a real setup, the surface can resist motion; a visible hand push alone is not a complete account of all horizontal forces. Keep the starting condition explicit as well: the model begins at rest. Balanced forces do not mean that an object already moving must automatically stop.

Plan the next activity

1. Draw an arrow for each opposing push and explain its direction and relative strength.
2. Predict the motion for the initially resting block without inventing numerical force measurements.
3. Plan repeated observations with the same block and surface, changing one aspect of the pushing arrangement at a time.

Use an adult-guided classroom demonstration to compare arrangements and record whether the object stays at rest, starts moving, or changes direction. Decide how to make the pushes comparable and account for resistance from the surface; human pushes are difficult to reproduce precisely. Record uncertainty instead of declaring a perfect balance from one attempt. Discuss how the observations support or challenge the simple model. This activity focuses on qualitative comparisons, rather than calculations with force units.